The recently introduced lenses for the M-line of cameras continue the Leitz tradition of introducing bold and daring designs. We have to return to the year 1961 to record the introduction of the first Summilux 1.4/35mm lens, to 1942 to see the Summarex 1.5/85 and 1966 for the Noctilux 1.2/50mm with two aspherical surfaces. The Summilux and Summarex lenses offered high speeds but not the ultimate in definition, and the Noctlilux, excellent as it was wide open, did not deliver superior imagery at smaller apertures. For a very long period (more than one designer generation at least) Leica eschewed daring designs and focused their resources on top class lenses with moderate apertures. It is indeed the designer's rule of thumb that for the best all-round performance "modest" apertures around f/4 are required. This seems to be a kind of iron regularity and when we look at the simply best lenses in the world, apertures from f/4 to f/5.6 are the norm.
Photographers are not always and certainly not singularly interested in the best optical performance. They want best-practice lenses that can be used under every possible lighting condition and preferably handheld. Then a very high speed lens is a must, but also because a wide aperture adds a compositional element to the picture.
On the other hand, there are photographers who prefer good quality lenses for an acceptable price.
Leica now has three lens lines in progress: The Summilux designs for best overall use, the Summarit range for the more budget oriented user and the Elmar line that offers ultimate performance. In the past the Elmar designation was reserved for lenses of modest performance and good price-performance relation, but that role has now been delegated to the new Elmarit designs. The Summicon line, once the ultimate in quality, seems to become squeezed between the Summilux and Elmar designs.
There is a certain logic in this approach. In the recent past a logical progression was 1.4 and 2 and 2.8 as the maximum apertures for a system of lenses. But the performance differences between lenses, just differing by one stop is not that great anymore. Now that the ISO speed of current sensors is easily changed upwards without quality losses as you get with film when switching from ISO100 to ISO400, just one additional stop is not worth a much larger lens and a more costly design.

The Leica design goals have to be balanced between ultimate quality and cost and handling, read small size. Even Leica cannot command a hefty price tag on every lens, however good.
Optical designs then must also be created with production limits and restraints as controlling parameters. A possible structuring of the Leica lens scuderia might be a concentration on 1.4 designs as the standard best-of-breed lenses, on 3.8 designs for ultimate quality and on 2.5/2.8 designs for cost-effective and very compact designs.

Summilux -M 1: 1.4/21mm ASPH and 1:1.4/24mm ASPH
These lenses share many design elements, but the optical construction is different. The mounts do share a number of parts to facilitate production. The 21mm lens has a more complicated design than the 2.8 version. A careful study of the several drawings will reveal the re-use of some elements, but it is a tribute to the designers that they did not fall into the easy option to combine groups of elements. The 2.8/21mm lens has the same front group, but without the aspherical surface and the back group is related to the Summilux design.

Below left: SX 21; below right: Elmarit 21

The 24mm versions are more difficult to compare. Below are the designs for the new lenses with 1.4 and 3.8 apertures. Down is the 2.8/24mm design. Note again the combination of groups. The 3.8 version is a mix of the 1.4 and the 2.8 designs. Remarkably the 1.4 version has the second group as floating, where ll other lenses have the last group as floating. For a 3.8 version the lens design is certainly overkill, but the performance is outstandingly good.

The MTF graphs for the Summilux versions of the 21 and the 24 designs can be seen below. Left is 21 and right is 24. Note that wide open the 21mm version is somewhat better overall, but that stopping down the 24mm gains more. The residual chromatic aberrations are a bit more obstinate in this case and the curvature of field must be controlled more tightly. If you compare the 1.4/21 with the 0.95/50mm you see comparable quality. I wrote earlier that the energy flow (or etendue) of both lenses s equal, but here I erred in my calculations. In fact the Noct has twice the energy flow of the 1.4/21mm, but the 21mm lens has a much wider field to cover and the flattening of this field at the required wide aperture is as complicated as designing a high speed 50mm lens.

