Originally posted by hatsofe ...I ignore the focus confirmation sign and sound and rely entirely on my eye believing that what I see sharp on the focusing screen...I can understand a problem with AF, as it's a separate optical system, but the focusing screen and the sensor are located in the same register distance.
The focus screen registration is supposed to be at the same registration, but whether care has been taken during assembly to properly place and confirm is difficult to assess. If calibration is wrong, one would see consistent front or back focus with ALL lenses.
Front/back focus with PDAF is not related to registration. The topic is complex, but it is probably enough to simply say that it is related to alignment and relative position of elements at various distances and zoom setting causing an ambiguous or skewed image on the AF sensor. As a result, sensitivity to the out-of-focus condition is greater on the near than the far or vice-versa.
Originally posted by hatsofe Can any one explain this phenomenon?
The phenomenon of two similar lenses performing differently? Without an optical bench probably not. Problems with mechanical lash (elements shift position when the focus ring is released) is a possible cause as might misalignment. Another might be the method of testing/comparison.
Originally posted by Adam Do you get the same problem in live view with focus peaking/magnification? If not, then it's probably just tolerance that's not discernible via the viewfinder.
Kudos to Adam for asking the critical question. The live view image is the gold standard for focus evaluation. I am not confident of focus peaking, but am very fond of the magnified view.
A few points to ponder:
- The optical viewfinder on your camera is less sensitive to detect out-of-focus (OOF) than that of a manual focus SLR from the late 1970s-1980s. The reasons are complex, but one can expect the ability to detect OOF to be no better than with about an f/3.5 aperture regardless of whether a faster lens is mounted. The result is lower precision regardless of how well calibrated the rest of the system is.
- Your ability to detect the plane of focus is highly dependent on whether your eyes are actually focused on the focusing screen surface. The viewfinder diopter adjustment is important. On screens with etched lines (most Pentax dSLR stock screens) the AF area bracket lines should be sharp.
- When doing your focus evaluations the intent is to make it easy, not hard. Use a high contrast target positioned parallel to the camera sensor. Do your testing in good light and on a tripod (inches count).
- Distance is important, though perhaps not in the way one might think. There is no virtue in testing focus at other than moderate distance. Being able to actually see the target in the viewfinder is one good reason, but more important is that the further away something is, the less degrees of throw are available on the focus ring. This is easily seen by looking at the distance scale.
I have in front of me a 50mm lens. There is probably less than 20 degrees of arc between infinity and 30 feet. Extrapolating a bit, I figure there is less that 10 degrees between 30 feet and 1000 feet. A little closer in and that same 20 degrees will satisfy the range of between 10 and 15 feet. How easy is it to make fine adjustment of less than a 1 mm of linear travel of that ring? That is with a lens having a full 180 degrees focus ring travel to cover 15" to infinity. Now consider a lens such as my Sigma 17-70/2.8-4 (C) which traces the range 8 inches to infinity in about 40 degrees of throw.
If it is physically difficult to bring the target into focus, move it close, much closer. - Focus technique counts. It makes a difference whether one is focusing near-to-far vs. far-to-near. DOF is shallower in the near direction and focus "pop" is more distinct working from that side, though possibly no more accurate.
- Accuracy and precision are two different things. A combination of short focus throw, long distance and inability to reliably detect OOF works against even the most accurately calibration. Because precision can be poor, multiple attempts should be made before claiming front/back focus.
I am quite curious to read what happens with your live view attempts. One last thing...Is there any chance your focus screen has been removed at some point or that any of the lenses in question might have been dropped?
Steve