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09-21-2015, 09:25 PM   #1
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front-back focusing lens and auto focus

Hello, I was wondering if someone could explain how an slr autofocus with a front or back focusing lens can signal that it is focused (green dot and beep) when the camera is taking a picture of a flat surface like a wall or a picture hanging on a wall.

thanks, this has been bugging me for a few days!

09-21-2015, 09:32 PM   #2
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Because the camera thinks that things are in focus. Viewfinder AF works by calculating where to focus, so it's subject to some degree of error. A more technical explanation can be found here:
Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles

If you want a more accurate focus check, use live view.

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09-22-2015, 12:55 PM   #3
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QuoteOriginally posted by Adam Quote
Because the camera thinks that things are in focus. Viewfinder AF works by calculating where to focus, so it's subject to some degree of error. A more technical explanation can be found here:
Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles

If you want a more accurate focus check, use live view.
Thanks for the response and the pointer to the article which is quite useful. My question could have been stated better. I can imagine that the autofocus system could focus on something in a picture that was closer to the camera than intended focus point. I am curious as to how it can front focus if there is nothing in front of the intended focus point, like a wall for instance.
09-22-2015, 06:48 PM   #4
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QuoteOriginally posted by Hiroshi Quote
Thanks for the response and the pointer to the article which is quite useful. My question could have been stated better. I can imagine that the autofocus system could focus on something in a picture that was closer to the camera than intended focus point. I am curious as to how it can front focus if there is nothing in front of the intended focus point, like a wall for instance.
You have to understand how phase detect autofocus works. The mirror in the camera is somewhat transparent and allows some light through. This light then is reflected off another mirror down to the autofocus sensor. Two images are formed by two small lenses on the sensor. The distance between these images is used to calculate how far the camera needs to turn the lens to achieve proper focus. Different camera models use different logarithms to perform these calculations. If the lens and the camera are properly calibrated this will work without issue. Unfortunately manufacturing tolerances factor in here. Sometimes if the tolerances on the lens and the camera can be far enough apart that a proper focus is not achieved. If the camera is not calculating the distance between the two images on the sensor the camera can move the lens to far or back and think the subject is in focus even though it is not. This is were in camera focus adjustments come in. You can override the camera default and add compensation for this to achieve proper focus. If this needs to be done it does not mean anything is wrong with the camera or lens. Just that with manufacturing tolerances, accuracy can only go so far. Back in the film days sometimes you would need to add shims to your focusing screen to ensure when something on the viewfinder appeared in focus it actually was.

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