Below: Elmar-M 3.8/24 mm
The Elmarit-M 2.8/21mm is not outclassed at smaller apertures as can be seen below for the seen below for the f/4 and f/8 aperture. You can infer from this diagram that generally the 2.8 version will deliver a somewhat crisper definition than you will get with the 1.4 version.

The 3.8/24mm ASPH and the 2.8/24mm ASPH are quite identical in overall performance. The 3.8 version has the edge in distortion, but has somewhat more vignetting at least as seen from the diagrams. The MTF graphs for the 2.8/24 version and the 3.8/24 version, both at f/4 show almost identical image quality, but the new 3.8 version has improved edge performance, the only weak point of the 2.8/24mm lens. Left Elmarit 2.8/24, right Elmar 3.8/24mm. At f/8 both lenses are of the same high level, with a slightly more even performance in the field for the 3.8 version. The 3.8 version is more compact, but demands a higher amount of design effort to get the same results. The improvement at the edges is of course only visible when using a film loading M camera.

The Leica Summilux-M 1:1.4/21mm ASPH. and the Leica Summilux-M 1:1.4/24mm ASPH. are now trickling into production and an evaluation is now possible. The SX 21 is a world-first and while the SX24 has some companions in the SLR world, for a CRF camera it is indeed a nouveauté. For generations of photographers high speed lenses implied a restricted usability, quality compromises and a high price. For general purpose imagery and high performance a standard Double Gauss system is the first choice. At least, that is the common knowledge position. Leica proved with the SX-A 50 that it was possible to design and manufacture a high speed general purpose lens with outstandingly good performance over the whole range of apertures and distances. All optical tricks were employed to make it possible: aspherical surfaces, floating elements, special optical glass and a sophisticated manufacturing process.
The high cost of these measures is not well-known. I have on my desk one tiny glass element (the middle element of the 2.8/24mm lens) and for the cost price of this single element (ready to be mounted, so after grinding, polishing, centring and coating) I could buy a good quality zoom lens of a reputed Japanese maker for their high-end cameras.
One is justified to ask if all these efforts make sense, when the cost price is soaring to a new level. In 2005, the Noctilux had a price tag of Euro 3000 and was with the 1.4/75 the most expensive lens in the Leica M stable. Now the Noctilux is still the most expensive lens at E 8000 and the SX line has been extended with a 21 and a 24 lens, both at a price tag of E 5000.
The mechanical sophistication of these new lenses has been brought to a new plateau: the tiny movements of the floating element must be very precise and consistent over years of use to have effect on the lens performance. The reason why the lenses do not have an aperture of 22 is related to this level of precision. The circular hole that the aperture blades have to create is very narrow and cannot be consistently assured with the required accuracy.

In the course of this review, I will present the optical performance of these lenses and I can state that the SX 21 and 24 open up new vistas for creative photography with high speed wide angle lenses.The community of Leica aficionados seems to be upset about the current pricing of Leica products, lenses in particular. This attitude is in striking contrast with the imperviousness to high price tags that can be found in the mountain bike world. Most mountain-bike riders are singularly focused on top-class engineering, cutting edge technology and subtle differences between components that determine rider satisfaction and above all rider success. MB's are a discerning and competitive lot of people. For a bike to be really competitive, a price tag between Euro 4000 and Euro 8000 is not uncommon. Bikers pay these prices because they feel the advantages of these products while riding over the terrain and trying to stay ahead of the rest. In fact price is not an issue if and when you experience the surplus value in demanding situations.
Leica cameras are often owned as icons and less often used as intended. This is a sad fact, but it is reality. If you wish to own the whole range of Leica M lenses, then the current price level might be an impediment. If you have the goal and intention to use the optical properties and pictorial characteristics of the high-speed-wide-angle combination, then the price is not a hindrance. The style and quality of your pictures will reflect your expertise and success in creating the images you want. As with bikers, high quality material is only justified if you are really putting it into action.
The handling and use in practice.
Both lenses are impeccably manufactured and finished. The aperture click stops operate with just enough resistance for speedy selection, that can be done without looking. The distance ring operates form infinity to closest setting with a homogeneous smoothness. Only with the finest sensitivity of touch can you detect the floating mechanism come into action. With normal handling, turning the distance ring from one position to another swiftly, the smoothness of the movement is amazing as these lenses offer a very solid feeling. Especially the SX21 is a weighty piece of optics in both senses of the word. Attach the lens to the camera body and you get a very solid package, with the weight of the lens making the system front heavy. With your hand giving the right support for the lens, quite slow speeds of 1/8 are possible with good image quality. Walking around with the camera hanging on the neck-strap, the lens is pulling downwards and you have to get accustomed to this position. At about 600 grams the SX21 is not a feather weight. The SX 24 is somewhat smaller and brings about a hundred grams less on the scale. The large front lens diameter of the 21 demands a Series VIII filter, whereas the 24 needs a Series VII filter.

