Issues in rangefinder engineering
Intro.
The Rangefinder mechanism, the attainable accuray, the required precision of engineering and assembly make for fascinating reading and study.
Let us first review the simple mechanism in the SLR to get an idea what is involved.
Generally we have two separate actions when focusing the lens on the object: 1. setting the distance by physically moving the lens relative to the film plane 2. checking the setting by visual inspection to ensure the movement gives acurate focus.Both acts are indeed separate and do not need to be connected. When we use a fixfocus box camera, we skip both and concentrate on framing. When we set the distance ring of the lens to some estimated hyperfocal distance, we do action 1, but not action 2.
The SLR system.
With the SLR mechanism both actions are combined in a direct way. That is: moving the lens and visually inspecting the sharpness are one act and we use the same mechanism.
In the SLR the lens transmits the rays which are focused via a mirror to a groundglass. We assume correct focus, when the image on the groundglass is sharp, that is when contrast is highest (coarse screen) or details can be seen clearly (fine screen). This principle is identical to the one used in a technical camera. As we can always move the focusing ring of the lens backward or forward over a large distance, we can always find a point of sharp focus. We do not have to know the focal length of the lens or bother about deviations from the nominal or true focal length. Or even about the true distance setting of the lens.
All we need to know is the simple fact: is the object in sharp focus? And this we can establish by looking at the focusing screen.
There are no mechanisms to transmit any mechanical information from lens to body and none are needed. That is why you will not find focusing groups engraved on a R-lens. The manufacturer will of course stay within the tolerances of the focal length, but for the focus mechanism it is irrelevant whether the actual focal length is 51.3 mm or 49.9 mm. Both lenses will give accurate focus at the specified focal plane (51.3 or 49.9), when the projected image on the ground glass is found to be sharply focussed by visual inspection.
As long as we can ascertain that the distance from the mirror to the groundglass (reflected ray upward) and the distance from the mirror to the film plane (transmitted ray horizontal) are the same, we will locate the focal plane image at the same location where the film plane is located.
Technically the Leica R body is partly assembled, including the chassis and the lens flange, and then at the end of the assembly line, the film guide rails are machined to the required specifications and automatically aligned parallel to the lens bayonet flange. The mirror box is machined and adjusted such that the two distances (mirror-ground glass and mirror-film plane) are within specifications.
Separately, any lens has its own distance from bayonet flange to focal plane and this distance is the same, irrespective of the actual focal length as a 19mm lens should focus the rays to the same physical location as does a 600mm lens. In the R series this is given as 47mm (see Osterloh books, where he names it wrongly "distance to film plane").
Specifications?
What are the required specifications? The film plane is not, contrary to what you will read in most books and hear in the publicdomain, a fixed location. Film support, remarks Goldberg in his famous book: Camera technology, is very complex. Sensitized surface must be located at the desired distance from the lens (our flange to focal plane definition) and lie in a plane coinciding with the image plane. Between exposures the film is transported and may not be scratched or subjected to mechanical pressure. Film surface location is the most difficult and makes "camera manufacturing an art as well as a science" (Goldberg)
Film is put through the camerabody inside a film channel (invented by Zeiss and not used in the TM Leica bodies), comprising an outer guide rail, and an inner rail. The film is held in place by the outer rails and the pressure plate that rests on the outer rail. The inner rail should hold the outer edges of the film in a flat position. BUT: The distance between both rails is 0.2mm. Exact distances vary as manufacturers have different tolerances. But on average a film has a thickness of 0.13 to 0.18mm. Thus the film has a clearance of 0.02 to 0.07mm and this is enough to jeopardize ideal film flatness. Films tend to bulge forward.
So the designer has some options when he has to locate the exact focal plane for his lens. Use the outer rails (lens flange to pressure plate distance) and be sure that you will miss the emulsion of the film where the image should be located. Use the inner rails and the flatness of the film is a problem. So here we have the real problem of locating the image plane. Every manufacturer has its own ideas and in the case of Leica they have deceided that a distance of 27.8 mm from the bayonet flange will locate the image plane inside the film gate dimensions and take care of the curving of the film it self. The wellknown dimension f 27.8mm is often described as the flange to film plane distance. Correctly described iwe would have to say: distance from flange to focal plane, which might be identical to the film plane under a certain set of assumptions.
Let us look at actual figures. In the leica M-series the distance from bodyflange to outer film rail is 27.95mm. The thickness of the film channel is 0.2mm. (I measured 0.21mm). So the area where the film might be located ranges from 27.95 to 27.75. With a lensflange to focal plane distance of 27.80mm the lens, when correctly adjusted would focus onto the film emulsion layer.
For the Konica, I measured film channel depth of 0.24mm, distance from body flange to outer film rail of 27.95mm. This last dimension is identical to the Leica figures but outside the official Konica specs of 28.00 +/- 0.03mm. More measurements would be required, but this example indicates that the Quality Control Criteria at Konica are somewhat more relaxed than the factory specs indicate OR I had a version which had been adjusted already
The M-camera RF system.
If we now focus on the M-camera, we see that the rangefinding act of the SLR cannot be duplicated. We have an indirect relation between the two actions. We focus manually by moving the lens mount, but we check the distance setting with a separate act and mechanism: the rangefinder. There a number of identical engineering elements between the M and the R: film channel distance from pressure plate to body flange distance from lens flange to focal plane (27.80mm with the M)
The new element is the coupling between the lateral rangefinder movement (the distance that the ragefinder patch moves) and the physical axial movement of the lens mount. This is done through a very complicated engineering trick: the roller cam and arm on the RF side and the steepness and angle of the distance curve on the mount. Disregarding here the issue of the focal length groups (where you match the pitch of the curve to the exact focal length), we need a mechanism to translate the axial displacement of the lens mount to the axial displacement of the RF patch. The roller arm and cam do the job and this humble instrument is the Achilles heel of the RF system.
