Out of boredom, I decided to test all my lenses for front-focus and back-focus problems. The test did not go very far since the results were very confusing.
My test setup:
0) Camera: K-x
1) I'm using the focus chart found here focustestchart.com/focus21.pdf
2) Lighting: CFL (not tungsten)
3) AF point is center. I tried both AF-S and AF-C mode, no difference
What I observed is that all the following three lenses: FA 77mm, FA 43mm, FA 31mm are back-focused quite a bit using phase detection AF, from 6mm to 10mm. When I switched to LiveView, all the focuses are almost spot on. In LiveView, I tried two different ways of focusing: (i) zooming in and (ii) just relying on the small circle square. Both methods were consistently producing correct focus.
My questions are: Is my setup correct? Assuming it is, is it the problem with the camera? What kind of tests should I do more before contacting Pentax to have it adjusted?
I was actually quite happy with the real life results from the FA 43mm and the FA 77mm, but I wasn't as impressed with the real life results from the FA 31mm.
Update 1:
I've just adjusted the focus of the K-x to +90um to fix the back focus issue and it seems like four out of five of my lenses are focusing correctly now: FA 31mm, FA 43mm, FA 77mm, and F 100mm macro. The DA 15mm is now a tiny bit front focus, but since it starts at f4.0 and I usually use it for landscape where I shoot from f8 anyway, it's not that much of a problem. I wonder if the indoor lighting actually affects the lens' focus system? I will probably try it outdoor tomorrow to see if the results are still the same
Update 2: Took the camera and the focus test chart outside with plenty of natural light, no detectable difference in test results. All lenses are almost spot-on. My next question is: Is it better to be back-focused or front-focused? Say if I can't find a perfect setting for all my lenses (since the K-x only allows one global adjustment) and have to make some compromise between them, is it better to have FF lenses or BF lenses?
Last edited by vtqanh; 11-04-2010 at 08:49 AM.