If accurate framing is required, the additional finders are a must. The new finders are beautifully finished, show the frame lines for the original 35mm format and the 1.33 crop factor for the M8 and M8.2. Size and logo on the finder are of classical proportions and blend very well with the camera. When medium apertures are selected, one can be a bit more careless about focusing accuracy and work with preset distances. Then the double action of framing with the accessory finder and focusing with the in camera rangefinder is not needed. With the wider apertures and certainly with the 1.4 aperture and closer focusing ranges, the rangefinder focusing is mandatory. It is a matter of habit how you will work with the sequence: first focusing and then framing or the other way. I prefer the second option as this also the way to go with moving objects.The SX21 and SX24 on the M8(.2) offer the field of view of a 28 and 32mm lens. The frame lines of the SX24 are available in the camera finder and the additional finder is not required. If you live with, or can guestimate the approximate framing, you could use the SX21 too without additional finder, but then you are walking on the wild side.

The 30% reduction in field of view is not a major problem when using lenses from 35mm to 90mm. Quite often, you will see more on the picture than you had intended. The 50mm lens (as example) on the M8 has the magnification of the normal 50mm lens, but the picture-box of a 75mm lens. For the 21mm and 24mm lenses, on the other hand, you want the full view. It is at least mentally a bit strange that you are using a 21mm lens on the M8 and expect (from past experience) the wide angle of such a lens and then to see a cropped picture with a narrower 28mm view.
Both the SX21mm and SX24mm lenses offer a new and fresh pictorial experience to Leica photography. The wide angle lenses are traditionally seen as lenses for landscapes and architecture (inside and outside the buildings). Bill Brand (among others) showed that you could create intriguing views on a subject when you exploited the other characteristic of the wide angle lens: the rapid shrinking of the size of subjects in the scene from foreground to background. Get very close to the subject and you will notice the elements behind the main subject in the scene receding rapidly in size and distance. The new characteristic that the SX wide angle lenses bring into the design is the possibility of a high degree of unsharpness in the background elements while isolating with great sharpness the main subject. This isolation and attention to the main subject while the background envelopes the main motive to bring it into perspective without overwhelming it, might be the primary attraction of the new SX lenses. With smaller apertures the main subject will be incorporated (or integrated) into the background because of the wide sharpness gradient that is created with the extended depth of field. This description is valid for pictures with the M7 (as the proxy for all film loading M cameras) and the M8, but with the M8 the background envelope is narrower because of the restricted field of view. You will need to experiment with both kind of cameras to explore what is suitable. While on this topic, I may note again that working with film is still a very pleasant and rewarding workflow. Film emulsions with speeds around ISO100 (bw) to ISO200 (color) offer better resolution than the current crop of Dslr’s with around 20 Mp and microfilms far exceed the capabilities of these digital tools. The main drawback of film is not the more leisurely pos-tprocessing, but the transfer of the quality from analog to digital: normal scans do not extract all information and drum scans are notoriously expensive and difficult to use.
The high speed of the Sx21 and SX24 are welcome in dimly lit interiors or outside in dusk or twilight, but can be a problem when the scene brightness is very high. Especially on the M8 with the lowest ISO speed of 160, you might be running into over-exposure sooner than you like when setting the 1.4 aperture. A neutral density filter is advantageous in this situation, but then you have to switch with the IR filter on the M8, which might be a problem depending on the subject matter. Perhaps Leica can design a filter that offers both options (IR and ND) at the same time.
Summing up for now: the SX 21 and SX 24 do represent the cutting edge of current optical and mechanical design and manufacture. Their use opens new creative possibilities for the Leica photographer. Handling of the lenses is easy and supported by the excellent ergonomics of the current generation of Leica M designs. Put the SX wide angles on a Leica body and you are formally being urged to start taking pictures. As with biking: you only get the satisfaction and the results with actual use.
It is standard procedure for most high speed lenses with apertures of 1:1.4 and more that the wider apertures have to be used with some consideration for the performance characteristics at full aperture. In most cases, stopping down does improve the quality markedly. Specifically overall contrast, definition and the crisp rendition of quite fine detail get visibly better when you move from full aperture to smaller stops. The classical fingerprint of the high-speed lens with a performance jump from full aperture to one or two stops smaller and then a gradual improvement to the optimum diaphragm is nowadays less prominent. Most current designs have a good overall contrast wide open, but lack some bite when recording finer details. A high speed lens cannot be optimized for the whole spectrum of demands one would list for a universal lens: uniform performance over the whole image field, at all distances and for every aperture. At shorter distances the image quality deteriorates, but is often explained with the fact that the high speed lens is for reportage use. For a medium tele-lens this argument may be appropriate, but a wide angle lens is often used at quite close distances.