The roller cam and the curve on the lens mount should match exactly and any movement on the cam or the fact that the cam does not follow the curve exactly (tolerances, non-parallel surfaces) is a source of trouble. The mechanism of the coller arm and cam in the Konica are not as sophisticated as the one in the Leica, and any misalignment here could well explain in part the focus errors.
Leica-Konica compatibility.
It is clear from the facts that the dimensions and tolerances between Leica and Konica differ, even if the M-bayonet and the KM-bayonet fit.
The mismatch that has been reported, may be caused by any of the factors involved: differences in flange to pressure plate distance differences in film channel thickness differences in cam/curve engagement differences in lens flange to focal plane distance differences in tolerances differences in engineering solutions (the Konica roller arm and cam are more sensitive to changes and tolerances than the Leica version differences in film flatness between several film types.
It is quite rash to identify one of these spects as being the sole source of the reported mismatches between Leica lenses and Konica bodies.
Verification in the "field": the many pitfalls.
Several individuals have checked that their Leica lenses on a Konica body do not deliver results that differ from what they expected to get from the combination Leica lens - Leica body.
This is a minefield, really. The checks as reported used short focal length lenses or close focus distances or any combination that was available.
FIrst of all: these checks without a proper and methodologically sound lab situation with controlled comparisons and predictable results, are quite subjective and without merit I am afaraid to say. If one does not know what the results have to be from a set of quantified paameters, how can one reliably remark that the problem dos not exist?
Checking a Konica body and a Leica lens and noting that the combination gives correct results at a certain distance, because the tester by visual inspection gets a sharp picture, is not a proof. The conclusion depends on what the observer accepts as "sharp" and without an objectified definition of that most elusive concept, we are lost in the desert.
Depth of Focus.
This is defined as the difference in distance that the film plane or the focal plane may move axially without disturbing the sharpness impression. Lenses are habitually checked in the design stage for the tolerance in through-focus MFT values. How does the contrast suffer when the ideal plane of focus is shifted for a certain amount. Lenses can even be tuned to be rather insensitive to changes in depth of focus by optimizing the through-focus behavior. But in general: if we do not know anything about depth of focus in addition to what we noted as elements of possible mismatch, the entire discussion and "proofs" of (in)compatibilty is vapour ware and more misleading than enlightening.
Goldberg uses a simple rule of thumb to estabish the depth of focus as the product of blur diameter and and aperture. If the blur diameter is taken as 1/30 or 0.03mm and the aperture as f/2 we have a depth of focus of 0.06mm. So if our combination of tolerances would be off by 0.06mm or 60 micron, a drop in contrast and image quality would result. As the drop in quality would be more noticeable as a drop of contrast than as a drop of resolution (see my Hexanon 2/35mm test), it might go unnoticed by many who check the sharpness of the image, by looking at a single negative, trying to check visually the plane of best sharpness. If we use a more realistic blur circle diameter of 1/100mm (0.01mm) to cover the improved definition of current leica lenses and the bigger enlargements that are possible, we get these figures:
1.4 at 10 micrometer (0,01mm) = 14 micrometer = 0.014mm
2.0 at 10 micrometer = 20 micrometer = 0.02mm.
We see that in the domain of critical and perhaps limiting demands, small differences are indeed significant for image quality.
A more sophisticated equation relates the depth of focus to object distance and here we see the opposite of what we assume: depth of focus increases at the shorter distances and so will cover any imaging defects, caused by a possible mismatch. The differences however are quite small and we may conclude that for photographic purposes the depth of focus is the same at all distances from 0.5m tot infinity.
Why such resistance among Leica users to accept the implications.
It is clear from the various discussions and "quick tests" on the many discussion forae that most people are more interested in preserving the status quo (no incompatibilities), than finding the facts (there are incompaibilities, here is why and that we can do about it).
That is bad for the ones who want to research, present and discuss the facts. But if we would estabish the matter onece and for all, a large part of the casual discourse on the newsgroups would have to vanish. Popular discourse (it is called coffee table chat in Europe) thrives on the basic idea of tolerance and ambiguity. Relativity may be difficult in science and pysics, it is common place in the flow of exchange of opinions. If we had nothing to argue about and to relativize what coud we talk about? If we agreed on a set of parameters for lens performance, we could end the discussion which Summicron 50mm is the best. But we would run short in topics to discuss and chat about. Scientific precision is anathema for the relative leniency we need to continue to chat. We can engage in a long discussion if we are able to say that it may be the case that person A has noted that lens X is better than lens Y, but person B says the opposite and A has used a different test target than B who is a real photographer. Imagine that we would be able to fix on a scientific basis (once and for all) that tennis player Anna Kournikova is more beautiful than film star Sophia Loren. 90% of all our talk would be suferfluous -:). So we need to engage in ambiguities and relativisms to continue to chat about our beloved topic: the leica and its surroundings.
As long as we are not interested in fact finding and accepting the consequences of these facts for our quest for best leica imagery, we will stay in a backward society
Additional info about the Hexar-Leica compatibility problem.
