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10-02-2010, 01:14 AM   #16
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QuoteOriginally posted by Lowell Goudge Quote
...I don't know how the phase detection works...
It was a mystery to me also, until I found this explanation:

Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras

So, in essence, phase-detect AF is similar to a split-prism focus aid - and therefore:

a). The algorithm knows which direction to alter the focus.

b). The algorithm should be able to decide whether or not the lens is acceptably focused.

Note though that focusing is achieved with the lens wide open. When stopped down to the set aperture, it is quite possible for focus to change (owing to spherical aberration). In fact, I've noticed this effect with my DA 50-200, which re-adjusts focus as you change the aperture setting (when mounted on my K-m/K2000). So the Pentax engineers seem to be aware that the DA 50-200 needs focus-tweaking unless shot wide-open.

(I tested that focus had actually changed by trying it stopped-down in manual-focus mode, having auto-focused it wide-open; the shot was marginally less sharp than when shot stopped-down and auto-focused.)

10-03-2010, 05:18 AM   #17
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What I am saying is that they may know the criteria, in terms of thresholds etc, but they may not know, down into the lens manufacture detail, the tolorance variables or design details that will gaurantee critical focus is achievable on each and every lens.

They may also, during the development of a lens, make adjustments between the theoretical design, and what can actually be produced, in order to ensure that they can meet the standards repeatidly, with real products. This may mean, for example, accepting that a lens in the production phase may be deliberately set a few microns one way or the other , nominally to get the highest yeild of production within an absolute tolorance range.

As I said earlier, if manufacturers coould control everything perfectly 100% of the time, there would be no need for FF/BF adjustments, and certainly no need to have customizable MF/BF adustments for 20 different lenses.

Either the manufacturers simply dont know/can't control the process, or it is grossly expensive to do so.
If their technical people are any good (and I mean, first-year science college good), they know how to calculate tolerances and uncertainties.

You are correct to assume that making perfect products would be grossly expensive, simply becaise the yield would be near zero. Not because they don't know their stuff.

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