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09-07-2009, 10:02 AM   #1
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Manual lenses and focus adjustment

I've been trying my K20D for a while now, with the kit lens, also a 135/2.5 and a 28/2.8, the A version. The first two are spot on in focus detection. That is, the kit lens is perfect both in autofocus and in manual focus confirmation, and the 135 focus is perfect in manual focus confirmation. I get a solid green focus confirmation at the best point.
I thought the 28 was a dud. Because it's hard to manual focus with the standard screen, and reliying on the green light I got soft images (at 100%, really fine stuff). It didn't quite beat the kit zoom in terms of resolution.
After trying it with live view on, I discovered the perfect focus point is a little off, like one and a half millimeter on the lens scale. On files it makes a world of difference. The lens is extremely sharp even at 2.8. But the difference was impossible to notice in the viewfinder with the naked eye. Trusting the green light would throw the focus off every time.
As I see it if I change the focus adjustment on camera for that lens it will throw other lenses off, and the manual ones won't be individually correctable.
Is there any solution besides looking for focus confirmation and then correcting (that works).
Thanks for any answers.

09-07-2009, 10:22 AM   #2
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There have been a couple of other recent threads with questions similar to yours. If I remember correctly, the short answer is that you can't tune the AF on your camera to individual non-AF lenses.

In regards to using the AF system to provide focus confirmation, I think you will find that focus confirm has inadequate precision for fast (f/2 or wider) lenses when shooting at wider apertures. That has been my experience anyway. Depth of field is so shallow and the sensitivity of the system is limited. Unfortunately the stock focus screen is no help here either since its design limits the ability to fine focus as well.

The standard answer on this forum for anyone wanting to use manual focus lenses is to suggest an aftermarket focus screen having some sort of focus aid (split-image and/or microprism).

Steve
09-07-2009, 10:29 AM   #3
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i think you may find that due to th erelitively good DPF of wide angle lenses, it is difficult to know when you are actually in focus with a wide angle lens.

a split image screen is a big help
09-07-2009, 11:48 AM   #4
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Thanks Steve, you're right, probably a split screen is the way to go.
Lowell, I'm aware of the wide angle focus issue. What impresses me is the amount of sharpness you can get out of this lens and sensor. With 35mm film I rarely care that much because at the level of magnification I'm talking about there were so many issues that you never knew what would be the optimum. With this sensor and a good lens the game seems more like medium format than 35mm, were you can get amazing results, but you have to put a lot of attention to it. Evidently, a good option for critical focus is using live view at 8x.

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