GEAR TALK - Troubleshooting a Couple Lenses

During the past 2-3 months, my keeper rate in terms of critical sharpness has been lower than expected. Something is amuck. So, this past weekend I dusted off the tripod and set about to figure out why the keeper-rate had declined.

LEICA 28MM F2 SUMMICRON-M ASPH (II) - FOCUS SHIFT?

The Leica 28mm F2 Summicron-M ASPH (II) is one wickedly sharp lens. In basic 'around-the-house' lens testing the lens was sharp wide open, showed good edge sharpness and exhibited no focus issues. All good stuff, but in the field that that goodness was not always there. This made no sense because there is so much depth of field with a 28mm lens at F8 and F11. Time for a focus shift test. This is easy to spot in a controlled test because plane of focus will increasingly shift behind the dark line at 0mm:

F2		F2.8		F4		F5.6		F8		F11

In this sequence the focus stays perfectly on point at the center mark. There is no focus shift here; however, the placement of the additional DOF is essentially going behind the focus point. Normally, when a lens is focused on a point and then stopped down, 1/3 of the added DOF (from stopping down) is placed ahead of the critical plane of focus, and 2/3 behind the plane of critical focus. In this case it seems like less than 33% is being placed ahead of the focus plane.

For me, this is a very tricky lens to use since really no DOF is added ahead of the focus point. The lenses were well calibrated and accurate at F2, so if re-calibrated to add some front focus, then the F2 and F2.8 shots would be front focused. After trying three 28mm Summicron V2's and seeing the same behavior, I dispensed with the 28mm Summicron-M Version II’s - too much headache.

LEICA 135MM F3.4 TELYT-M APO - SHUTTER-SHOCK?

Normally the Leica 135mm F3.4 Telyt-M APO is a pretty easy lens (for me) to shoot, but with the Leica M Monochrom there was a repeating pattern of slightly blurred images, or part of the image showing some faint blurring in dense patterns and textures.

Using a red filter costs ~2.5 stops of light, so if shooting at F8, the net result in terms of shutter speeds is akin to shooting F22. That can translate into 1/90th shutter speeds during midday sun. While 1/90th is too slow for a handheld 135mm shot, it should be plenty fast for a tripod based shot. Or so I thought... anecdotal testing revealed:

• 1/90 = blurred image
• 1/125 = blurred image
• 1/180 = marginal image
• 1/250 = marginal image
• 1/320 = okay sharpness

Slower shutter speeds due to the red filter was the likely the culprit, but I had not made the shutter-shock connection. In hindsight that conclusion is obvious. With the color cameras the shutter speeds during the day are much higher (no red filter to zap light), and the night shots have a very long shutter times, thus masking the shutter-shock.

CONCLUSIONS

As noted above, to fix the 28mm Summicron problems, I simply stopped. I do not know what the Leica 28mm F2 Summicron-M ASPH II's exact focus and calibration phenomena are, but I would label it as some type of focus shift. Thus, If I wanted to a 28mm F2 lens again, I would buy a used Leica 28mm F2 Summicron-M ASPH (Version I) and avoid the “Version II”. Or I would buy the Leica 28mm F1.4 Summilux-M ASPH - which is an absolutely stunningly good lens.

The solution for the 135mm APO is much less dramatic - boost ISO and keep shutter speeds in the 1/320 range. The same suggestion applies to the Leica 90mm F2 Summicron-M APO and for any lenses longer than 135mm (I use alot of adapted telephoto lenses). Hopefully Leica will incorporate electronic shutter into its next M bodies - that is a great shutter-shock work around.