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06-16-2013, 02:41 AM   #1
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Focusing troubles...

I don't know what it is...but for some reason I have been having a lot of trouble lately getting subjects in SHARP focus. I definitely don't trust my own eyes, so I really do rely on auto-focus, or at least the focus indicator according to what the camera says. For awhile now, I've been trying to leave my camera on center-point focus....then I will focus my subject in the center of the frame, then I usually move the frame off to one side (rule of thirds, you know), then take the shot. I guess I'm really just not sure of what types of situations require center-point, select-point, or auto focus. Is it better to use one over the other in certain situations? Last night I took a shot of my wife beside a friend...I'm pretty sure they were almost on the same plane (and distance away from the lens)...so I focused on only one of their faces, then moved the frame to where I wanted it, and took the shot. One of their faces is more in focus than the other. How does this happen? Granted, the photo was being taken in somewhat low-light, ISO was at 3200, and my shutter speed was 1/40. I suppose camera shake could have affected this, but still...one face is definitely sharper than the other. They were both holding still.

Any thoughts / advice are greatly appreciated.

Thanks

06-16-2013, 03:10 AM   #2
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Re-framing the subject does not guarantee it staying in focus as you are effectively moving the focus point in an arc vs. the subject plane.

Try moving parallel to the plane of focus so the focus distance remains the same.

That said, this could prove impossible/impractical, so ideally frame/compose your subject as you want it, then use select-point to focus on it.

Also, there could be an issue with narrow dof and/or lens distortion, as I presume from the circumstances you describe the lens would have been wide-open with minimum dof and possible at it's worst performing point?

Last edited by JohnX; 06-16-2013 at 04:30 AM.
06-16-2013, 07:10 AM   #3
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QuoteOriginally posted by JohnX Quote

Also, there could be an issue with narrow dof and/or lens distortion, as I presume from the circumstances you describe the lens would have been wide-open with minimum dof and possible at it's worst performing point?
For the photo in question, my lens was not wide open (F5.0), but almost I guess. And as far as movement, I generally do try move parallel to the focus plane (at least I think I do).

Thanks for the suggestions.
06-17-2013, 08:55 AM   #4
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The recompose technique works better at longer distances. As John says, it's an arc. At 1000 meters, the section of the arc used to recompose is close enough to a flat plane so it doesn't matter. At one meter, you can't pretend they are equal any more. Two other benefits of long distance: you can move your whole body in an approximate arc, like you would when taking a handheld panorama. And the distance included in your depth of field is greater.

In your example, if you rule out subject motion, then the faces must not have been within the focus plane and covered by the depth of field. But taking a better shot may have been tough with the settings you had. Stopping down more to increase depth of field probably required increasing ISO; you couldn't go much slower on shutter speed.

The main advantage to using a single AF point only is predictability. Auto focus is not always smart enough to know where your subject is, so when the camera can choose any point it wants, it will be wrong sometimes. If you like the rule of thirds, you could try using one of the points that is not in the center. I generally think the center AF point has the best performance but that doesn't apply to all cameras.

06-17-2013, 09:54 AM   #5
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QuoteOriginally posted by slr_neophyte Quote
I don't know what it is...but for some reason I have been having a lot of trouble lately getting subjects in SHARP focus. I definitely don't trust my own eyes, so I really do rely on auto-focus, or at least the focus indicator according to what the camera says. For awhile now, I've been trying to leave my camera on center-point focus....then I will focus my subject in the center of the frame, then I usually move the frame off to one side (rule of thirds, you know), then take the shot. I guess I'm really just not sure of what types of situations require center-point, select-point, or auto focus. Is it better to use one over the other in certain situations? Last night I took a shot of my wife beside a friend...I'm pretty sure they were almost on the same plane (and distance away from the lens)...so I focused on only one of their faces, then moved the frame to where I wanted it, and took the shot. One of their faces is more in focus than the other. How does this happen? Granted, the photo was being taken in somewhat low-light, ISO was at 3200, and my shutter speed was 1/40. I suppose camera shake could have affected this, but still...one face is definitely sharper than the other. They were both holding still.

Any thoughts / advice are greatly appreciated.

Thanks
That is the situation (to have Rule of Thirds) I would use select point.
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