Apo-Summicron-M 2/50 mm ASPH, part 2

The Apo-Summicron-M 1:2/50 mm ASPH. is the best standard lens for the Leica rangefinder camera, film-loading and sensor-equipped. It is easy to become convinced of this claim when one does base it on looking at pictures and examples on the internet. We should however be aware of the mechanics of the psychology of vision. We assume that seeing is believing, but in fact the reverse is true: believing is seeing. What we want to see is that what we see. And we should address the very tricky problem of visual comparisons and the manifold ways that a setting intended for comparisons introduces errors or looks at the wrong aspects. When comparing pictures made with the ASCR50 and another lens, both mounted on a digital camera (the preferred option currently) we are not looking at the characteristics of the lens, but at the system performance of the lens/camera combination. Here many conditions that may influence the result may stay unattended. As example: if you focus on an object with two different lenses (stationary object and camera on tripod) the chance that the distance is accurately set in both attempts is zero. A 5% margin of tolerance is always possible and that difference may ruin the comparison. 

The MTF measurement is accurate and objective and most importantly evaluates the lens in itself. It is still the best measure for the optical performance of the lens. The practical implications of the MTF results are a different matter. The importance of the MTF results has been a bit exaggerated over the years as the correlation between MTF and practical imagery (a word that Mike Johnston hates) can only be established indirectly. 

As a benchmark for evaluation purposes and optical quality, the MTF graphs are still unrivaled, but one should be careful not to read too much out of the numbers. 

The MTF results are always presented for the tangential and sagittal image planes (better curved surfaces). As can be seen from the image below, the idea of a flat plane surface is a theoretical construct. In reality there are three differently curved surfaces that join each other at the central axis of the lens. (P = Petzval surface, S = sagittal surface, T = tangential surface.) A small detail of the object will be focused more or less unsharp, depending on the surface it is projected onto. Because most objects in real space have depth, we will have a different depth impression when looking at the reproduced detail. The MTF graphs show nicely the difference in these curves, but one should realize that we are mostly looking at a mix of both representations. 


Modern Leica lenses are very precisely manufactured with tight tolerances because of the fact that the plane of sharp focus is very precisely defined and because of the fact that the throw of the focus ring is quite small: a slight shift of the focus ring will have its effect on the sharpness. 

The MTF diagram below compares the two best standard lenses on the market. The difference in performance is not that great, but it is clear where the ambition of the designers lies: the highest possible contrast transfer at the 40 linepairs/mm limit. Zeiss has set the target very high: a contrast transfer of 70% for the 40 lp/mm for a high speed lens at f/2 is rather ambitious. The Zeiss designers did succeed as the graphs show, but at a price: the lens mount is very substantial in size. Attach it to the M and the camera is dwarfed. 

Blue = 10 lp/mm, red = 20 lp/mm, green is 40 lp/mm, purple = 80 lp/mm

Horizontal axis is image height from 0 to 21.5 mm 


The performance of  the ASCR50 at f/2 is equally good. Note in particular the very homogeneous quality over the whole image area. A difference of 10 percentage points on this level is most likely not noticeable.  Because the ASCR50 will be used preferably on the sensor-equipped Leica M bodies, the behavior at the 80 lp/mm is quite interesting. The contrast transfer is above 20% overall and mostly above 30%. This is a major accomplishment and well above the Summicron-M 50 mm.  

The size of the ASCR50 is quite small and here lies the true difference between this lens and the Distagon. Leica has always claimed that making a good lens is not a problem nowadays, but making a good small lens is. Indeed, it has to be noted again and again: the superior performance of the Leica M lenses in combination with the s all size is the true accomplishment. The graphs show how well they have succeeded.

A very important characteristic of a new lens is its correction of the secondary spectrum. Normally lenses are best corrected for the green and red part of the spectrum, but the blue and purple part is neglected. Below is a table of the wavelengths:

380                440                purple

440                483                ultramarine blue

483                492                iceblue

492                542                sea green

542                571                foliage green

571                586                yellow

586                610                oange

610                780                red

780                …                   IR-A

Because of the relative insensitivity of silver-halide emulsions for the blue region, most designers in the past assumed that one sdid have to pay attention to this part. This attitude has been referred to as the 'cursed 435 wavelength'. Even a talented designer as Dr. Mandler evaded this wavelength. In the diagram below one can see the strongly under corrected blue part of the spectral graph. The modern approach is to pay much attention to this wavelength as it is responsable for the blue color fringing, so visible in digital photography. The Apo-Summicron-M shows the much improved correction of the blue region. The correction is four times better than what one should expect normally.

As a comparison, note that the new Distagon is equally well corrected. 


 © Erwin Puts 2013