Now the continuing saga of the Hexar/Leica lens compatibility. First a few remarks: You can not measure the actual distance from bayonet flange to pressure plate by using the pressure plate itself as a reference. The slightest and unnoted pressure from the instrument itself on the pressure plate will give errors and the pressure plate itself is hardly ever a plane itself. So additional errors. The only way to do it is to remove the pressure plate and insert a device that is calibrated to be at the same distance where the pressure plate ideally has to be. To start from here. The distance from the bayonet flange to the pressure plate or more accurate the top of the outer film guide rails ( pressure plate rails) in the Leica M is 27.95mm. This distance is also (but wrongly referred to as register. But this distance and measurement is used to check if the guide rails and the bayonet flange are parallel to each other and have the correct distance. The second important measure is the distance from the film rail (the innermost film guide rails) to the bayonet flange. In the Leica this is 27.75mm. The film gate then has a distance of 0.2mm. In every Leica book I know of there is a reference to the filmplane/flange of 27.80mm. What is this. Rogliatti, Roger Hicks, Collectors Checklist, Hasbrouck you name them, all refer to flange to film plane distance or flange to film register. Now in German the word is "Auflagemass". This can be correctly translated as "flange focal length" or "flange focal distance". But this measurement is done for the lens itself on a collimator where the lens is adjusted such that the distance from the lens bayonet flange to the true optical focal plane (focal point) is indeed exact 27.80mm. First lesson: NEVER believe what is written about Leica in books that are focussed on history or collecting: these persons are no engineers. In every other book, check, double check, triple check to make sure the person knows what he talking about.
To sum up: we have an optical measurement done on the lens to adjust the flange focal distance and that distance should be 27.80mm. We have a mechanical measurement on the Leica body, which is the distance from bayonet flange and the pressure plate rails which is 27.95mm. The film gate is 0.2mm. If we now use a film with a total thickness (emulsion plus base) of 0.13mm (APX25 as example) the thickness of the film will not fit into the film gate. There is some play and therefore the film will curl and curve inwardly (away from the lens). By using a focal distance of 27.80mm, Leica will ensure that the film when bowed a little, still will be correctly aligned in relation to the focal plane. It is intriguing to note that thick colour neg films of about 0.27mm will fill the film gate completely and the pressure plate will press the film to a plane position, instead of the curved position with thin film emulsions. Theoretically a thick film would have a better flatness than a thin film. Of course more research is needed, but these investigations do show that the information in the public domain is at best scanty or at worst misleading.
Now for the Konica Hexar. Here I have only one official fact: that is the bayonet flange to the pressure plate rails of 28.00mm. But I do not have official info about the flange distance to the film rails (or film gate distance). Nor about the lens flange focal length. My own measurements on one Hexar body and lens showed that the film gate had a thickness of .24mm and the lens a flange focal length distance of 27.71. On the basis of these measurements the flange to film rail distance is 27.76mm. These results are however not reliable enough to draw firm conclusions. What I do know from discussions with Konica people is that their tolerances are wider than with leica and are choosen such that the best fit of Hexar body to hexar lenses is assured. The many inconclusive reports about problems or the lack of problems with fitting a leica lens on a Hexar body is partly to be explained by these tolerances and partly by the unreliability of the reports themselves. The Konica people at the factory told me that the Hexar is designed for use with the Hexanon lenses and that all dimensions inside the Hexar are based on that fact. If a hexar user fits a leica lens and he has problems, than it is caused by these different dimensions and/or the chain of tolerances add up unfavorably. If he has no problems: than he is plain lucky as the tolerances are such that they are close to what is expected for leica bodies and/or his demands are such that they are below the visibility threshold for the mismatch to show up.
Conclusion:
Konica and Leica bodies and lenses are made with different dimensions and tolerances and different engineering solutions and implementations. While the overall approach is comparable (Konica did copy most of the mechanisms that reside inside a Leica rangefinder, but added some unique engineering that it presumably tuned to their ideas about the RF mechanism.
These dimensional and engineering differences would be ample argument, for me at least, not to use Leica lenses on a Konica body: the manufacturing and design philosophies differ too widely. The Konica is a fine instrument in itself and should be considered a system with its own lenses. Who would put a BMW engine in a Mercedes-Benz even if it would fit??
Rangefinder accuracy
The limits of the eye.
Now to more important matters: rangefinder accuracy. Obviously any measuring instrument has some tolerances, mechanical and optical/visual. The RF of the Leica measures the distance of an object by superimposing two images of that object and noting the degree of coincidence of both images. If both images fully align, the distance measured is correct. As our eye is the critical factor here, the limit of accuracy is dictated by the eye's visual resolution. So any equation that tries to compute the RF accuracy has this limit of visual resolution incorporated. The necessary accuracy is also defined by the blur circle that relates to the depth of field. The eye has a maximum limit of resolution of 0,06 mm at a viewing distance of 25cm, translating to 8 linepairs/mm. Mostly we use a more practical limit of 0,1mm, that translates to 5lp/mm. Even this limit is too fine for most uses and so the industry settled to a more convenient 2 lp/mm as the norm for optical formula. These 2 lp/mm translate to a distance between two adjacent objects (points or lines) of 0.25mm (1 mm divided by 4). As we are talking here about the print or transparency , we need to translate this figure to another one on the negative. Assuming an 8 times enlargement factor we divide the .025 mm by 8 and we get at 0,03mm: the famous diameter of the blur circle. The importance of this blur circle is this: as long as a point on the negative is smaller than 0.03mm AND we limit our enlargement to about 8 times , all points will be visually sharp as perceived by the eye.
The depth of field distance is based on this assumption. We know that in reality we only have an infinitely small sharpness plane that is 'artificially' extended into three dimensional space by this DOF mechanism, combined with the resolution limit of the eye.
