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07-15-2013, 04:13 PM   #1
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A-lens problem


I have a 50 1.7 A-lens that differ when it is in A mode on lens or when I manually choose the same f-stop.
It always differ minus one step exposure in A-mode on the lens.

When I turn from F 22 to A it looks like the aperture turns a little smaller.

Mats W

07-15-2013, 07:13 PM   #2
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Could be slow/oily blades. That'd be my guess, shine a light into it with the aperture closed and move the lever back and forth, if you see more than just blades, you've probably got the reason.
07-16-2013, 04:19 AM   #3
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Hi !

I think I found the problem.

It works fine to 2.8 then it will underexpose 1 step.
I try the lens on a Pentax Kr instead of my Kx, now it only underexpose 0.5 step.

At last i found this article:
Shooting manual lenses on Pentax DSLRs «
and this quote.

Another metering issue is a bit more complicated. It seems that the metering system in the modern Pentax DSLRs is not optimized for use with the older manual lenses. I have read a lot about this and still can’t quite explain what is going on. Bottom line is that you may need to adjust your exposure up or down depending on the combination of camera/lens/aperture/subject being used. When shooting my A series lens in Av mode (as I typically do) I generally dial in around +0.7 to +1.0 EV compensation at f/1.7. If I shoot at smaller apertures I can often back off this setting depending on the subject matter. However, I do pretty much the same thing with every lens I use on my K10D so this is an area where experimentation is definitely required (on a lens-by-lens basis).

Mats W
07-16-2013, 04:27 AM   #4
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Note that manual aperture lenses do not meter perfectly on Pentax

See the graph below using a K50/1.4, and comparing it to an A50/1.2 in and out of A mode. I wanted other camera owners to do the same test and send me the data using a 50/1.4 so we could compare all the bodies. Thetest is shooting a uniformly lit concrete surface using green button metering at each aperture . Then measure greyscale value in the middle of the frame (or send me shots and I will do it). Shoot in JPEG with all settings neutral


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