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07-22-2018, 11:11 PM   #1
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AF problems shooting at an angle

Just noticed on my camera if I shoot something straight on, the focus is pretty good; for example a test pattern against a wall or a subject that I'm standing directly in front of who is squarely facing me.

But if I'm shooting something obliquely, the AF tends to front focus quite a boot. So for example standing up and shooting angled down, or shooting something that's angled toward me while shooting level. This is very pronounced on my K-50, and slightly less but also noticeable on the K-3 II.


I'll have to test my Canon gear with this test, but does anyone else have this problem? Is it because the focal points are larger than what the dot suggests? Anyone else experience the same issue?

07-23-2018, 12:08 AM   #2
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QuoteOriginally posted by keos Quote
Just noticed on my camera if I shoot something straight on, the focus is pretty good; for example a test pattern against a wall or a subject that I'm standing directly in front of who is squarely facing me.

But if I'm shooting something obliquely, the AF tends to front focus quite a boot. So for example standing up and shooting angled down, or shooting something that's angled toward me while shooting level. This is very pronounced on my K-50, and slightly less but also noticeable on the K-3 II.


I'll have to test my Canon gear with this test, but does anyone else have this problem? Is it because the focal points are larger than what the dot suggests? Anyone else experience the same issue?
Oh, they're larger than the dots, alright.

But the main thing to do with a 3d object is to choose the focus point over the part you want in focus, instead of what's closest to the camera. Eg the eye instead of the nose.
07-23-2018, 04:46 AM   #3
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@keos

What AF area selection have you made? If you need more accurate AF on a specfic point then select the Spot or 5/9 point settings to prevent the camera choosing its own prefered focus point.
07-23-2018, 07:00 AM   #4
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Yes your AF might be a bit more indecisive when the subject is angled because the sensor is presented with part of its area in-focus and part that isn't, since all those points can't be in focus at once for an angled subject. Square on, most or all of the subject is in the same plane and the sensor sees a good deal of its area in agreement, thus allowing the AF to work better. As mentioned, spot focusing narrows the area looked at reducing the amount of out-of-focus points the sensor sees from an angled surface so it helps in those cases.

Differences between cameras may be due to the size or placement of the AF sensor in the field you're trying to focus on.

07-23-2018, 07:30 AM   #5
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I'm using center only to test; not recomposing or using any other focus point. shooting some shoe boxes that was lying around on the UPC code. If I shoot it straight on, I get the UPC code in focus.
If I angle myself so that the UPC code is at an angle (so if the UPC runs parallel to the wall and I faced the wall perpendicularly, I move myself about 60 deg clockwise so now the UPC code and the lens would make a 30 deg angle), and center focus the UPC code, the shoe name (to the left of the UPC code, so now in a plane that's closer to the lens) would be in focus but the UPC code is out of focus.
07-23-2018, 12:39 PM   #6
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You would have to show us pictures, but remember that it's looking for an obvious line under the focus point and that might be a letter's edge rather than the mass of smaller lines making up the barcode.

You can explore where the AF point limits are by taking repeated shots of an interior wall and a doorway and seeing where the AF lock kicks in.
07-23-2018, 01:55 PM   #7
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Even if I use single point AF on my KP, if I shoot in about 45 degrees angle the results are dissapointing, there is as keos tells a tendency of unpredictebel front focusing. With manual focus there is no problem when you dont let you disturb of the visual and acoustic focusing aid but trust your eyes

07-23-2018, 02:06 PM   #8
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QuoteOriginally posted by niels hansen Quote
Even if I use single point AF on my KP, if I shoot in about 45 degrees angle the results are dissapointing, there is as keos tells a tendency of unpredictebel front focusing. With manual focus there is no problem when you dont let you disturb of the visual and acoustic focusing aid but trust your eyes
As a test, rotate the camera so the line you're focusing on is now at a right angle to the rows of pixels that make up the focus point.

That's maximum performance.

Note for other situations that only the central AF points are of the cross type, too.

07-23-2018, 09:36 PM   #9
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Look at Camera help central/ product suggestions and feedback
08-25-2018, 09:54 AM   #10
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Maybe camera AF point in few mm away?

Last edited by pxt; 08-25-2018 at 10:00 AM.
08-25-2018, 10:15 AM   #11
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QuoteOriginally posted by niels hansen Quote
Even if I use single point AF on my KP, if I shoot in about 45 degrees angle the results are dissapointing, there is as keos tells a tendency of unpredictebel front focusing. With manual focus there is no problem when you dont let you disturb of the visual and acoustic focusing aid but trust your eyes
I agree. We must keep in mind that even the best automation is not 100% reliable. This is a good illustration of how the Pentax quick-shift MF touchup system can be so beneficial.
08-26-2018, 01:16 AM   #12
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pxt
very informative video
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