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I prefer to have a very narrow depth of field. Whenever I am focusing for portraits of too people standing near to each other, the focus-point-selected person is sharp, while the person standing next to him is not sharp.

What should I do?

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    Please provide a sample photo, but I strongly suspect the answer is "stop down". – Philip Kendall Mar 13 '15 at 19:20
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You have quite a few options here, but in practice I've found one that works best for me.

  1. Rearrange your subjects so that their eyes are at the same distance from your lens
  2. Use a narrower aperture for greater depth of field
  3. Choose a focus distance between the two subjects rather than a focus point on one specifically
  4. Use a wider focal length(at the same distance)

The first option is what I would recommend. If you only have two subjects it is quite possible to arrange them so that they have their eyes at the same distance from the camera and still are in a pose that is desirable. It does depend on your posing, focal length, and aperture of course as well. But you should be able to shoot at quite wide apertures i.e. f/2.0 and still get enough depth of field with a standard focal length portrait lens to get adequate depth of field in the eyes and face of pair of subjects. Now if you want to shoot a group of people in the 3-4 subject range, you will have to step into the narrower apertures(f/4-5.6) to really get acceptable depth of field and focus across all of their faces.

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  • Don't you mean use a narrower aperture for greater depth of field? – Flying Trashcan Mar 13 '15 at 21:34
  • Consider rephrasing "focus between subjects", because on the image, between two subjects there is usually background and focusing on that will throw both subjects out of focus. I know that's not what you meant, but I have the feeling it is easy to misunderstand it that way. Maybe "choose a focusing distance between both subjects". Or maybe it's just me interpreting too much into the phrase =) – null Mar 13 '15 at 22:10
  • @dpollitt Great answer byt note that the distances from the subjects eyes and the lens are irrelevant. It's whether they are in the same plane perpendicular to the lens axis or not. Can you perhaps clarify on that? – Hugo Mar 14 '15 at 8:26
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You should use a different aperture.

If you are using a lens that has an aperture with a low number, say 1.4 or 1.8 for example, you should change that to a higher number, say 4.0 or 5.6

The higher the aperture number is, the more closed is the aperture. This reduces the amount of light and increases the are that is within acceptable sharpness.

I know it can be tempting to use a lens wide open, for some reasons

  • it is usually considered more beautiful to have a narrow depth of field, because the often distracting background is reduced to a blur
  • opening the aperture allows more light to reach the sensor, which helps getting a proper exposure (short shutter speed) in dark situations (e.g. at night)
  • one doesn't spend a fortune on a 1.2 lens, to use it at 4.0, right?

But if the situation asks for a higher aperture, don't be afraid to stop down your lens. If the shutter speed is becoming to long in that process, use a flash (this can improve the portrait a lot) or use a higher ISO value.

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Instead or in addition to increasing the F-number, you can also consider increasing the distance to the persons or if you use a zoom lens to zoom out. The DoF when focussing at an object a distance d away is given by

DoF = d^2/H

where H is the hyperfocal distance, assuming that d is much smaller than the hyperfocal distance H (typically H will be of the order of many dozens of meters or even a few hundred meters, so this condition is met when taking pictures of people nearby). So, if you take your pictures from 4 meters away instead of 2 meters away, then that alone increases the dof by a factor of 4.

The hyperfocal distance H is given by the formula:

H = f^2/(F r)

where f is the focal length, F the F-number and r the circle of conusion (which i.m.o. should be taken equal to the pixel size to get an objective measure here).

You can then see that decreasing the focal length has the same effect as moving away, the combination of the distance and the focal length in the formula for the DoF makes the DoF only depend on the F-number and the fraction of the field of view occupied by the object. So, if you move away and then zoom in to make the object as large as it was originally, you leave the DoF invariant.

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    Even when shooting larger groups, I'm not sure you want to go hyperfocal. This can cause the subjects to get lost in an almost equally sharp background. You really only want to increase your DoF just enough to include both the nearest and most distant individuals at roughly the same amount of blur (compared to the point of true focus). – Michael C Mar 14 '15 at 2:42
  • Right, but then the hyperfocal distance only enters in the equation DoF = d^2/H to calculate the desired DoF here. – Count Iblis Mar 14 '15 at 3:26
  • That's an awful lot of work when you could just plug in your focal length, desired DoF, and desired circle of confusion into a DoF calculator... – Michael C Mar 15 '15 at 3:10

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