Leica Noctilux-M 50mm f0.95 ASPH. , part 2 (June 7, 2009)
Some optical aspects
The current generation of CRF lenses has reached a very high level. Automatic lens improvement programs are universally employed and their use has reduced the design period for a lens from years to months. The programs are constantly improving and research into new designs brings new insights. A lens database with 25.000 different designs is not uncommon. Still there is a wide gap between theory and practice. If you add physical requirements like size and manufacturing cost into the equation, the number of useful designs shrinks rapidly.
All superfast CRF lenses are based on a basic Double Gauss design, but with the number of lens elements increased to seven and even nine elements. Within this limited design space the number of options is infinite, but the challenge is the same: to deliver excellent definition at all apertures and object magnifications (distances from camera to object).
Let us take a look at one of the first superfast lenses for CRF camera to understand what is at stake. Below is a series of MTF measurements of the Zunow 1.1/50mm from 1953. For unknown reasons the lens has assumed a kind of cult status. The graphs are not calculated but are based on physical measurements of an actual lens.
Zunow 1.1
Zunow 2.0
Zunow 4.0
Zunow 8.0
Wide open at 1:1.1 the lens exhibits a very low overall contrast. There is a small area in the center of the image where subject outlines are recorded with soft delineation. Finer details are not recorded at all or are quite blurred. The lens shows strong spherical aberration, quite visible astigmatism, curvature of field and coma too. The better MTF values at the edge of the image are the result of a strong portion of vignetting, that reduces the number of marginal rays reaching the film plane and this in itself improves the definition. On stopping down to 1:2 the central portion of the image crispens visibly, but fine detail is still quite blurred. A significant focus shift is visible and the outer parts of the image hardly improve. At 1:4 the focus shift is very pronounced, especially in the definition of fine detail. Astigmatism is clearly visible and the edges are fuzzy. Curvature of field is hardly corrected. At 1:8 overall contrast is good and fine detail, at least in one direction, is crisply rendered in the outer zones of the image.
When we evaluate this performance with modern eyes, it would not be acceptable. Set in the context of the period, the image quality is not that bad, compared to other lenses. As a comparison, you may look at the MTF graphs of an older Nikon 1.4/50mm lens from the same generation of lens designs.
Nikon 1.4
Nikon 4.0
Note that this lens wide open at 1.4 has a very low contrast too, but astigmatism, and curvature of field are well controlled. When stopping down to 1:4, there is a major improvement in definition of very fine detail and only a small occurrence of focus shift. The outer zones show good delineation of subject outlines but fine detail is fuzzy and blurred. Overall this lens shows excellent performance and is even better than the Leitz Summilux-R 1/4/50mm lens from 1969! We might take this date as the moment when Japanese optical design forged ahead and Leica designers retreated in their cocoon.
Aspherical revolution
The aspherical revolution within Leica started with the arrival of Mr. Koelsch, originally from Zeiss, and since then the Leica CRF lenses have been improved beyond the wildest dreams.
High speed lenses share a number of difficult challenges: Spherical aberration (SA) occurs when marginal rays (the rays entering the lens at the edges) focus on a different plane than the paraxial rays (the rays centered around the optical axis of the lens). This error becomes more pronounced when the lens diameter is bigger (as is the case with the high speed lenses). Coma is basically the same phenomenon as SA, but for skew rays entering the lens in the principal plane (the plane that contains the optical axis). The worst villains are the oblique rays entering the lens from object points that lie outside the optical axis and its plane. These rays create astigmatism, a problematic aberration for high speed lenses. Add to this list the correction for curvature of field, the vignetting and the chromatic aberrations, particularly in the blue region of the spectrum and you may see the challenge. Remember too that many aberrations grow in magnitude by the third power when increasing the aperture. In the case of the Leica CRF lenses an additional requirement is the physical size of the lens. The smaller the lens size, the more difficult the aberration correction.
Since around 1990, the Leica design approach has been to deliver CRF lenses with optimized performance over the full aperture range and with a strong emphasis on the wider apertures. This approach has driven the MTF values to very high values over the whole image area, in many cases approaching the theoretical maximum. The resulting image formation favors overall clarity, crisp definition of fine detail, fidelity over an extended subject depth and subtle reproduction of tonality in hues and greys. This fingerprint is markedly different form that of previous generations of Leica CRF lenses. Some Leica users look favorably upon this classical fingerprint and would like to see modern designs with this classical look. This is precisely the gap that is being filled with the Voigtlander lenses from Mr Kobayashi. Zeiss with the ZM lenses occupies a middle ground which is located between both extremes.