There is a widespread tendency to characterize wide angle lenses as useful for landscapes and architecture and for capturing much of a scene in a confined environment. In fact one cold argue for the opposite position. Wide angle lenses are preferable whenever you need a dramatic composition with the main object prominently in the foreground and the background enveloping around the central object for contrast or substance.
In such cases the quality at closer distances must be high, even at full aperture, in order to make good use of the unsharpness gradient.
The new Summilux designs for the focal lengths of 21mm and 24mm follow the standard set by the Summilux 50mm with floating elements and aspherical surfaces.

Film Orthopan
A full assessment of the performance of these lenses requires the use of film in order to study the image quality at the outer zones and edges. In this case I used the M7 body, loaded with Spur Orthopan film, exposed at EI 20 and developed in Spur Nanospeed. This choice in itself is already a sign of the potency of these two new lenses. Comparison tests with the M8 body indicated that the limit of resolution of the sensor would be reached before the optimum lens performance could be displayed. All tests were done on tripod and in low contrast situations to establish a bottom line. Specifically in sulky weather the demands on the quality of the lens does increase. It is no big problem to have good contrast in sunny weather.


SX21: definition and vignetting
At full aperture very fine detail is crisply rendered over a large part of the image frame. Edge contrast of the major subject outlines is high. Towards the edges and corners of the frame fine detail is becoming fuzzy, but the outlines hold on with a good edge contrast.
Stopping down improves the outer areas of the image visibly and at 1:4 the optimum is already reached with outstandingly good definition over most of the image area, but the edges lag behind. Stopping down to smaller apertures (1:11) does improve the edges of the image to match the quality in the center. A small softness in the rendition of very fine detail can be detected, but you really must be focused on these details at big enlargements.
In practical use, you will not notice big differences between the wide open performance and stopped down to 1:11. This behavior is exemplary and sets a new standard for the performance level of high speed very wide angle lenses. Many high speed lenses do improve on stopping down, simply by reduction of the flare factor. The SX behavior is also a sign that flare is very well controlled.
At full aperture there is a visible darkening at the edges of the frame. Stopping down to 1:4 removes most of the vignetting. It is really difficult to give useful information about vignetting as the occurrence of vignetting depends on several factors: the exposure itself, the characteristics of the scene, the characteristics of film and so on. In most circumstances the vignetting of the SX21 at wider apertures is hardly obtrusive and only occasionally a real nuisance. The vignetting you see on negative will be partly compensated in the darkroom. On the M8, the software (when the lens detection option is activated) does a good job in at least reducing the darkening to quite acceptable proportions. The published curves which show vignetting values of 2 stops are correct, but this amount of darkening is only visible at the extreme edges.
Vignetting is stronger when making close up pictures. Wide open you see a brighter area in the middle of the picture. At smaller apertures the density is homogeneously distributed over the whole image area.