The rangefinder in theory measures a point in space at only one exact distance, which is not feasible in real life. There is always a certain latitude in measuring accuracy: the focusing error. Slightly before and slightly behind the real distance the instrument will give identical readings. So as a bottom line for rangefinder accuracy we must state that the distance of the focusing error is at least equal or less than the DOF distance. This is the minimum demand.
As the rangefinder is based on triangulation, we do not use in our equations lp/mm but angular resolution.
For the limit of 0.06mm the angular resolution is 1 minute of arc. For the mostly used 2 lp/mm the angular resolution is 3,4 minutes of arc. The former figure relates to optimum viewing conditions and the latter one to normal conditions.
We are almost there! The triangulation method obviously is more accurate as the base length is larger. Leica has an effective baselength of 49.86mm for the M2/4/5/6 and 58.863 mm for the HM series. Contrary to most authors I must state that the physical base of ALL Leica bodies from M1 over the M3 to the latest M6 is identical (69,25mm). The only difference is the magnification (0.58, 0.72 or 0.85 or 0.92). Any equation that computes the RF accuracy will use at least three variables: Effective base-length, visual resolution in angle measurement and blur circle diameter. These are intimately related. In my earlier computations I used an angular resolution of 1.6 minutes of arc, as an average between the minimum and maximum figures. I now use a somewhat more complicated equation too to reflect the realistic figures.
The correct results using a more narrow angular resolution and the other equation are as follows.
Focal length | Aperture with 0.58 | Aperture with 0.72 | Aperture with 0.85mm |
| 50 | 0.42 | 0.34 | 0.30 |
| 75 | 0.98 | 0.79 | 0.67 |
| 90 | 1.4 | 1.13 | 0.96 |
| 135 | 3,2 | 2.55 | 2.16 |
This table gives the limits of accuracy when all variables are ideal: high contrast image, the eye at its best etc.
It is clear that up to 50mm the accuracy is well above any critical demand. Thus for all the rest of this article we limit ourselves to 50mm and above. It does not make sense to present figures for focal lengths, shorter than 50mm as the limiting value for focal lengths from 21 to 50mm is from an aperture of 0.13 to 0.34. This table tells you that even with a magnification of 0.58 (the new Leica M-viewfinder model) the theoretical aperture for accurate focusing with a 90mm lens is 1:1.4. Remember however that these figures are based on optimum engineering accuracy, optimum eye condition, high contrast image etc.
There is another approach for the determination of RF accuracy.
The question now is: given a focal length, a maximum aperture and a defined diameter of blur circle what is the value of the effective base length. When using this approach we need to distinguish between the resolution of the eye when point objects are involved and when lines are involved. The eye is much better at determining when a broken straight line is not aligned that at determining the distance between two object points. The former property is called vernier acuity and is employed in the Leica RF, explaining its uncanny accuracy.
Doing some more calculations, based on equations that are used by Leica we get this table;
Based on point distance discrimination
Focal length | Aperture | Effective base length needed |
| 50 | 2.0 | 12.5 mm |
| 50 | 1.4 | 17.9 mm |
| 50 | 1.0 | 25.0 mm |
| 75 | 1.4 | 40.2 mm |
| 90 | 2.8 | 28.9 mm |
| 90 | 2.0 | 40.5 mm |
| 135 | 4.0 | 45.6 mm |
| 135 | 3.4 | 53.6 mm |
| 135 | 2.8 | 65.0 mm |
The vernier acuity is 6 times more accurate than the point distance discrimination. In theory! But the tables are based on a conservative blur circle of 0.03mm. If we would like to use the optical quality of leica lenses to the most, we need a blur circle diameter that is 2 to 3 times smaller. So the figures presented could be halved again to represent theoretical accuracy. Here I am conservative and would use the table above as reference. But be aware that the accuracy now is good enough for the rendition of extremely fine detail at enlargements of 15 times for critical close view inspection.
Yet another way to get a feeling for RF accuracy is a tabulation of the focusing error at several distances for point and vernier acuity
Point acuity.
Distance | Magnification 0.58 | Magnification 0.72 | Magnification 0.85 |
| 1 m | 7.45 mm | 6 mm | 5.0 mm |
| 2 m | 29,79 mm | 24 mm | 20.0 mm |
| 3 m | 67,03 mm | 54 mm | 46.0 mm |
| 5 m | 186,21 mm | 150 m | 127.0 mm |
| 10 m | 744,83 mm | 600 mm | 509.0 mm |
| 50 m | 18672,83 mm | 15042 mm | 12741.0 mm |
The figures for 10 and 50 meter are NOT a typing error. It tells you that at a distance of 10 meter the focusing error is 60 cm plus and minus. At a 50 meter distance the error can be up to 15 meter. This last figure tells you that long distance focusing is a game of chance. Enter now the vernier acuity in the Leica RF
Vernier acuity.
Distance | Magnification 0.58 | Magnification 0.72 | Magnification 0.85 |
| 1 m | 1,24mm | 1.0 mm | 0.8 mm |
| 2 m | 4,97mm | 4.0 mm | 3.4 mm |
| 3 m | 11,17mm | 9.0 mm | 7.6 mm |
| 5 m | 31,03mm | 25.0 mm | 21.0 mm |
| 10 m | 124,14mm | 100.0 mm | 85.0 mm |
| 50 m | 3112,14mm | 2507.0 mm | 12124.0 mm |
This table tells you that when circumstances are ideal even the most accurate range finder at 50 meter could be off for 2,5 meter plus or minus. Note also that at a more realistic distance of 10 meter we can expect accurate measuring within 10 cm plus or minus or a range of 20 cm. As it makes more sense to give a percentage, we note here that accuracy is ± 1.5% at 10 meters and at 50 meters we have an error of 6%.
