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I have tried taking a few portraits in my living room, using studio flash. I'm still new to it, so my lighting setups may not be great.

I have found sometimes that I can shoot 3 or 4 shots in a row - essentially identical lighting, pose, camera settings. Sometimes one shot can be superbly sharp while the others seem to have focus problems.

This led me to learn more about how aperture affects depth of field, and in turn how distance from subject plays a part also.

I found an online calculator that will tell you the depth of field given an aperture, distance to subject and camera model.

I have just bought Nikon's 80-200mm 2.8 specifically to work on portraits. I searched Flickr for examples and found this one (not my photo).

The exif info tells me it's a D300 at f/2.8, 80mm and it has approximate distance to focal point = 2.51 (I presume it is metres).

When I enter this info into the focal length calculator it tells me that the depth of field for acceptable level of sharpness is 11 cm.

When I measure my own head, it's approx 22cm front to back. However, in the example photo almost all of the model is in focus. I see some slight blurring at the back of the head but even the fingers seem sharp and these are in front of the face.

How did this photographer manage to get almost all of the model sharp and in focus?

Does it appear sharper because the image has been downsized?

Has sharpening been applied in post-processing?

Am I totally missing something with the maths, or with the photo-taking process? I am almost certain that if I attempt a shot at f/2.8, 80mm at 2.5m from the subject I will have focus problems. (Testing this is my next step of course!)

Thanks in advance for any help, suggestions or insights you can provide.

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    I haven't had a chance to look at your example photograph, but if you want your subject all sharp and are shooting in the studio with strobes, there's no reason to shoot wide open. – mattdm Oct 28 '14 at 1:01
  • Good point. But I'm still curious as to why the maths suggests that not all of the model should be in focus, and yet all of the model appears to be in focus except the very back of his head. – youcantryreachingme Oct 28 '14 at 1:16
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There are a few things going on here:

  1. The subject's hand may be considerably closer to the photographer, but not so much to the camera's sensor plane. The image was shot from just above the subject's eye level (looks like about mid-forehead to me), so the camera was tilted down.
  2. You can only see the front half of what you're looking at, assuming it's a 3-dimensional shape with just about as much in front of as behind the widest point. But like the old Hollywood wild west town, it only has to look good from the front.
  3. Depth of field calculations assume a given image size and distance, and while this photograph may look tack-sharp front to back when displayed on screen at this resolution, that tells you very little about what an 8x10 or 8x12 inch print will look like when seen up close and personal. (That's the print size that most DoF calculators are based on; they are overkill for web resolution and not nearly strict enough for larger gallery prints.)

As mattdm pointed out in his comment, there is no good reason (usually) to shoot wide-open in studio in any case. You may want to restrict depth of field for effect, but f/2.8 at 80 or 100mm is pretty meh for that at typical portrait distances; you've really got to go wider than f/2 to make the effect look like it was deliberate (and if you're using strobes, you may not be able to get down to the power you need without an ND filter, depending on the brand and model). Ordinarily, you would be shooting at something more like f/5.6 or f/8 to coax the maximum amount of goodness out of your lens (and to make up for any slight movement of the subject or the camera between focus and shooting — a couple of mm is all it takes to go from tack sharp to slightly mushy when you're shooting at very wide apertures, even when it's not bad enough to say you missed focus).

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    This response is really helpful for me because it looks at a broad range of possibilities for me to consider in turn as I do my next shoot; also because it's very specific about which settings will have which effect. Thanks. Am still definitely open to others' thoughts on this too. – youcantryreachingme Oct 28 '14 at 2:35

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