f/1.4

f/1.4 removes the world around the subject. The sharp band runs a few centimetres deep on a close shot, so a face arrives and the room behind it turns into colour.

It also buys light. The stop passes roughly four times as much light as f/2.8, which is what lets a night scene play on practical lamps alone.

Depth of field shrinks as the aperture opens. At f/1.4 on an 85mm lens at portrait distance the sharp band is a few centimetres, so the near eye lands and the far eye softens.

Focus becomes a job rather than a setting. A subject who leans forward by a hand width leaves the plane, which is why this stop is hard on a moving actor.

Most lenses render softer wide open. Contrast drops, the corners smear a little and bright edges bloom, and a lot of the look people call cinematic is that softness.

Use it for night interiors, for a close portrait that has to own the frame, and for any background you would rather nobody studied.

Skip it when two people at different distances have to be sharp in one shot. At f/1.4 the second face is gone.

f/1.8 is one third of a stop behind and far easier to hold. The background is almost as soft, and the focus survives a subject who moves.

The test is the far eye on a close portrait. If it is readable you are nearer f/1.8. If it has melted you are at f/1.4.

Fast glass is what put candlelight on screen. Stanley Kubrick went past this stop on Barry Lyndon (1975) with an f/0.7 Zeiss lens built for satellite work, and shot whole scenes lit by the candles on the table.

The model returns a soft background but keeps the whole face sharp, which is a portrait look rather than f/1.4. Say the sharp band is a few centimetres deep and name where it sits.

The other failure is a background that is soft and still busy. Ask for the background dissolved into tone, so shapes stop reading as objects.