It should be clear that these figures are the theoretical most optimistic and most pessimistic values. In reality the values would be in between.
I did some practical measurements and at a distance of 3 meter I arrived at an error of 15mm, above the theoretical figure of 9mm, but still very good.
I hope this discussion will be helpful in assessing the RF problems we all encounter and to take into account the many variables that are needed to analyze the RF accuracy in a scientific matter.
Rangefinder engineering from M3 to MP
Intro.
The Rangefinder mechanism is the most famous component of the M-series of cameras and its study is of some importance to understand recent discussions about rangefinder flare. This phenomenon can be experienced by a whitening or washing out of the rangefinder patch. Let us first make a few statements:
(1) Many users of Leica cameras of all models do not experience this phenomenon at all. It does indeed only occur in situations where a large area of high luminance is obliquely positioned relative to the camera. It is not necessary to use a spotlight or another high intensity light source. Even a diffuse but large area of light (a window as example) will do. Not every one uses the camera in this situation and then there is no flare problem. When the eye is focused close to the ocular and is aligned to the optical axis the chance of flare occurring is even less.
(2) Several users report the occurrence of flare. With the exception of the M3, ALL models of the M will under some circumstances have the whitening of the finder patch. I have reports from users who state that M2, M4, M5 will in some conditions show the whitening of the patch, but less intense than users of M4-2, M4P and M6 will experience.
The flare phenomenon is certainly annoying sometimes, but one should be aware that it can be overstated too. In this report I willt try to clarify the situation with respect to the several models of the M-series
The rangefinder flare.
To understand the origin of the flare, look at the picture below. The middle window illuminates the finderframes (shown in black): the incoming rays are angled,and directed to the flat mirror and redirected from there to the mask with the framelines. In the center of the mask you will see the rectangled hole that will form the rangefinder patch in the ocular. The user sees in the ocular the projection of the framelines and the superimposed rangefinder patch from the image that is coming from the outermost rays in the right.part of the diagram. In fact he sees two images: one coming straight from the finder window (left rays) and one coming from the right through the prism. When focusing on the object is correct, both images coincide. If not you see a double image in the rangefinder patch.
If you want to have very bright lines for the delineation of the finder frames, you must focus as much energy as possible onto the finder mask area. Then the chance for the occurrenc of flare is higher.
Look at the Konica Hexar RF finder, that is a very good copy of the finder in the M-camera. The Hexar finder has somewhat dimmer frames and the whole finder is not so bright overall. But the propensity for flare is lower.
The explanations that the addition of the light diodes in the M6 or the curved mirror are part of the cause are not correct. The MP is flarefree and still has the light diodes and the curved mirror!
The path of the light through the system can be followed quite well in the colour picture below. The yellow beam is the diffused lght from the frame illunination window. The red beam then crosses the yellow beam and in this interference we see part of the problem.
To see the rangefinder patch quite clearly, you need to look through the hole in the reflecting mirror. (follow the red arrow). To get as much light as possible onto the frame lines of the rangefinder mask, the mirror must reflect as much light as it can. Preferably there should be no hole at all.
Why the M3 has no flare?
The M3 finder is a totally different construction. Now the framemask is parallel to the window and the rays do not have to be angled several times. Any angle will cause problems! And there is no mirror for the illumination of the frames. There get the light directly from the finder illumination window.
The construction of the M3 finder is dedicated for a viewing angle of 50mm and higher. You cannot go to 35mm as example. That is the reason why the M2 finder had to be of a different construction. All subsequent models (M2, M4, M5 etc) share the same basic design.
M2 and later models share the same basic design
See below the construction for the M4. It is very similar to the design of the M4P and M6 and later models. The only exception is the small field lens on the framemask that collects the rays to illuminate the 90mm frame lines. There is some mystique here that this small part has been deleted from the construction to save the Leitz company so much money that they could survive. This is not true. The cost of that part and even the additional cost of assembling is a fraction of the cost of the whole camera. What is known is that in later models the separate mirror and framemask were redesigned as a unit. This changed the geometry and I assume that the designers were confident that this would be as effective as the field lens solution. The M-models from M3 to M4-2 had glass masks on which the frames were etched. Later models (starting with the M4P) had a metal mask to accomodate the six frame lines and dropped the field lens (it could not be fitted in the assembly). To compensate for the loss of light to the 90mm frame, the designers came up with the curved mirror. They changed the geometry of the finder assembly and the result of this is the occasional occurrence of flare. When you do not look at the ocular at the line of the optical axis, you may look at the mirror in stead of through the hole. That is the basis of the flare. Therefore every model from M2 can show some flare. If you focus in a situation where there is a large amount of oblique or contre-jour light this light is reflected from the curved mirror and adds to the basic deflection of the light path.
The current situation: M7 and MP
A solution to the flare problem is generally not that simple, without returning to the classical M3 finder design. The magnification factor is another: The 0.85 is more flare prone than the 0.72 and the best here is the 0.58.
Improvements have been made incrementally.
The M7 has already a number of improvements: coated windows and coated prisms and lenses inside the finder mechanism. This reduces the flare substantially. Some persons assert that the M7 improvements have been in stages (first the windows, later the prisms). This is not true: all improvements were done at once and in all models from the beginning of the production of the M7.
The MP has additional improvements: the curved mirror has a somewhat different shape and the mask now has a kind of light collector in front of the mask. (as stated in the MP broschure). This mirror-mask system can be retrofitted in M7 models and I assume in M6/M6TTL too. In principle even an M4P could benefit from this solution.
As far as I know, Leica (or the distributors: anyone has its own policy I presume) can retrofit M7 models to a nominal cost and previous models to a fair price.
