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02-26-2009, 09:28 PM   #1
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What Does Front and Back Focusing Mean?

I see some discussion about the K20D that talks about the lenses being out of adjustment, or that they are front focusing. What does this mean and how can you tell?

02-26-2009, 09:58 PM   #2
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QuoteOriginally posted by GregK8 Quote
I see some discussion about the K20D that talks about the lenses being out of adjustment, or that they are front focusing. What does this mean and how can you tell?
It means that if you try to focus on one object, the object won't turn out to be in focus - something in back of (backfocus) or in front of (frontfocus) will be in focus instead. Note this has *absolutely nothing* to do with the pretty common issue people face when the camera actually *chooses* to focus on an object in front of ior in back of the one you intended - cameras are not mind-readers. It's only backfocus/frontfocus if you are 100% sure the camera actually did try to focus on the object you intended. And that's a hard thing to be sure of, which is there are various focus tests charts you can use for the purpose that try to remove some of the guesswork.
02-26-2009, 10:06 PM   #3
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Of course this only applies to Autofocus and Focus Trap functions where the machine judges focus.
Manual focus through the viewfinder and lens is fine.
02-27-2009, 07:54 PM   #4
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So is it caused by a faulty sensor? or is it just a misalignment.

02-28-2009, 11:05 AM   #5
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That's kind of an open question. Certainly, if the focus sensor is not aligned properly - it needs to be *exactly* the same distance from the mirror as the sensor - that would cause problems. But there is also the possibility that the problem would have to do with the mechanical linkage between the AF system and the lens - eg, turning the screw by X amount not producing *exactly* the right result, and the camera not catching on that it has failed to achieve focus. Probably other possibilities as well.
02-28-2009, 11:47 AM   #6
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QuoteOriginally posted by garth1948 Quote
Of course this only applies to Autofocus and Focus Trap functions where the machine judges focus.
Manual focus through the viewfinder and lens is fine.
Not necessarely...

The k20d I had to return for severe front focus did focus PERFECTLY in the viewfinder, with autofocus or manual lenses... but every picture came out front focussed by nearly 1/2 to 1 inch!

Pat
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