SX24: definition and vignetting
The publication of MTF graphs is a fine idea when one wishes to study and compare lenses in detail. It is easy however, to fall into the trap of attaching too much value to small differences. The MTF graph of the SX24 wide open looks not as good as the SX21 and extrapolating from theory to practice would imply a softer definition and lower contrast at the 1.4 aperture. Below you will find two extremely small sections from the whole negative, absolutely not manipulated: these are negatives as scanned with identical settings. The slightly higher softness of the image is just visible. The difference is not that big that the choice between either lens should be based on the wide open performance.
At full aperture very fine detail is clearly rendered over a very large part of the image frame. Edge contrast of the major subject outlines is medium to high. The edges and corners of the frame hold fine detail over a wider area than the SX21, partly due to lower level of vignetting.

Stopping down to 1:4 brings the contrast and detail definition to levels equal to the SX21 at the same apertures and due to the slightly larger magnification (at the same distance of course) slightly finer detail is just detectible. Stopping down to 1: 11 improves the edges and corners as usual. Compared to the SX21, the overall image quality is now very homogeneous over the full image frame from 1: 4 to 1:11. Vignetting is slightly higher in the SX24 images, but disappears after stopping down to 5.6. A slight corner darkening will however be visible even at 1:11. (same with the SX21).
The comparison with the Elmar-M 3.8/24mm asph. at apertures from 3.8 to 8 is informative: At 1:4 the Elmar shows identical definition in the image center, but slightly crisper definition in the outer zones till the edges and far corners. Vignetting is also lower. At smaller apertures this fingerprint holds and the Elmar has just a fraction better definition and edge contrast than the SX24.
In the not so distant past, one could hardly compare with decency two lenses with apertures so widely different from 1.4 to 3.8 and only detect small differences. In is certainly a tribute to the Leica designers that the new 1.4 designs can be approached as fully normal lenses without making reservations.

SX21: distortion
Distortion, interpreted as the curvature of straight lines at the outer zones of the image area, is visible, but the effect is quite moderate for a 21mm high speed lens. As with the aspect of vignetting, the distortion is more or less visible and disturbing depending on the scene. The picture here has strong geometrical lines (rows of stones) and in this case distortion (seen in the curvature at the top edge of the image) is visible, but not obtrusive. On the M8 this effect is much less an issue, due to the reduction of the capture area. With "honest" film one has to be more attentive.
There is one other type of curvature that is prominent with wide angle lenses. When making pictures at closer distances of flat surfaces, like walls, there is the subtle spheroidal effect that the center part of the mage seems to push forward and the rest of the image seems to recede. This is natural for this type of lenses and not a defect.
SX24: distortion
Distortion is just visible in the corners and on he same level as the SX24. The Elmar 24 has the lowest level of distortion. All three lenses exhibit a low level of distortion that is certainly visible, but becomes never objectionable.
SX21: close up performance
Wide open very fine detail is rendered clearly over the full picture area, but with a slight veil of softness. The image has not the crispness of the pictures made at medium distances, but is fully usable even at quite large enlargements. The effect of the floating element is clearly present: without this device the image would be much softer. The residual smoothness is a sign that a floating element is no wonder oil, but a very effective tool to improve close up performance significantly. Stopping down does improve the crispness of the rendition but the gain is modest, but of practical importance. The fact that you can discuss the performance of a 1.4/21mm lens at a distance of 70cm wide open and stopped down without seeing a big difference in definition and contrast is remarkable.
SX24; close up performance
Wide open very fine detail is rendered clearly over the full picture area, but with a trace of softness. At smaller apertures the fine details are rendered quite crisply and one can recommend using the close focus range without reservation, form 1.4 to 16.
The comparison with the Elmar 24 at the same close up distance shows that the Elmar at 1:4 has a slightly crisper definition of the very small details. This small advantage holds over the aperture range from 3.8 to 8.