As far as I know all MP delivered from the start have the new unit and M7 models currently in production have that unit too, When the transition occured cannot be related to a serial number as the Leica policy (since the start of the R8) has been to see the serial number as an individual identification number and not as a number that identifies the production run and date.
Rangefinder scene: the others
The first golden age of the rangefinder.
When the Leica Standard arrived on the market in spring 1925, cameras with viewfinders in all kinds of constructions were already abundantly available. It was, so to speak, the natural engineering solution for the topic of framing the object that would have to be photographed. The revolutionary ideas behind the Leica were the filmformat, that allowed a small body and a string of 36 pictures, and the interchangeability of the high quality lenses with fast apertures. The all metal body gave dimensional stability, that allowed for a higher degree of accuracy and also durability and reliability. In its wake came the redoubtable Contax I, certainly over engineered, when compared to the sleek and effective Leica, but in optics and accuracy the Zeiss people were as good as, and many claim, even better than the Leica.
With the Contax the combined rangefinder/viewfinder and the bayonet mount introduced the last evolutionary step in the rangefinder camera growth to domination. In its wake many German companies, produced variations and simplification, including the extremely popular Kodak Retina, which in fact did more for the candid photography and the acceptance of small format photograpy than the outrageously expensive, but leading marques, like Zeiss and Leica.
This era started in around 1930 and ended in 1950, when the Japanese entered the market.
The Second World War (some would argue with good reason, that this was just the continuation of the First World War or Great War) made an abrupt end to the era of 35mm-rangefinder based photography, simply because the resources were no longer available and people has something more pressing to do.
But behind the scenes, amidst great turmoil of East and West tensions, the Dresden factory of the Zeiss company had developed the pentaprism and could incorporate this into the Contax D, the first ever single lens reflex camera. Anyone had noted that the system expansion of the rangefinder camera was quite cumbersome. It is now the delight of collectors of course, but one cannot help wondering how the endless range of gadgets and accessories for the great system cameras of Zeiis and Leitz could have ever been designed and produced.
The SLR-concept offered much easier possibilities for system expansion and this potential became fully realized in the japanese system cameras that surpassed the rangefinder concept: the Nikon F, the Canonflex, the Pentax and the Topcon, to name a few.
From 1950 to 1970 however the rangefinder had its second golden age,
The second golden age of the rangefinder.
After the war, the industry picked up the evolution where it was temporarily halted and fell back, naturally to proven designs and evolved from there, with or without a stage of simple copying. During the war, the research of course had been continued, but these results were not yet available for civilian use. (An example is the coating of lenses).
Now the German industry no longer dominated the RF market, or even the photographic scene. Canon and Nikon elaborated with great success on the designs of the Leica and Contax. Some would call the designs of the Canon and Nikon RF's shameless copies of the originals, others (like myself) would argue that sound engineering solutions for the same problem, will be divergent to a common solution.
The Canon RF was more solidly built than the Leitz counterpart, the leica III series and the Nikon S, in several incarnations, showed more innovative solutions.
Zeiss made a quite lackluster attempt to revive the Contax with the Contax IIa and IIIa cameras, redesigned from scratch, but within the same mental frame of thinking as the predecessor. The result was an excellent tool, but no competitive advantages as we say it nowadays.
Anyway, Zeiss put all resources into the more promising SLR-concept and for a while became world leader with the Contaflex series. This leadership position was thrown away, in a way typical for the German Zeitgeist of that period with the amazing Contarex, which many collectors credit with the range of best ever lenses, which is not true. But then collectors are not wellknown for their knowledge of engineering or optical craftmanship.
Leitz on theother hand, answered the threat of the japanese RF's with the M3, that defined the essence and limit of the mechanical RF camera. We have this approach to RF-photography still with us in the M6 series, that inches slowly forward in a new evolutionary branch.
While the main battlefield concerned the Leitz, Zeiss, Canon and Nikon companies, there were many more companies, Japanese, Italian, French and German that produced rangefinder type camera. The most popular was the Japanese species, which evolved into the highly successfull point-and-shoot compact-cameras of today.
One of these was the Voigtlander company with the Prominent, a high class RF camera with interchangeable lenses and leaf shutters. A kind of 35mm Hasselbald so to speak. Its lenses were excellent and incorporated novel optical thinking, that could not be fully explored as the position of the leaf shutter hampered the passage of the rays.
Around 1960 one after the other of the great marques stopped production and so Leica was left alone as when it started 35 years earlier.
Photography as a cultural phenomenon exploded to stardom with that famous Antonioni movie (Blow-Up), but in fact photography was a major force in culture and society and the fashion photographer and photo journalist were role models. Nude girls and adventure respectively would have been additional imaginary rewards, no doubt.
With the Minolta AF innovation, the SLR reached its own limit of evolution and from there photography started to become a hobby and profession less and less pursued. Hobbyists embraced the video and the computer and TV became the new hero.
But that is not my story here.
Leica continued the RF tradition, capitalizing on the accuracy and stability of the body with ever better optics, that needed such a body and its narrow engineering tolerances to be exploited in the full.
The third golden age of the rangefinder?
Early in the 90's, the downward direction of the SLR market was evident to all to see. The P&S market was then and is still strong, but not everyone can be a large volume producer. And with a trend to retro-design and back to the roots, we now see a remarkable number of rangefinder models, beginning with the Contax G1 and G2.
