« on: April 23, 2014, 10:29:43 AM »
When I read back over TDP's review of the S50A and I think to myself on how to obtain the best focus using a tripod, using the AF from the camera with phase detect focusing is not how I'd do it.
The whole point of the test was to determine the accuracy of the SIR PDAF system with the Sigma 50/1.4 A under controlled conditions, with neither camera nor subject moving. Should be pretty simple for an AF system with an accurately-focusing lens. Using Live View would defeat the purpose of the test.With Canon lenses mounted on a Canon camera, I've watched it focus on something and lock, then I press the focus button again, it de-focuses and refocuses again. Why can't it just "know" that it has acquired focus and not move the second time?
How does the camera "know" that the subject has not moved? Having said that, with many subjects, the focus will not change as the PDAF sensor will determine that it is already properly focused. When the AF does defocus/refocus, that's usually becuase of the characteristics of the subject, for example with a cross-type AF point and a subject having different phase differences in the orthogonal orientations.The point I was making above is that if you've already focused the lens on X and press the button again, it is doubtful that the lens will end up in exactly the same position as before (and by exact, I mean exact, not some "within half a millimeter.")
Slight variances are one thing - you may see differences with image analysis (MTF/SQF values) that would be too subtle to detect visually. The OOF shots in the TDP test aren't subtle…the 50A just flat out missed focus on 4/10 shots.
Depends on brightness of scene too. I often focus/re-focus on a subject as I am shooting/ verifying. In good light, my canon lenses either don't change focus distance at all, or very very little, if using single point focus. Most focus changes are probably due to a subtle change in the camera position or change in light quality/contrast on the subject. Defocus/refocus only happens in very low light.