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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You have quite a few options here, but in practice I've found one that works best for me.
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.
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
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.
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.