What is the sharpest aperture on my lens?

Usually about two stops down from the maximum, so f/4 on an f/1.8 lens and f/5.6 on an f/2.8 lens. Cheaper zoom lenses tend to peak a little later, around f/8. The only way to know for your own lens is to shoot the same subject at several apertures on a tripod and compare the files at 100 percent.

Should I always shoot wide open for maximum blur?

No. At maximum aperture the depth of field can be so thin that the subject's own features fall out of focus, and many lenses are softer there. Stopping down one or two stops often gives a background that is still pleasingly blurred but a subject that is reliably sharp, which is usually the better photograph.

Does aperture affect autofocus accuracy?

Yes, indirectly. Cameras usually focus with the aperture wide open and then stop down to take the picture, so a very wide aperture means a very thin zone of acceptable focus, and small focusing errors become visible. This is why portraits at f/1.4 need the eye to be the focus point, and why stopping down slightly makes focusing much more forgiving.

What aperture should I use for night skies or video?

For stars, use the widest aperture your lens has, because every stop of light is a stop of ISO you do not have to pay for in noise; many photographers stop down a third of a stop from maximum to reduce coma at the corners. For video, choose the aperture for the depth of field you want and keep it constant between shots so the look does not shift.

Tutorials

EXP · Exposure

Aperture Explained: F-Stops and Depth of Field

Aperture controls both how much light enters your lens and how much of the scene stays sharp. Here is how to pick the right f-stop for any subject.

Diagrams

Aperture Explained: F-Stops and Depth of Field — Concept overview — 01Concept overview

Aperture Explained: F-Stops and Depth of Field — How it works in the field — 02How it works in the field

Aperture Explained: F-Stops and Depth of Field — Common mistake to avoid — 03Common mistake to avoid

Aperture is the adjustable opening inside your lens, written as an f-number such as f/2.8 or f/11. It works backwards from intuition: the smaller the number, the larger the opening and the more light it lets in.

That single control does two jobs at once, and this is what makes it the most useful and the most confusing setting on the camera. It decides how much light reaches the sensor, and it decides how much of the picture appears sharp. Learning to choose it deliberately is most of what separates a snapshot from a considered photograph.

The F-Number Scale and Why It Inverts

Each full stop on the standard scale — f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — halves or doubles the light. Going from f/4 to f/5.6 halves the light; going from f/8 back to f/4 quadruples it, because two stops are involved.

The numbers look inverted because an f-number is a ratio: focal length divided by the diameter of the opening. As the divisor grows, the opening shrinks. Understanding this once removes most of the confusion, because it explains why f/2 is "faster" than f/4 in both senses of the word.

Many cameras also let you work in thirds of a stop, such as f/3.5 or f/4.5. These make fine exposure adjustments easier, but the arithmetic of the full stops is worth knowing by heart.

Depth of Field: What Actually Controls It

Depth of field is the zone of the picture that appears acceptably sharp, and aperture is only one of three things that control it. The second is the distance between you and your subject: get closer and the zone gets thinner very quickly. The third is focal length: a longer lens at the same framing compresses the zone.

This is why the common belief that "wide apertures blur the background" is only part of the story. A wide aperture plus a close subject plus a long lens produces almost no depth of field at all, while the same aperture at a distance with a wide lens keeps a surprising amount sharp.

Sensor size matters too, mostly because it changes how close you stand for the same framing. A smaller sensor with the same angle of view and aperture gives more depth of field, which is why compact cameras appear to keep everything sharp.

Choosing an Aperture by Subject

Choosing a setting is a two-step decision. First ask what must be sharp: one face (f/1.8 to f/2.8), a couple (f/4), a full group or a wide landscape (f/8 to f/11). Then check your light and adjust the shutter speed or ISO around your choice.

For portraits, f/1.8 to f/2.8 isolates a subject from a busy background, but be careful with groups: at f/1.8, only the person you focused on will be sharp, and the two people beside them will not. For architecture and interiors, f/8 to f/11 keeps walls and window frames straight and crisp.

For close-up and macro work, depth of field collapses to millimetres, so most macro shooters stop down to f/11 or beyond and accept the loss of light, using a tripod and flash to compensate.

Aperture and Light: Fast Lenses and Slow Ones

In dim rooms a fast f/1.8 lens lets you keep the ISO low, which is the practical argument for buying one. The same scene at f/4 would need two stops more ISO, and those two stops are usually visible in the shadows.

In bright sun the problem reverses: you may need f/11 or a faster shutter to avoid overexposure, and if you are shooting portraits at f/1.4 in daylight you will hit the camera's maximum shutter speed. That is what neutral density filters are for.

Variable-aperture zoom lenses add a complication, because their maximum aperture changes as you zoom. A 18-55mm f/3.5-5.6 is f/3.5 at the wide end and f/5.6 at the long end, so the light drops by more than a stop as you zoom in, and your carefully chosen exposure drifts with it.

Diffraction, Sweet Spots and Testing Your Lens

Beware the extremes. Many lenses are soft wide open, which is why a portrait at f/1.2 can look dreamy rather than sharp, and most lenses lose sharpness past f/16 to diffraction, so stopping down to f/22 for a landscape gives a softer image than f/11 would have.

The usual rule is that most lenses are sharpest about two stops down from wide open, which puts a typical f/2.8 zoom at f/5.6 and an f/1.8 prime at f/4. That is a starting point for your own lens, not a law, because designs differ.

Test your own lens at f/2.8, f/5.6 and f/11 with the camera on a tripod and the same subject, and compare the results at 100 percent. Ten minutes of testing will tell you more about your equipment than any review, and it will stop you blaming the lens for a setting choice.

Key points

Small f-number means a large opening, so more light and a shallower depth of field.
Portraits: f/1.8 to f/2.8. Groups and street: f/4 to f/5.6. Landscapes: f/8 to f/11.
Each full stop doubles or halves the light: f/4 to f/5.6 halves it, f/4 to f/2 doubles it.
Most lenses are sharpest two stops from wide open and soften past f/16 from diffraction.

Frequently asked questions

What is the sharpest aperture on my lens?: Usually about two stops down from the maximum, so f/4 on an f/1.8 lens and f/5.6 on an f/2.8 lens. Cheaper zoom lenses tend to peak a little later, around f/8. The only way to know for your own lens is to shoot the same subject at several apertures on a tripod and compare the files at 100 percent.
Should I always shoot wide open for maximum blur?: No. At maximum aperture the depth of field can be so thin that the subject's own features fall out of focus, and many lenses are softer there. Stopping down one or two stops often gives a background that is still pleasingly blurred but a subject that is reliably sharp, which is usually the better photograph.
Does aperture affect autofocus accuracy?: Yes, indirectly. Cameras usually focus with the aperture wide open and then stop down to take the picture, so a very wide aperture means a very thin zone of acceptable focus, and small focusing errors become visible. This is why portraits at f/1.4 need the eye to be the focus point, and why stopping down slightly makes focusing much more forgiving.
What aperture should I use for night skies or video?: For stars, use the widest aperture your lens has, because every stop of light is a stop of ISO you do not have to pay for in noise; many photographers stop down a third of a stop from maximum to reduce coma at the corners. For video, choose the aperture for the depth of field you want and keep it constant between shots so the look does not shift.