M8 part 5: Performance of Leica and Zeiss ZM lenses
Introduction
In previous parts I have compared the M8 performance with a high-end DSLR and with film. The results lead to the summary conclusion, that the M8 image quality is superior to film, with the exception of microfilm technology and on a par with the high-end segment of the competition. It was also noted, and that is for me at least an important fact that the quality of definition is very close to the silver-halide based capture medium. Tonal scale, sharpness, definition and fine texture reproduction are reminiscent of the film based era. In this respect too, the Leica M8 is a heritage camera, transmuting the Barnack/Berek approach into the 21-century with digital means.
In all comparisons I used top-quality modern Leica M lenses (SL-A 1.4/50 and ASC 2/75). The nature of the digital capture is such that the recording of the finest structural details is limited by 0.7x the Nyquist frequency and the low frequency components of the image are artificially enhanced by the post recording algorithms. One might expect then a levelling off of absolute performance differences when comparing lenses, used on the M8. Using test charts is the best method for comparing lenses to study relative and absolute quality differences. The chart used is the extra fine Siemens star chart.
For easy comparison I have segmented the charts in four parts, together producing the complete chart. The presentation is always the same. The top row has the full aperture and the bottom row has the stopped down apertures, from 5.6 to 11, depending on the widest opening of the lens. The left column shows the centre portion of the image and the right column the edge position. See scheme below.
full aperture center and corner
stopped down center and  corner
The lenses I selected are the Summicron-M 2/35 ASPH, compared with the Biogon 2/35, the Summilux 1.4/35 compared with the Summilux-M 1.4/35 ASPH, the Elmarit-M 2.8/21 ASPH compared with the Biogon 2/35 and the C-Biogon 4.5/21, the Planar and identical the Summicron 2/35 with the Summilux-M 1.4/50 ASPH, and without comparison the Ultra-Wide Heliar 5.6/12, the Distagon 2.8/15 and the Apo-Summicron 2/75 ASPH.
The methodology for this comparison was: pictures made in DNG mode. Imported with Camera Raw with all settings on zero and transformed in Photoshop to JPEG, again with all settings as efault: no addition sharpening or contrast boost has been done. The goal was to see the pure or raw performance of the lens as recorded by the M8 sensor.
Apo-Summicron-M 2/75 ASPH
The SCA 2/75 performs as planned. Extremely fine structures are recorded with crisp micro contrast. Hardly any difference between stopped down (f/8) and wide open performance. A slight drop in quality in the edge position wide open. Note the dust spot at the edge of the image. 
UltraWide Heliar 5.6/12mm
The focal length of 12mm is the shortest available for the M camera. The performance is quite respectable, especially at the smaller apertures (f/11). Note the strange artifacts in the centre of the image at the small aperture. Vignetting is quite low and this shows the effectiveness of the special construction of the microlenses of the M8.

Distagon 2.8/15mm
This lens has more lens elements and is much heavier than the 12mm. The performance at 2.8 however is already better than what you get from the UWH. Quality wise the ZM Distagon is a very fine lens indeed. Stopped down is f/11. A pity it is so heavy. The level of vignetting at the corners (not shown: I used the edge position) is high, at least three stops.



Elmarit-M 2.8/21 ASPH and Biogon 2.8/21mm and C-Biogon 4.5/21mm
Can we see differences between the lens design with and without aspherical surfaces? If there is a difference, it can be found in the curvature of field, which is better in the Zeiss version. And the contrast wide open which is better in the Leica version. The digital images show the same type of quality difference that has been reported in the film based images. The Biogon 4.5/21 should be pronounced the winner in this contest. This lens exhibits excellent behavior at all apertures and field positions. It has an 'unfair' advantage of 1.5 stops of course, but if you can live with this, the lens itself can hardly be faulted. Stopped down is f/11 for all three lenses.

Summicron-M 2/35 ASPH and Biogon 2/35mm
I noticed a change of character in the most important focal length of the M camera: the 35mm. Now in the M8 it will be the replacement for the 50mm lens in the film-based environment. The Summicron 35mm ASPH has been the standard bearer of lens performance for a long time. But the Zeiss friendly revenge has created a challenge: the Biogon 35mm is in most aspects the better performer. In the centre and wide open the Summicron is still unequalled as contrast goes, but measured on most other criteria the Biogon is the current winner. Note the higher level of flare for the Summicron in the corners.

Summilux 1.4/35 and Summilux 1.4/35 ASPH
This is no contest. The older version of the 1.4/35 wide open has low contrast, low resolution and a lot of flare. Stopped down the performance is much better. Now you know why you need the ASPH version.

Planar 2/50 and Summicron 2/50 and Summilux-M 1.4/50 ASPH
Wide open at 1.4 and at the edge the new Summilux design shows some weakness in the contrast and resolution area, compared to the standard f/2 designs, but do not forget that there is a full stop difference! The new 1.4/50 Summilux deserves the conclusion as the best high speed standard lens in the world. The Planar and Summicron are identical in performance!

Vignetting
A full study of the vignetting behaviour needs another article. For now I can conclude that the film based versions have an advantage here, specifically if we take into consideration that the M8 has a reduced angle of view and should have a natural advantage. My measurements indicate that a 1.4/35 mm ASPH lens has more vignetting on the M8 than the Summilux 1.4/50mm ASPH has on an M7 loaded with film. Both lenses have roughly the same angle of view on the M8 and M7.
This result is typical for all the other lenses tested above. In all cases the film based behaviour was better than the sensor based behaviour. And the M8 has a restricted angle of view. The new microlens design is really needed, but even this construct cannot cope fully with the light fall off at the extreme corners.
Conclusion
The smaller angle of view of the M8 implies that a lens, designed for the full 35mm format will not be asked to bring into effect the extreme corners and edges. So most lenses will operate with a safety margin and the results above do testify this effect. In a sense the M8 does reduce the inherent differences between lenses, as the effects of the problematic edges and corners can be neglected.. Differences do exist as can be inferred from the examples shown above.
My earlier conclusion that in this digital age the optical designs should be concentrated on vignetting, flatness of field and reduced astigmatism and flare wide open and even performance over the image field are substantiated. The ZM lenses from Zeiss have done this already and the comparison between the ZM 35 and the Summicron 35 is an indication of this changing battleground