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12-23-2010, 06:30 AM   #16
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QuoteOriginally posted by sewebster Quote
To me, that says that the acceptable tolerances were chosen incorrectly.
Best practice in any rigorous industry (and approved by the ISO commitee) says that any product should be within tolerances 100% of the time with a 95% confidence level. Just like the odd DOA will occur from time to time, extreme tolerances matches will also occur. It's also more likely to happen wieh many companies are matched together (Pentax camera, Tamron lens). In this age of internet, you're bound to hear some horror stories from time to time. People also have more ways to nickpick about their pictures, going crazy over things they would never have noticed before.

12-23-2010, 11:55 AM   #17
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QuoteOriginally posted by bdery Quote
Best practice in any rigorous industry (and approved by the ISO commitee) says that any product should be within tolerances 100% of the time with a 95% confidence level. Just like the odd DOA will occur from time to time, extreme tolerances matches will also occur. It's also more likely to happen wieh many companies are matched together (Pentax camera, Tamron lens). In this age of internet, you're bound to hear some horror stories from time to time. People also have more ways to nickpick about their pictures, going crazy over things they would never have noticed before.
Yeah, sure, that makes sense. I guess we don't really know exactly what the tolerances are and how they test etc. anyway.
06-08-2012, 07:21 AM   #18
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QuoteOriginally posted by yusuf Quote
What lens ROM or electronics has to do with focus since camera is adjusting the element, isn't lens is just a passive element as far as focus is concern?
This is my current understanding:
  • FF/BF is not caused by a lens overshooting or undershooting, regardless whether the lens is screw-driven or SDM-driven. Pentax AF definitely has a confirmation step, making sure that focus has been obtained. This sometimes leads to small "last minute" corrections, aka, focus hesitation.
  • The main issue is that the AF system sees a different image than the sensor. The AF system works with a f/5.6 aperture. Most lenses have some focus shift, i.e., they shift their focus plane when the f-ratio changes. That's where the lens ROM comes in. It contains data as to how much compensation is needed, depending on the f-ratio the image will be taken at.
  • Decentering and other issues may invalidate the standard ROM compensation values. I do not believe this is due to "phase shift" because the AF system does not care about the phase of wave fronts. It cares about focus and measures the latter by transforming amount of misfocus into phase shift (spatial shift) by using prisms.
I have not seen AF algorithms and cannot be a 100% sure that my assumptions are correct. However, I have not come across a better explanation yet.
06-08-2012, 02:23 PM   #19
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Several causes for BF/FF. In SLR cameras with AF sensor typically the camera causes the trouble, but under some circumstances the lens is involved as well.
It really gets interesting when comapring BF/FF issues for different F-stops as the camera always uses the max. f-stop during focusing (unless you use live view...)

Besides that you can raed a lot of BS on the web.

06-08-2012, 03:44 PM   #20
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i guess it depends on the SAFOX version too : my DA*55 need a +180Ám correction on the K-x, but when i tried it on a K-r and K5 it didn't need ny adjust.
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