f/1.8

f/1.8 buys the shallow look without the cost of holding it. The background is almost as soft as at f/1.4, and the sharp band is deep enough to carry a whole face.

That margin is why so much portrait work sits here. The subject can breathe, lean and turn, and the shot stays sharp.

The sharp band on a head and shoulders crop runs several centimetres, so both eyes and the tip of the nose usually hold together.

Lens character improves by a third of a stop. Contrast lifts, the corners tidy, and highlights stop blooming as much as they do wide open.

Light stays plentiful. The stop passes about twice what f/2.8 does, which is enough for a room lit by windows or by a couple of lamps.

Use it for interviews, talking heads and portraits where the face has to be whole and the location does not matter.

Move off it when the room is part of the story. At f/1.8 a background reads as tone rather than as a place.

f/2.8 keeps the same subject sharp and lets the background come back as recognisable shapes. f/1.8 dissolves them.

Look at the wall behind the subject. If you can name what is hanging on it you are at f/2.8 or smaller.

A fast normal lens at this stop is the cheapest route to a portrait look, which is why the 50mm f/1.8 has been the first lens most photographers buy for forty years.

The model treats it as f/1.4 and melts half the face. Say both eyes stay sharp and only the background falls away.

The other failure is a flat, sharp frame. Name the background falling soft behind the subject, because the number alone is weak wording.