New lenses for M cameras: first analysis (October 6, 2008)
Introduction
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.
Let us look at the new lenses.
Noctilux 1:0.95/50mm ASPH.
This lens is still referred to by Leica advertising as faster than the human eye. This is not true: any 2.8 design has a wider aperture than the human eye, but the eye is unsurpassed in its retinal ability to capture light photons. Nonetheless: such a wide aperture is still a big challenge for any designer. The Leica team has created a lens that is certainly close to the theoretical optimum of this type of high speed lenses. Leica has burdened itself with the drive to develop high performance lenses in the smallest footprint possible and this goal adds additional demands on the designs. If you could work without space limitations and without the fact that a lens needs to be produced at a certain cost and within existing production facilities, the sky might be he limit. But that is not the case. The Noct ASPH is a true Double Gauss system and one needs to stay in this kinship when designing such highspeed lenses. The older Noct version shows the resemblance, but that is only superficial.
The true surce of the design is the Summilux 1.4/50mm ASPH as can be seen from the diagram below.
Note that the back group (behind the stop) of the Noct ASPH is derived from the Summilux ASPH, but that the front group has a split front lens with an additional air lens. This is quite difficult to mount with the required precision in a production situation, but Leica excels in this domain. The required precision of all new lenses is much higher than what you may know form other designs. See below the construction drawings for the Noct and Summilux
The performance wide open of the Noct ASPh is not significantly improved compared to the old Noct. Indeed it is difficult to improve on the severe vignetting of the older Noct as the requirements for the mount (M bayonet and the front diameter of 60mm are identical). It is some feat that the lens could be stretched to 0.95, after all an enlargement of more than 10%. When using these lenses on the M8(.2) the issue of vignetting is less important, but still very visible.
Wide open the definition of the Noct ASPH and the older Noct is of comparable low contrast, but the new version shows some visible improvement. The 40 lp/mm have a higher contrast at the center of the image and the corners and edges are improved too.
The main improvement can be seen from the graphs when the lens has been stopped down. Below are examples at f/4 as being representative of the general behavior. Note the much improved contrast for the 20 lp/mm and 40 lp/mm. Note too the remarkable bulge of the 20 lp/mm ad especially the 40lp/mm for the older Noct. This is the visual representation of the focus shift that affects the older design. The new design is outstandingly good at f/4 and this is absolutely an achievement.
The claim that the new Noct is as good as the Summilux 50 ASPH at the smaller apertures is substantiated by the diagrams below showing the MTF at f/5.6. The differences in the curves are not substantial and not visible in practical photography. Left: Noct; right Summilux
At f/1.4 the Summilux however has the edge and if you are often working in the critical range from 1.4 tot 2.8, then the Summilux 50 ASPH is the better option, at least rationally speaking.
Left: Noct; right Summilux, both at f/1.4
Summilux 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.
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.
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 to get the same results. The improvement at the edges is of course only visible when using a film loading M camera.