The floating element in the SX24 (and 21) does not necessary improve the image quality at close distance. This is not the role of the element. The point is this: the correction of a lens is normally done for rays that enter the lens more or less as parallel rays. And the aberration correction is derived form this condition. At closer distances the rays are inclined to follow quite different paths and the more so when the aperture and the angle of view get wider. A designer needs to find a balance between these two extreme positions and often the compromise is set to favor the longer distances. The close up then drops in performance. The floating element is used to compensate this compromise and create good performance even in a high speed lens at very close distances. The floating element is used to get the best quality that you can get with a comparable lens at normal apertures. This comparison shows that the floating element in the SX24 functions very well.

SX21 and SX24: at infinity
"Infinity" is a somewhat ambiguous concept. True infinity is physically impossible, but the distance to the moon from earth is definitively close to practical infinity. In optical handbooks "infinity" is defined as that distance of the light source to the lens where the incoming light rays from that point are parallel. In photographic handbooks the concept of infinity and circle of confusion are related and the following rule of thumb can be found: any distance beyond "x" is interpreted as infinity because then the CoC is so small that one can speak of an infinite distance. "x" = (the square of the focal length) divided by (the product of CoC and aperture). For the SX 21 at 1.4 this formula amounts to a distance of 18.5 meter. Any distance beyond that is infinitely far away. As a very loose approximation one can also work with the 1000 times the focal length rule. Landscape pictures are however often taken when the horizon can be at several kilometers distance. When you take a picture of an object at such a distance (a skyline or the rim of a mountain), different considerations must be weighted. When you enlarge such a picture, it is not uncommon to find the details lacking or being unsharp. One could assume that the lens is not correctly set at infinity. Assume you take a picture of a scene at a distance of 200 meters. Then the magnification is roughly 1/9500. An object with a height of ten meters will be reduced on the negative to a size of 1mm! The sharpness impression of the outlines of the object are defined by the depth of field as defined by the CoC diameter and the true location of the plane of sharpness at the infinity position of the lens mount.

It is unlikely that you are going to take landscape photos at very large distances with the SX21 or the SX24 at full aperture. For my infinity tests I used a distance of 100 meters. Wide open the objects at that far distance were small, but clearly rendered. The grain structure of the emulsion started to break up the edges of the objects, but no trace of fuzziness. Stopping down did tighten the grain clumps and this did improve the definition of outlines. Both lenses perform commendably at infinity settings. A check with the Elmar 24 brings identical results.

SX21 and SX24: unsharpness behavior and perspective.
Wide angle lenses offer some peculiar characteristics, that one needs to know for effective use. The first characteristic is the rapidly shrinking size of objects from near to far. This behavior is an optical illusion, because the viewing angle is different from the picture taking angle. It is often assumed that a change of focal length will alter the perspective. This is not true. Look at the following calculations. A 50mm focused at a distance of 1.5 meters will reduce the object to a size 1/30 of the original size and at 5 meters the object will be shrunk to about 1/100: a ratio of 1:3.3.

