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11-07-2008, 09:33 PM   #1
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explanation

can someone explain front/back focusing of a lens to me?

11-07-2008, 10:48 PM   #2
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search for "focus test chart". Those terms are explained there.
11-08-2008, 11:27 AM   #3
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QuoteOriginally posted by DanLoc78 Quote
can someone explain front/back focusing of a lens to me?
These terms are usually used to describe an auto focus system - camera & lens. It's usually difficult to decide which part of the system is at fault.

Front focus = camera/lens reports it has focused on some particular object, but in the actual picture, focus appears to be in front of that object.

Back focus - same, but focus appears to be in back of the reported object.

Note this has nothing to with the more common issue of the camera simply not choosing to focus on the thin you wanted it to focus on, which is kind of unavoidable sometimes. That is, if you try to focus on a person standing 3 feet behind a flagpole, there's pretty much way to force the camera to focus on the person rather the pole. It will focus on whichever it feels like focusing on, even if you try to force the camera to use the specific focus point closest to the person (the focus points are too broad to guarantee it won't focus on the pole instead).

So in order to test FF/BB, you need to use a chart with a foolproof focus target, one that removes any possibility that the camera is actually focusing on something other than what you think it is focusing on. The one at Home is very good.
11-08-2008, 05:17 PM   #4
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i typically use spot focus, focusing on what i want with the center AF point, then recompose. I keep it on AF.S so that once recomposed the focus is locked. However, I feel like I have seen quite a few of my own photos with this front/back focus problem (which really is as obvious as it sounds). I still attribute it to user error...but thank you for the explanation.

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