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I recently shot some family pictures for a friend's family event (thank god I didn't get paid for this job). The location was problematic with the light - very strong late afternoon light and I don't have a good setup with flash to fill in the shadows.

My main frustration looking at my photos was the focus, which seemed very off. I have had very good luck with the sharpness using this lens, but the vast majority of the pictures this time were really off, except for one.

Because of the difficult light I shot using manual exposure (shutter and aperture) but used auto focus. because of the relatively busy background, I used f4 to get a relatively short DOF. Some of the times not all of the faces where on the same plane so some of the faces were in focus.

By far the best picture was enter image description here

WHich is what I am used to. This was an F4 1/3000 focal length of 50mm

A picture from 12 seconds earlier is: (same settings exactly): enter image description here

In both cases, I focussed on the mothers face and then repositioned.

Another example is: enter image description here Where the fence is nicely focussed, but there is no way I remotely focussed on that.

Again, just to prove that I can take good pictures using these F stops and lighting conditions: enter image description here enter image description here

In all these example low lighting isn't an issue. I was also using a very high speed, so shake shouldn't be an issue.

Any help is welcome.

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    What's the camera body/lens combination we're talking about here? Some of the Sigmas are notorious for AF issues. Also, does the body you're using allow for autofocus microadjustment/calibration? – inkista Aug 27 '14 at 22:23
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    The above could be caused by a number of things really and without more information it would be hard to provide an answer. Could you add info about whether you were using single shot or tracking autofocus and a single focal point or auto selection please? f/4 might well be too shallow, especially if people are on different focal planes and it is possible you're experiencing front or rear focus combined with shallow DoF – JamWheel Aug 27 '14 at 22:24
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    F4 1/3000? What was your ISO and why would it be so high? - Unrelated to the sharpness issue but still, that would be a seemingly unnecessarily high ISO. – dpollitt Aug 28 '14 at 1:42
  • @dpollitt Not so fast, my friend! The NR normally associated with high ISO does cause a reduction in detail. – Michael C Aug 28 '14 at 2:48
  • Thanks for the attention. I have a Pentax D10 and the Sigma is an 18-50 F2.8 EX Macro. I selected f4 in order to blur a busy background. In retrospect, the different focal planes was an issue in some of the images, but doesn't seem to be the issue in the images I posted. – Josh Golbert Aug 28 '14 at 6:21
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In the second image there's a metal-bar-looking-thingy right between the mother's and father's head that is very much in focus (at least it looks like that on the web-sized image) - so I would guess the AF focused on that bar and not the mother's face.

The area covered by the AF sensor is substantially larger than the rectangle you see in the viewfinder so it's highly possible that the AF system focused on that nice contrasty horizontal line when you aimed it at the camera-right eye of the mother.

I'm not sure why the fence in the 3rd image is so nicely focused but it's way too in-focus to be an AF error, it looks like the AF successfully focused on the fence, in an AF error usually nothing is in focus.

Also, I just want to mention that - while I don't think this is the situation in those images - the "focus & recompose" technique can cause focus problems on wide apertures because when you recompose after locking focus you are changing the distance between camera and subject, if you have shallow DOF and you focus on something at the side of the frame this change can throw the subject out of focus.

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1/3000 Such bright light is crazy fast! What is your ISO here? General rule of thumb suggests you want the slowest ISO possible for the conditions (to avoid noise). Another rule of thumb (for handheld photos), if your shutter speed is faster than your focal length you should be fine (ie., 24mm and 1/30 sec or 200mm at 1/200 should produce shard imagery). No need for 1/3000.

You may have an issue with your lens.

You may have an issue with a filter.

Your diopter may have gotten bumped and is affecting what you're seeing through the lens.

More likely though, the bright light was affecting your ability to see clearly and you just didn't focus correctly.

Also, f4 is pretty shallow for group portraits beyond 2 people. Looking at your image comparisons too, you're not getting that creamy f/4 glow to your background in the problem images so it makes me wonder about the lens again.

Run some tests.

  • I was using 200 ISO (no great reason, unfortunately). I will test my lens. Any recommendations other than taking my daughter and running through the Fstops? Also, I suspected my equipment (the refuge of all mediocre photographers, right?) but I did get that one good picture, so I am wondering if it was something I did that screwed things up? – Josh Golbert Aug 28 '14 at 6:32
  • Rather than using your daughter as a subject I would use some objects that don't move. Otherwise you may be contending with the "wiggle" factor. Bottle don't move...kids do. And remember, f4 is pretty shallow for a group. I tend to hang around f/8 or higher (depending on the size of the group). You may have just gotten lucky on that one frame and nailed your focus. Just had a thought, what kind of focus were you using? If you weren't using spot than your camera may have been confused and settled on a focal point behind your subjects. – Rob Clement Aug 28 '14 at 16:58
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Not necessarily the cause, but I have the same lens (on Nikon mount) and it has been reported to suffer from relatively bad autofocus and sometimes front-focus issues.

I saw it happen sometimes with 50 mm focal length, wide aperture (f/4 may still be considered wide in this respect) and close subjects. Your pictures are not actually close-ups, so I would be more inclined to blame the AF.

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