The Contax G-series is beautifully engineered and styled and has a range of lenses with excellent optical quality that challenges the Leica range as it did in the first golden age. The handling however is not so pleasant and the small viewfinder, even with its zoomfinder facility is small and more slr-like in its feeling. You really miss the direct vision quest that is so prominent with the Leica M-series. Overall proportions are small, with brings a bit crowded feeling when using the buttons and trying to control the functions of the camera. As a very capable compact with a range of fine lenses is has no equal. My personal feeling for this camera is mixed. The imagery of is a high order as is the engineering. Handling is more akin to a P&S camer, but this can be used as a bonus too and for unobtrusive hand held candid shooting its AF- and AE-facilities give advantages that the M-user can only acquire after lengthy experience and dedication.
There is little progresss in the G-series, with the exception of the introduction of a zoom lens, that slots into the intended use of the camera. Trying to cover a niche between the high-end P&S cameras and the auto-pilot support of a classical rangefinder may be to demanding for one product without losing design integrity.
The Voigtlander Bessa-series, now with an L- (finder less), R- (finder with frames) and T- (rangefinder, but no viewfinder) version approaches the rangefinder niche with a different kind of design and philospophy. The choice of a modest-specification-body with the tradition oriented Leica-thread mount allows for a relatively low price. The body is a basic slr camera, without the mirror box, but offers enough functionality (shutterspeeds, auto-exposure) for a wide range of photoraphic goals. Handling of the camera is close to that of a simple SLR body, often called the student-SLR, like the Pentax K1000 and while adequte, does not inspire. It suffices however for its tasks.
The amazingly fast expanding range of lenses and finders and all kinds of accessories closely resembles the frenzy days of the first golden age, when Zeiss and Leitz quickly produced the tools for a total system concept. The several Besa versions can even be mirrored in the several stages of the Leitz bodies from the O-series to the III-series.
The recent T-version has M-type bayonet mount, with the other two being Thread Mounted. The Bessa line is in several ways challenging the Leica territory, but also expanding it, so it should benefit the growing interest in the rangefinder tradition of photography. It brings affordable access to this exciting species of cameras, which may even outlive the SLR, which sooner or later will be killed by the digital image capture instruments.
While many Voigtlander products are innovative and/or re-inventions of products from the glorious past, the company has its own set of engineering criteria and here we have to make a few remarks. There is a strong toy-like and must-have-one appeal in the Voigtlander offerings, but one may sometimes question the instrumental validity and mechanical qualities.
Still the Bessa system is an innovative and thought provoking line of products, that sometimes seem to mock the slow moving solid engineering progress of the German manufacturer of high grade RF-systems.
The Konica Hexar RF is the offering of one of the oldest and also very innovative japanese companies (AE in an SLR is a Konica invention, as is the motorized transport for SLR cameras). It uses the M-mount from the Leica, but not to the finest specs and so calls it their own dedicated mount. It also has the same shutter as is found in the Contax G-series, but for the rest it is a close copy of the M-camera. The rangefinder is almost a true copy of the one found in the M6. Superficially it seems as if most of the parts of the G2, Minolta CLE and some sprinkling of its own background have put successfully amalgamated into the classical M-concept.
Whatever the view of this approach, the RF is a very harmoniously integrated camera, and excudes the feel and sound of a finely engineered instrument.
Specs in a nutshell: body dimensions: a few millimeters bigger than the M6. Auto Exposure with electronic shutter to 1/4000 and the same white patch on the shutter curtain as the M6. Rangefinder base is also the same:69.2 mm. Magnification of finder: 0.6x. Manual focus. Integrated motorwinder. This mix of electronic, automated and manual features may certainly appeal to the longtime Auto-SLR user and simplifies the migration into the RF world. On the other hand: the manual features and the M6-type rangefinder might smooth the transition from longterm Leica users, who wish themselves sometimes a higher degree of automation support and higher speeds.
In this perspective is the strategy of the Hexar RF closely related to the Contax G approach of trying to seduce P&S and SLR users to the new simplicity.
As a photographic instrument, the Hexar RF is a joy to use. Well thought out functionality and good ergonomics make for effortless photography even in difficult situations. TTL-flash facility is lacking, which may or may not important for some users.
The rangefinder accuracy (baselength 69,2 and viewfinder magnification of .6) is good enough for almost any Leica lens and certainly well above the norm for the lenses offered by Konica, The newly introduced 1.2/50mm lens can be focused accurately with this base. The only limiting factor of the Konica rangefinder is the fact that is not as bright as the Leica version, which on the one hand helps to reduce the flaring in the rangefinder patch but on the other hand brings a slight reduction in vernier acuity and a somewhat reduced ability to find the exact focus, specifically in dull or low light. The framelines are not as bright or sharply defined as with the Leica design. It is horses for courses at the current state of the art.
Generally the Hexar is a very capable camera that adds many fine and useful features to the classical definition of the mechanically and manually operated RF-camera. Its electronically assisted ease of operation may allow for pictures that escapes one if not fully versed in RF style of photography. On the other hand it may be exactly the dilution of craftmanship that this camera has been designed to compensate that is part of its limitation to conquer the defined niche.
The lens lines: design approaches and results.
No one today designs a lens in a vaccuum and many optical design departments use the same (american) software. The criteria for what constitutes good image quality are also wellknown and often translated into the requirements of a certain level of MTF performance. So it will not be surprising to discover that the Contax, Bessa and Konica lenses perform very well and in many instances will are or seem to be in the same league as current Leica lenses.
Most users will never use their lenses to the breaking limit of its potential and when these lenses are deployed in handheld photography at moderateley stopped down apertures, one would need to be very critical to detect general or significant differences. In a sense it is true that none of these lens lines will show a structural advantage versus any other one.