A 24mm lens focused at the same distances will have size reductions of 1/60 and 1/200 and a 21mm lens will give reductions of 1/66 and 1/230. Here the same ratio occurs. If we enlarge a section of the negative of the 21mm lens to the same size as the negative of the 50mm lens, the sizes of the two objects at 1.5 m and at 5 m are identical. The imaginary pictures have been made at the same distance of course. If you now change the distance in order to maintain the size of the the main subject, you need to set the distance with the 21mm to 0.75 meters to get a reduction of 1/30. Now the object at the distance of 5 meters is again reduced to 1/230 and the relative sizes do differ. The perspective has apparently changed. Would you be able to look at the negative taken with the 21mm as close as 5cm then the perspective would be "normal" again. The second characteristic of the wide angle lens is the extended depth of field, a fact that allows for some relaxation in the focus accuracy (the snapshot position!), but adds the impossibility of isolating the main subject from the background and foreground.
The SX 21 and 24 offer interesting options for isolation the main subject when the lenses are used wide open. See the examples that show that this aspect is indeed one of the important characteristics of these lenses. Keep in mind that the effects of isolation, even at 1.4, do diminish when the taking distance increases beyond 3 meters.
The unsharpness gradient wide open allows for good isolation of the main subject. Stopped down to 5.6 shows the increased depth of field.

SX21 and SX24: veiling glare and secondary reflections
The SX21 has excellent control of secondary reflections. The picture below (sun obliquely into the lens) shows only one reflection close to the light source. Note however, that the branches loose contrast and blackness, a clear sign that there is some veiling glare. This picture is with the lens wide open. (SX21).
Stopping down does improve the image significantly, but does not remove the effect completely.
Below is a picture that is in itself not related to the lenses discussed here, but is a defect of the sensor.


The vertical bar in the picture (small section of the full scene) is a wrong interpolation of the software (I assume) of the radiation pattern of the bright light source. It does not happen often, but is does occur occasionally.
The SX24 under extreme situation scan show a secondary reflection, but has a better control of veiling glare at wide apertures,as can be seen in the dark branches of the tree.
Both lenses perform very well in this critical discipline and behave as well as lenses with longer focal length and the same high speed. Perfect they are not, but there are limits to what a designer can accomplish and we should also take into consideration.

The SX21 delivers outstandingly good imagery and when one takes into account the high speed of the lens, it is close to superb. You can always find aspects that can be criticized, but criticism because of criticism is not the best approach. And this lens can hardly be faulted. One would wish somewhat more definition in the outer zones at the wider apertures, but to restate the position, the wider apertures are not meant to be used for architecture. For documentary work and the art-snapshot the critical definition over most of the frame and the fine unsharpness gradient are a big benefit. The lens hood is a must however and should be always in position.
The performance of the SX24 is very even over the whole image area, but wide open the overall contrast is slightly less good than what we have in the SX21. One should not over-emphasize this behavior, as it can be compensated quite easily with film (steeper gradation and selection of paper grades) and with post processing in the M8 domain. The SX24 compares favorably with the E24. This lens (the E24) delivers overall outstandingly good quality and it is an achievement of first order that the SX24 with the high speed of 1.4 can be compared without problems with a 24mm lens of more modest speed.
The choice between the SX21 and SX24 can be made solely on intended use and artistic considerations. The SX24 is the more compact lens, and the angle of view is very pleasant for the film domain where you can use the full potential. On the M8, the 24 gets cropped to a viewing angle of 32mm and is close to the classical Leica focal length of 35mm. In this respect the new SX 1.4/24mm has better performance than the SX1.4/35. The SX21 on the film loading Leica cameras brings a new experience in the wide angle domain and is really a joy to use. On the M8 the practical angle of view is 28mm. For reportage, documentary, snapshots and location photography for fashion and portraits this is quite often appropriate. In these situations the high speed allows a new type of imagery and combined with the compositional advantages of the wide angle, the photographer is to blame if the pictures are not visually pleasing and arresting.
A nice combination would be the SX21 and the E24. Then you can use the full range of options on film and with the M8. The other way around would be the SX24 and the very compact Zeiss ZM 4.5/21mm. If you prefer to stay into the Leica scuderia the Elmarit 21 might be a consideration, or the Tri-Elmar 16-18-21.