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05-06-2013, 02:28 PM   #1
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Back Focus / Focal Length correlation ?

While reviewing recent images from my grandson's little league game, I noticed that pics taken at longer focal lengths of my Pentax 18-250 lens from considerable distance showed back focus discrepancy. Images taken at closer distances and shorter focal lengths demonstrated sharpness at the subject intended. AF-S set to center focal spot for all.

My question before I attempt FF/BF adjustments of my K7, " Do various focal lengths or distances from subject affect FF/BF issues while using the same zoom lens ? " and " Does distance affect reliability of AF ? " I was at least 50 yards away on images with BF problems.

Thanks for the help.


Last edited by zman; 01-21-2017 at 06:46 AM.
05-06-2013, 02:36 PM   #2
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It might be - MIGHT BE - the size of the centre focus spot. The fence has sharper contrast than the batter. In this case, the lower centre spot might have been better. Try it with the focus spot selection by camera. It might be worse, but it might work.
05-06-2013, 02:46 PM   #3
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QuoteOriginally posted by zman Quote
Does distance affect reliability of AF ?
Possibly, especially if you are using center point focus. The further away from the subject the more information is inside the center point 'box'. It is not a point, rather an area within which the sensor selects something to focus on. So up close possibly the only thing in the box is the subject's face, but farther away maybe the box contains some high contrast pixels from the fence behind the subject.

QuoteOriginally posted by zman Quote
Do various focal lengths or distances from subject affect FF/BF
Note that on a prime lens you can adjust the AF fine tuning with some confidence. With a zoom, you have only one adjustment but many focal lengths. So in theory your lens could back focus at 18mm be spot on at 100mm and front focus at 250mm (or some other configuration, that's just an example). With a zoom you have to either pick an approximation that works for you or tune to the focal length that you use the most.

In your case (from the image posted) you don't have a FF/BF issue. The lens did focus correctly just not on what you wanted it to focus on. It looks like it decided the high contrast fence was the better target. Try keeping the AF 'box' on your target if you can and then recompose after focus has locked. You might also try zooming in to focus, lock focus, and zoom out to shoot. Assuming that works on that lens, I don't have it so I'm not sure if the focus changes with focal length on it.

FF/BF tuning only helps if the lens is not in focus where the AF sensor thinks it is. In your case it appears the lens is focusing properly (just not where you want) and if that's the case then messing with the AF tuning will cause more harm than good. Set up a tripod and a proper AF target and run numerous tests before mucking about with AF Tuning.
05-06-2013, 04:14 PM   #4
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I'll agree with what the others have said. The camera picked the fence, not the batter. The AF point is not as small as the little LED would have you believe, sorry to say.

QuoteOriginally posted by jatrax Quote
You might also try zooming in to focus, lock focus, and zoom out to shoot. Assuming that works on that lens, I don't have it so I'm not sure if the focus changes with focal length on it.
Just to clarify the comment -- some lenses are parfocal, some are varifocal. Parfocal lenses stay in focus no matter what zoom length. So an old technique when manually focusing some older pre-autofocus lenses it is to zoom in, nail the focus, and zoom back out. But you can't do this with a varifocal lens. Not sure if yours is parfocal or varifocal.

QuoteOriginally posted by jatrax Quote
FF/BF tuning only helps if the lens is not in focus where the AF sensor thinks it is. In your case it appears the lens is focusing properly (just not where you want) and if that's the case then messing with the AF tuning will cause more harm than good. Set up a tripod and a proper AF target and run numerous tests before mucking about with AF Tuning.
+1, agree here too.

05-06-2013, 04:56 PM   #5
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QuoteOriginally posted by zman Quote
Images taken at closer distances and shorter focal lengths demonstrated sharpness at the subject intended.
I agree with others who've already said it's likely that the camera picked the fence or other background for autofocus.

I would like to confirm what you meant by "shorter focal lengths" though. I understand "shorter" to mean a lower focal length. Shorter = less zoom. In other words, 50mm is shorter than 250mm. If your camera was focusing on the background instead of the batter, shorter lengths would have more focus problems, while longer focal lengths like 250mm would cause the batter to fill more of the autofocus sensor and decrease the problem.
05-06-2013, 05:49 PM   #6
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PARFOCAL.
Got my attention on that one when I realised that would be the best video type of lens for my K-5.
Sooo I went to ebay and a search revealed about 140 of them but they're all telescope eyepieces.
I see on another discussion there's a list of Pentax parfocal lenses.

Last edited by chromo; 05-06-2013 at 05:56 PM.
05-07-2013, 06:04 PM   #7
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Appreciate the education. I do think I was mistaken believing the little red point on my viewfinder represented only the area of specific focus. I get it now that it is indeed greater. Next game is Thursday and I'm going to bring my A* 300 for the distant shots. Thanks again for all the help.
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