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08-17-2009, 04:56 PM   #1
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Join Date: Aug 2009
Location: Hawaii
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Front/back focus or focus shift?

Hi All,

These days I experienced much focus issues with my new K-7 and FA 50 1.4 and DA 16-45. I searched the forum and saw others also have the same problem with these two lenses. Here after some research I got some hints to get sharp images with them.


1. For AF tune -10 I still have front focus with these two lenses when shooting wide open;
2. But when stopping down to f2.8 for 50 and f5.6 for 16-45, I have the focus spot on. This was confirmed both by chart and real life shooting;
3. Stopping down more, they will back focus but it is OK if you have your object not so far away since the DOF is thick enough.


1. If you google focus shift or spherical abbreviation, you may find some articles discussing focus backward shift when stopping down. This is exactly what's happening to K-7 and the two lenses mentioned above;
2. The problem is the image sensor is not consistent with the AF sensor. Image sensor seems to be a little further from the lens than it should be. This gives us a good focus when stopping down. Let's call it a threshold aperture for best focus. So if I have AF tune weaker than -10, I need to stop down more to get a good focus. A different case is my Nikon D90 with AF 50 1.4d. The similar focus shift exists but the solution is different that it get best focus at f1.4 and all back focus when stopping down. I don't know if Pentax did this on purpose to have best focus on best apertures, but it would be nice if we can tune the AF spot on when wide open and choose our best aperture. It just need a little offset! Maybe -5 is enough. Or if we can have a larger range of focus adj in next firmware I will be absolutely happy!

Pls test yours and see if you agree!



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af, aperture, focus, front/back focus, k-mount, lenses, pentax lens, sensor, shift, slr lens, spot, tune
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