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03-20-2012, 06:04 AM   #16
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QuoteOriginally posted by Laurentiu Cristofor Quote
Focus shift is one design issue that is lens specific and can cause misfocus. Cameras focus wide open, then stop down to take a shot. A lens with focus shift would focus in the wrong place once it's stopped down, even if the camera correctly focused it wide open.
Focus shift tends to be more of an issue with fast lenses. The shift is always to move the focus plane farther away as the lens stops down. I did a series of tests on the K50 f1.2 that shows this lens is affected more than the f1.4, but still slight enough that the increasing DoF mostly hides the shift.
However, the 1.2 remains hard to focus on the stock K-5 screen, so mis-focus wide open is more of a problem than shift. I don't think the "shim adjustment" method is accurate enough for lenses with such a thin DoF. Older Pentax used adjusting screws on the screens.

03-20-2012, 06:47 AM   #17
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So I've been playing with the lens and I noticed that if I adjust the camera and it looks good at 2 feet wide open. At 15 feet it looks out of focus wide open. I think it's now not focusing properly at infinity. I'll try to play with it more to see if that's really the case.
03-20-2012, 10:01 AM   #18
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QuoteOriginally posted by Laurentiu Cristofor Quote
Focus shift is one design issue that is lens specific and can cause misfocus. Cameras focus wide open, then stop down to take a shot. A lens with focus shift would focus in the wrong place once it's stopped down, even if the camera correctly focused it wide open.
In which case, testing would only show the issue when stopped down - but it would be difficult to see because DOF would be increasing. Whereas most people's test results appear to show the biggest focus issues wide open - consistent with what would happen if the camera were the problem (or testing error that causes the camera to consistently choose to focus on a target other than intended).

QuoteQuote:
There could also be other aspects related to the AF implementation, where the camera tells the lens to focus a certain amount, but the lens changes focus more or less than it was asked to do it - if the camera doesn't double check (I know Pentax AF is supposed to, but maybe it avoids that in certain situations), then it will misfocus.
Even if the camera neglected to double check (which it does), it would FF half the time, BF half the time, and the degree of BF or FF would not be constant but would be proportional to how far "off" the initial focus was. You'd see an error going from infinity focus to say, portrait distance, but even if off by a couple of mm on first attempt, second attempt would then be off by only the tiniest fraction of a mm.

So yes, these are the two physical phenomena people use suggest as explanations - but neither would cause *consistent* BF or FF. They would cause "random" discrepancies that would tend not to actually show themselves in practice and would yield the sort of consistent test results that cause people to believe that the lens has a problem.

Bottom line: if you have test results that appear to show consistent BF or FF - same direction, same approximate amount - then it is either the camera's focus sensor that is out of alignment, or the test was flawed. In the seven years I have been following these forums, I have yet to see even one convincing argument to the contrary. And again, I realize this flies directly in the face of many people's beliefs. But without science to back up those beliefs, I remain the skeptic. I know from experience how easy it is to stuff up a focus test, so I know that explanation comes into play a lot, but it is also trivially simple to show how a misaligned focus sensor would cause consistent focus errors. Those two explanations I have no problems with whatsoever. But without scientific basis, asking me to believe in lenses that magically cause cameras to consistently misfocus them is like asking me to believe in, oh, say, bad luck that magically follows black cats around.

Last edited by Marc Sabatella; 03-20-2012 at 10:06 AM.
03-20-2012, 06:45 PM   #19
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QuoteOriginally posted by Marc Sabatella Quote
But without scientific basis, asking me to believe in lenses that magically cause cameras to consistently misfocus them is like asking me to believe in, oh, say, bad luck that magically follows black cats around.
I am not asking you to believe anything, but you made a statement that is factually incorrect (because it is too strong):

QuoteOriginally posted by Marc Sabatella Quote
There is absolutely nothing that could possibly be physically wrong with a lens to cause it to front focus.
So I just pointed out that *some* focusing issues can be indeed due to lens design.

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