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We could have used this one during the rangefinder wars.
 
Kobalux 21mm f/2.8 revisited (Leica M8)
 

Original shot at f/8 (ISO 160), DNG converted in Adobe Lightroom, auto-leveled, saved for web on Photoshop.  Focus point is the lion head in the center.  I don't think that shadow is me!

Today I was screwing around and decided to see what the Kobalux 21mm f/2.8 (final version) could do on an M8.  The following are 100% crops of the same frame, shot at f/2.8, f/4, f/5.6 and the f/8 shown above.  In each instance, the left picture is with no sharpening in Lightroom or Photoshop.  The right picture is the same frame, run through Photoshop's Unsharp Mask function at a very light setting (100%, 0.3 pixel radius, threshold 0).

I'll leave you to see the results with this lens.  My own observations (aside from satisfaction) are:

nThis lens peaks on an M8 at f/4-5.6, which seems a little bit unusual for a wideangle lens of the old-school design (usually these peak at f/8 or smaller).

nIn terms of design, this is a symmetrical superwide, the nightmare scenario for the M8's design team.  Lenses like this are the reason why a 24x36mm sensor is not a priority for Leica.

nShooting at f/4 versus f/5.6 is a tradeoff; at f/4, the center has higher contrast than the edges; at f/5.6, the center and edges are more even (although both slightly less contrasty than the center at f/4).

nThe vignetting at f/2.8 is significant enough that the camera does not see f/2.8 as twice as fast as f/4.

nI really need to clean the sensor on this camera.

This kind of thing really adds insult to injury in terms of the discontinuance of the Kobalux lens line for Leicas.

Above left: center, f/2.8, 1/6000 sec, no sharpening

Above right: center, f/2.8 (USM, 100%, 0.3 pixels, threshold 0)

Above left: center, f/4, 1/4000 sec, no sharpening

Above right: center, f/4 (USM, 100%, 0.3 pixels, threshold 0)

Above left: center, f/5.6, 1/2000 sec, no sharpening

Above right: center, f/5.6 (USM, 100%, 0.3 pixels, threshold 0)

Above left: center, f/8, 1/1000 sec, no sharpening

Above right: center, f/8 (USM, 100%, 0.3 pixels, threshold 0)

Above left: right third, f/2.8 (no sharpening)

Above right: right third, f/2.8 (USM, 100%, 0.3 pixels, threshold 0)

Above left: right third, f/4 (no sharpening)

Above right: right third, f/4 (USM, 100%, 0.3 pixels, threshold 0)

Above left: right third, f/5.6 (no sharpening)

Above right: right third, f/5.6 (USM, 100%, 0.3 pixels, threshold 0)

Above left: right third, f/8 (no sharpening)

Above right: right third, f/8 (USM, 100%, 0.3 pixels, threshold 0)

DAST