Portrait DOF: Choosing the Right Aperture

Opening up to f/1.4 does not automatically make a better portrait, and stopping down to f/8 does not ruin one. Choosing an aperture for a portrait is a matter of matching the depth of field to what you need sharp, which depends mostly on how many faces there are and how far apart they sit front to back.

How deep the zone really is

Aperture does two things in a portrait: it sets how much of the subject is sharp and how far the subject separates from the background. Start with the first. Here is the zone in focus for an 85 mm lens on full frame at two common working distances:

At 2 m the figures are smaller: 4.3 cm at f/1.4 and 5.5 cm at f/1.8. A face turned a little away from the camera has its far eye perhaps 4 or 5 cm behind the near one, so at f/1.4 from 2 to 2.5 m, with focus on the near eye, the far eye is on the edge of soft. That is the look, and also the risk.

f/1.2 and f/1.4

The maximum separation and the maximum risk. It works for a single face parallel to the camera, tight head shots, and low light where the aperture is a necessity. Focus on the near eye and avoid recomposing. If you focus and then swing the camera 15 degrees to recompose, the focus plane moves by about 8 cm at 2.5 m, more than the whole zone at f/1.4. At these apertures, move the focus point onto the eye with the joystick or touchscreen instead.

f/1.8 to f/2.8

f/1.8 is where many portrait photographers spend most of their time. At 2 to 2.5 m the zone is 5 to 9 cm, enough for both eyes on a face turned slightly toward the camera, and the background still separates well. Recent 85 mm f/1.8 primes from Sony and Nikon, and Canon's RF 85 mm f/2, are sharp at these apertures. f/2.8 gives more margin, around 9 to 13 cm at 2 to 2.5 m, which suits a subject who moves a little, a 70-200 f/2.8, or a couple standing close together.

Couples

The question is the depth between the two faces, plus a margin. Standing side by side and square to the camera, there may be only a few centimetres between them, and f/2.8 is fine. If one is half a step behind, the difference can be 20 to 40 cm, and you need f/4 to f/5.6, preferably from further back, since the zone at 3.5 m is twice that at 2.5 m.

Groups

For three or four people in a line parallel to the camera, f/2.8 to f/4 can work. Once they stand in a V or a staggered row, the faces can be 40 to 60 cm apart front to back, and f/4 to f/5.6 is a safer starting point for a tight group. Wider groups behave differently. They make you stand further back, and at a fixed framing the zone in focus grows with the square of the width of the group, so twenty people across 6 m have several metres of depth at f/4 to f/5.6, and f/11 or f/16 is rarely needed. Background blur falls away as you stop down, so do not go smaller than you need to. The group photo guide has the numbers.

Focal length and distance

If you keep the framing the same, the zone in focus is about the same whatever the focal length. At f/1.8, a 50 mm lens from 1.5 m, an 85 mm from 2.5 m and a 135 mm from 4 m all give roughly 9 cm. What changes is the background: the longer lens blurs it more, and makes it look larger, as the bokeh guide and the aperture versus distance post explain. So for a blurrier background with a face that is still sharp, use a longer lens from further back rather than just opening the aperture.

Focusing

Below f/4, focus on the near eye. Viewers look at eyes first, and a sharp near eye with a slightly soft far eye reads correctly, whereas the opposite looks like a miss. Eye detection autofocus on recent Sony, Canon, Nikon and Fujifilm cameras usually picks the nearer eye, and at f/1.4 to f/1.8 it is the most reliable way to land focus on the right plane. It often works in continuous mode as well as single-shot, so try both on a still subject.

Common mistakes

The big one is shooting groups at f/1.8. The back of the viewfinder looks lovely, and then one face is soft on delivery. Another is using f/8 for a single person in front of a close, busy background, where nothing separates the subject. And wide-open shooting in bright light: by the sunny 16 rule, ISO 100 at f/1.4 needs about 1/13000 s, beyond the 1/8000 s limit of many mechanical shutters, so you need a neutral density filter (three stops brings it to about 1/1600 s), a higher shutter speed from an electronic shutter if your camera has one, or a narrower aperture. The sunny 16 post shows the arithmetic.

By portrait type

These are starting points. Check the zone for your own lens, distance and sensor with the calculator before a shoot. And f/1.4 portraits are not better portraits, only different ones.

Related reading: Background Blur (Bokeh): How to Maximize It, Mirrorless vs DSLR: Does It Affect DOF?, Aperture vs Distance: What Affects DOF More?.