Below is the MTF for the NX 0.95
NX 0.95 wide open
The NX 0.95 is the most recent addition to the Leica CRF scuderia employing aspherics and floating elements. When comparing the MTF graphs you can appreciate the quantum leap forward at the widest apertures. At 1:0.95 astigmatism is well controlled as is curvature of field. Overall contrast is low to medium, but the focus shift when stopping down is very well controlled. This implies that spherical aberration has been reduced to low amounts, but one cannot expect a lens with such a wide lens diameter to have crisp definition of fine details. And where fine detail is soft, the delineations of major subject outlines will be soft too. To get sharp edges at subject outlines, we need lots of high frequencies to create this sharp boundary. The performance of the old Zunow lens at 1:1.1 can only be described as a last resort when you desperately need that picture. The NX at 1:0.95 performs so well that you hardly will guess that the picture was made with a superfast lens. At this point in the report, we need to take great care in examining the performance profile of the NX. When testing a lens, and even when just examining the results of the lens, it is quite easy to select and present the good, the bad and the ugly aspects of a lens. The occurrence of vignetting is such a case. The technical data sheet reports a vignetting value of 20% at the edges or more than two and a half stops. There are picture situations where you will very clearly see this strong darkening in the edges. But there are also many instances where the vignetting is negligible. We should also be aware of the optical specifications. A superfast lens with a wider angle of view will exhibit vignetting, caused by a number of factors: size of mechanical mount, cosine 4 law, and image distortion. There is a trade off between distortion and vignetting and the Leica design favors a low level of distortion, in my view a fine choice. This discussion is not meant to diminish the effect of vignetting in the Nx 0.95 but to indicate that we should differentiate between an optical error and an optical given. Focus shift on the other hand is an error that can be corrected without severely effecting other aberrations.There is slight coma at the outer zones of the image field. see below at the left side of the picture.
Wide open the NX 0.95 is sensitive to the occurrence of flare and secondary reflections. See picture below. One should take some trouble to prevent strong light sources to shine obliquely onto the lens surface. The previous Noctilux design was not better in this respect and one should have hoped for an improvement in this area.
The lower overall contrast wide open has already been mentioned. Residual aberrations and internal reflections may be responsible for this behavior. The contre-jour scene below shows that the black stems of the flowers get a blue cast In higher contrast scenes the lower contrast wide open is not detectible and the image offers excellent quality, and one would not assume that this picture is made with a lens at aperture f/0.95.
THis is the full frame picture
Below is a street scene at f/0.95 with the focus on the group of bicycles. Note the well reproduced luminance differences, but also the slightly soft edges of the outlines.
this is the full picture
Here is another picture, now at a wider distance, to show that the rangefinder - lens calibration is spot on. The image of the bird is greatly enlarged and still shows good subject outlines and medium detail.
here is the full image
In situations with dark areas in the scene, the penetrating power of the lens to detect and reproduce detail is reduced. In this picture we see also the occurrence of the blue fringes at the edges of black/white transitions. This phenomenon is a chromatic aberration and is almost impossible to avoid in high speed lenses. The correction of the blue part of the spectrum for this type of lenses cannot be accomplished by simply selecting exotic glass types from the glass catalogues: it is a balancing act between all aberrations, focal length, back focus and so on. The previous Noctilux lenses also showed a significant amount of blue fringing. But is was less visible. Many pictures were made on black and white film, color film was less responsive in the blue region, most pictures were taken in artificial light where red is over-represented.
The full picture is here
The close up performance wide open is very much improved compared to the previous version. Note in the picture below the three dimensional representation of the rounded subject details.
Bo-ke(h), one of the prime characteristics of the previous Noctilux has been well preserved in the new NX 0.95. Unsharp highlights are rounded and have homogeneous illuminance.
Performance at smaller apertures.
Stopping down a fraction to f/1.2 increases the crispness of the image visibly. And from aperture f/1.4 the NX 0.95 behaves as if it were a current Summilux-M 1.4/50 Asph. with the exception of the flare propensity and the color fringing both of which diminish when stopping down. At this point we may note the dilemma for the Leica marketing department: one could have introduced a new 1.2/50 design with outstandingly good properties, but such a lens would be very close to the Summilux 1.4/50 Asph design and offer only a half stop more speed which in the current digital world is less impressive than it was in the past. Stretching the aperture to f/1 or a bit wider, makes more sense, but then some compromises are required. These have been discussed in this text.
Conclusion.
The new NX 0.95 combines many of the characteristics of the previous Noctilux 1/50 and the Summilux-M 1.4/50mm Asph in one design. Stopped down it delivers state of the art image quality and wide open the performance has been optimized in such a way that it can be used at all distances and ambient light levels (clear daylight and near darkness). In particular I would like to mention the improved imagery at close distances which allow for strong portraits with a minimal depth of field, and a clearly defined sharpness plane while maintaing pleasant out of sharpness outlines. The manufacturing quality of the NX is superb and there is no reason to be afraid that the lens will not focus correctly on any Leica M body with a well adjusted rangefinder. The absence of focus shift is another characteristic that needs to be mentioned in a positive sense. The historical fact that the original Noctilux 1.2/50 had to be hand adjusted to find the best balance between focus shift and rangefinder alignment is a thing of the past.
Most classical Leica high speed lenses suffered from curvature of field and astigmatism. The new NX is practically free from these aberrations. The result is a pictorial smoothness over the full image (capture) area that especially wide open delivers a sense of depth and clarity not seen in a superfast lens before.
While the lens is a major improvement on the predecessor, some character traits are not changed: the propensity to flare, the blue fringes and the softer representation of subject outlines. It is a matter of great progress in a superfast lens that these are the only aspects that deserve critical attention.
This lens is a specialist lens, price tag and size/weight put it in a category of its own. If you can see the added value of the unique performance characteristics for your type of imagery and accept a lengthy learning curve to get the best out of the lens, a new pictorial style might be your reward.