Which f-stop is best for a portrait?

f/1.8 or f/2.8. Both keep a whole face sharp. f/1.8 turns the room into tone, and f/2.8 leaves enough of it to say where the person is.

Why does a small aperture need more light?

The hole is smaller, so less light reaches the sensor. Each full stop halves it, and f/16 passes about a thousandth of what f/1.4 passes.

What is the difference between an f-stop and a T-stop?

An f-stop is a calculation from the size of the hole. A T-stop is measured, so it accounts for the light a lens loses inside itself. Cinema lenses are marked in T-stops, and the two numbers are close.

Do AI models respond to f-stops?

Yes, because the numbers appear in photo captions everywhere. Back the number with a line about the background, since a stop on its own is weaker wording than a description of the depth.

Aperture and f-stops

The f-stops, and what each one does to the background.

The f-stop is the size of the hole the light comes through. It settles two things at once: how much of the scene is sharp, and how much light reaches the sensor. A low number opens the hole wide, so one thin plane stays sharp and the rest dissolves. A high number closes it, so near and far hold together and the set needs more light. Name the number and a model renders the depth that belongs with it.

Wide open

f/1.4 to f/2.8. The hole is large, the light is cheap, and one thin plane is sharp. This end of the scale is Shallow Focus with a number on it.

f/1.4

The widest common stop. One thin plane is sharp and the rest dissolves.

f/1.8

Nearly as soft a background as f/1.4, and far easier to hold focus at.

f/2.8

Separates the subject and still lets the background read as a place.

The middle

f/4 and f/8. The band of sharpness reaches past the subject, most lenses are at their best, and the frame still has a foreground and a background rather than one flat plane.

f/4

Holds a group sharp and keeps the middle distance readable.

f/8

The sharpest stop on most lenses, with the whole scene legible.

Stopped down

f/11 and f/16. Near and far hold together, the set pays for it in light, and diffraction starts to take the fine detail back. This end of the scale is how Deep Focus is made.

f/11

Near and far both sharp. This is the stop deep focus is shot at.

f/16

Maximum depth, and the whole frame goes a little soft with it.

Run one of them

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Other techniques

Print stylesMaking a poster rather than a shot? The same reference, for ink on paper.

A low number opens the lens and dissolves the background. A high number closes it and brings the background back into focus. The number is the fastest way to ask for depth.