Lens design is closely related to the mechanical tolerances of the mounts and the requirements of the wider apertures. It is a rule that a very high image quality at the widest apertures and over the whole image field demands manufacturing tolerances and careful assembly and testing that far exceeds what is needed if one can relax these demands a bit.
Zeiss Contax.
The Zeiss approach for the Contax is just this: design relaxation to a level where costs and quality requirements meet the required level. Zeiss assumes, quite sensible, that most lenses will be used at moderate apertures and that users will allow a slight reduction in image quality when using the lenses at their widest aperture, as these apertures will be choosen when photographic situations are critical and image demands may be not as high.
Zeiss Contax lenses then offer very high image quality generally and given the cost are excellent value for money. (the exception is the Hologon). In fact one has to wonder whether Zeiss really can make money on these lenses, as the moderate price belies their performance potential. At the widest apertures one notes a drop in overall contrast and a visible loss of definition of very fine detail. The mechanical construction is very good.
Konica Hexanon.
Three lenses are offered: a 2.8/28, a 2/50 and a 2,8/90 as generally available and one, the 1.2/50mm only as a kit with the Hexar Special Edtion body. In their documentation Konica stresses as design characteristics the smooth transition from sharp to unsharpness areas and the retention of shapes in the out of focus areas (good boke-h). As is now wellknown, these imaging characteristics can only be part of the design if one allows certain effects of aberrations to exist in the design. These characteristics however will degrade the image quality at the widest apertures, resulting in lower contrast.
Here the designer has a number of choices and the results will be balanced depending on the choices and given the mechanical quality of the mounts and the care of assembly. The lenses in the Konica line are wellknown specimen of the lensmakers craft and it will be no surprise that they are close to if not equal to the Contax versions.
Voigtlander lenses.
A good optical design needs a matching mechanical mount, choice of materials and assembly procesures. A lens that is sensitive to assembly errors needs more careful checking than a lens that is designed from the start to be insensitive (relatively speaking) to small errors in mounting. One can also choose the specs very carefully in order to ensure that smaller errors are not detectible. This is not a new approac in design: it is a time honored way of delivering good value for money. But nobody can expect a miracle: lens manufacture is in its basics the same all over the world. It is evident that a 75mm lens with an aperture of 2.8 is less critical to build to a good quality standard than a 1.4 design, even if at smaller apertures the performance is similar. It is not often appreciated by the user that at smaller apertures defects like decentring, a higher level of residual aberrations and a more extended bandwidth of tolerances will not be as devastating on image quality as at wider apertures.
A 12mm wide angle lens has such an extended depth of field that any defects in the focusing mechanism or decentring of lens elements will go unnoticed, even at critical inspection.
These remarks may be interpreted as criticism, but this is not my intention. When studying the line of Voigtlander lenses and even disassembling them, I got a good impression of the design approach of the company. Their lens line delivers excellent image quality and sometimes outstanding performance. Every company, Cosina included, has its own set of criteria and given a certain level of cost for a lens, the possibilities are limited. If Cosina needs to produce x lenses a month, a laborious and time intensive grinding of an asherical lens element is out of the question. So they have to settle for something else. The design choices of Cosina are clear: high quality optics at a cost effective price imply a a high volume production and that implies a wider spread in the tolerances allowed and a choice of material and mounting procedures that minimise the cost of assembly and production.
My tests do indicate that Voigtlander lenses are front rank designs that are consciously designed for high volume production and use materials and tolerances to keep costs low and performance at a level that is sufficient for its intended use. I am sure the Cosina designers are well aware of the typical deployment of their lenses and have the experience to deliver quality that satifies the customers.
Cosina is to be congratulated at delivering these lenses to the photographic community at a very affordable cost.
Leica RF specifications
| Rangefinder specifications for all Leica RF models since 1954 | Model | Enlargement (base for all is 69.25mm (except CL) | Frames | ||
| Leica M3 | (1954)0.92 | 50, 90 135 | |||
| Leica MP | (1956)0.92 | 50, 90 135 | |||
| Leica M2 | (1958)0.72 | 35, 50, 90 | |||
| Leica MP2 | (1959)0.72 | 35, 50, 90 | |||
| Leica M1 | (1959)0.72 | 35, 50 | |||
| Leica M4 | (1967)0.72 | 35, 50. 90, 135 | |||
| Leica M2-R | (1969)0.72 | 35, 50, 90 | |||
| Leica M5 | (1971)0.72 | 35, 50. 90, 135 | |||
| Leica CL | (1973)0.60 | 40, 50, 90 | |||
| Leica M4-2 | (1977)0.72 | 35, 50. 90, 135 | |||
| Leica M4-P | (1980)0.72 | 28, 35, 50, 75, 90,135 | |||
| Leica M6/TTL | (1984/1999)0.72 | 28, 35, 50, 75, 90,135 | |||
| Leica M6J | (1994)0.85 | 35, 50, 90,135 | |||
| Leica M6/TTL 0.85 | (1998/1999)0.85 | 35, 50, 75, 90,135 | |||
| Leica M6TTL 0.58 | (2000)0.58 | 28, 35, 50, 75, 90 | |||
| Leica M7 0.58 | (2002)0.58 | 28, 35, 50, 75, 90 | |||
| Leica M7 0.72 | (2002)0.72 | 28, 35, 50, 75, 90, 135 | |||
| Leica M7 0.85 | (2002)0.85 | 35, 50, 75, 90, 135 | |||
| Leica MP 3 0.72 | (2005)0.72 | 35, 50, 90 | |||
| Leica MP 0.72 | (2002)0.72 | 28, 35, 50, 75, 90, 135 | |||
| Leica M8 0.64 | (2006)0.64 | 24, 28, 35, 50, 75, 90 | |||