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Aperture (F-Number) Basics: How the Lens Opening Shapes Your Photo

Tap through each step to build a working understanding of aperture, from what the f-number means to choosing it for the shot you want.

  1. What aperture actually is

    Aperture is the adjustable opening inside a lens that controls how much light passes through to the sensor; the number used to describe its size is called the f-number, or f-stop.

  2. Why a smaller f-number means a bigger opening

    F-numbers work backwards from what most people expect: f/1.8 is a wide opening that lets in a lot of light, while f/16 is a narrow opening that lets in very little. The number is really a ratio of focal length to opening diameter, which is why it shrinks as the opening grows.

  3. How aperture controls depth of field

    Shooting wide open, at a low f-number, keeps only a thin slice of the scene in sharp focus while everything in front of and behind it blurs β€” the effect known as a shallow depth of field, or "bokeh."

  4. Why portraits often use a low f-number

    A wide aperture blurs the background, separating the subject from clutter behind them and pulling the viewer's eye straight to the face, which is why a portrait lens is often used near f/1.8 to f/2.8.

  5. Why landscapes often use a higher f-number

    A landscape usually needs everything from the foreground rocks to the distant mountains in focus, so photographers narrow the aperture, commonly to around f/8 to f/11 for the sharpest overall result β€” going much further than that can actually soften the image through a phenomenon called diffraction.

  6. Try aperture priority mode to practice

    Most cameras have an "A" or "Av" mode that lets you set the aperture yourself while the camera automatically adjusts shutter speed to match, which is a good way to build an intuition for the effect without juggling every setting at once.

Aperture is only one leg of the exposure triangle

Aperture works alongside shutter speed and ISO to determine how bright or dark a photo turns out, so opening the aperture wider for a blurrier background also brightens the shot, often requiring a faster shutter speed or lower ISO to compensate and keep things balanced.

Why very small apertures can backfire

It is tempting to assume the highest possible f-number always gives the sharpest photo since more of the scene is in focus, but past a certain point, often around f/16 or smaller, a phenomenon called diffraction actually softens fine detail across the whole frame. That is why most lenses reach peak sharpness a few stops down from their widest setting rather than at their smallest.

Frequently Asked Questions

Does a lower minimum f-number always mean a better lens?

Not necessarily better overall, but it usually means more versatile in low light and more capable of a strong background-blur effect. A lens with a higher minimum f-number can still be excellent for landscapes or well-lit conditions, where those two advantages matter less.

Why do two photos at the same f-number sometimes show different amounts of background blur?

Depth of field depends on more than just the f-number β€” the focal length of the lens, the distance between the camera and the subject, and the distance between the subject and the background all affect how blurred the background appears, even when the aperture setting is identical.