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08-25-2011, 02:55 PM   #16
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QuoteOriginally posted by debmalya Quote
Here is a link that demonstrates how passive phase detection auto focus system works: Autofocus: phase detection
Thanks, very nice resource. I like how it interactively illustrates the light path.

QuoteOriginally posted by Marc Sabatella Quote
I don't really understand how those pictures lesn defects would cause AF to be off if the sensors are aligned correctly. After all, whatever defect the focus sensor sees is the same defect the imaging sensor sees. If the image is correctly in focus at the AF sensor, how could it not be at the image sensor, since they are the same distance away from the focal point?
I'm not sure either. Just offering a possible version. What i'm thinking of is a case where each of both AF sensors does not see the same image, because the lens is not symmetric. If you look at the debmalya's link you see that AF uses light entering at each side of lens. That's why AF works only with F5.6 or faster lenses. This is because of aperture placement in AF sensor.
Of course optical defects like CA are symmetric at both sides of lens and can affect only quality/speed/precision of the AF system but not the absolute AF position. If the lens is not centered many effects produce unsymmetrical distortion. Some calculations should be done, to find out what effect this can have.

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