Aperture and depth of field
The aperture controls light and how much of the scene is sharp at the same time. It is the control that most changes how a photograph feels.
The aperture is a ring of overlapping blades inside the lens that opens and closes to change the size of the hole light passes through. It does two things at once, which is what makes it the most interesting of the three controls.
The numbering
Apertures are written as f numbers — f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — and the sequence looks arbitrary until you know where it comes from.
The number is a ratio: the lens’s focal length divided by the width of the opening. Because it is a division, a smaller number means a wider opening.
The steps go up by a factor of √2, which halves the area of the hole each time and so halves the light. Each step is one stop.
The second job
A wider opening also narrows the band of distance that comes out sharp — the depth of field.
Wide open, a portrait has sharp eyes and a background dissolved into bokeh. Stopped down to f/16, a landscape is sharp from the rocks at your feet to the hills behind.
Three things make depth of field shallower:
- A wider aperture.
- A longer focal length.
- Standing closer to the subject.
They compound. A long lens, wide open, close up, gives a sliver of sharpness a few centimetres deep.
It is not symmetrical
Roughly a third of the sharp zone falls in front of the focus point and two thirds behind it.
The practical consequence: for a group of people at slightly different distances, focus on someone in the front row rather than the middle. More of the depth will land where you need it.
Diffraction, and why f/22 is not the answer
Stopping down further and further does not keep improving sharpness. Past roughly f/11 on most lenses, light bending around the edges of the small opening starts to soften the whole image.
The sharpest aperture on a typical lens is somewhere around f/5.6 to f/8. Use f/16 or f/22 when you genuinely need the depth, not as a default.
Try it
Separate the subject
Focus is on the second marker. Open the lens far enough that the furthest marker dissolves into a soft wash, so the eye goes straight to the subject — and keep the exposure right.
Loads an interactive 3D camera simulator.
Key takeaways
- A smaller f-number means a wider opening and more light.
- Wider apertures give shallower depth of field; narrower ones give more.
- Focal length and subject distance change depth of field as much as aperture does.
- Very narrow apertures soften the image through diffraction.
Common questions
Why is a smaller f-number a bigger opening?
Because the f-number is a division: focal length divided by the width of the opening. A 50mm lens at f/2 has a 25mm opening; at f/8 it has a 6mm one. The number is the divisor, so bigger means smaller.
Is a blurrier background always better?
No. It isolates a subject, which suits portraits, and hides context, which does not suit a photograph about a place. A blurred background is a choice, not an improvement.
Next steps
The exposure triangle
Three controls decide how bright a photograph is, and each one changes something else as well. That trade-off is the whole of exposure.
Focus, and what it is for
Autofocus is very good at focusing accurately on the wrong thing. Deciding what should be sharp is the part it cannot do for you.
ISO and noise
ISO buys brightness when there is not enough light, and pays for it with noise. Knowing when that trade is worth it is the skill.