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One of the most common beginner questions is whether aperture and f-stop are different things. They are not. Aperture is the physical opening inside the lens. The f-stop is that opening expressed as a ratio of the lens focal length to the opening's diameter. That ratio is why the numbers feel backwards.
Why a smaller number means more light
Because the f-number is a ratio with the opening on the bottom, a bigger opening produces a smaller number. So f/1.8 is a wide opening that lets in a lot of light, and f/16 is a narrow opening that lets in much less. It feels upside down until you remember it is a fraction.
Depth of field: the blurry background
Aperture also controls depth of field, which is how much of the scene is in sharp focus front to back. A wide aperture (f/1.8) gives a shallow depth of field: your subject is sharp and the background melts away. A narrow aperture (f/11) keeps more of the scene sharp, which is what you want for a landscape or a real estate interior.
Fast versus slow lenses
A "fast" lens is one with a wide maximum aperture like f/1.4 or f/2.8. It can shoot in less light and blur backgrounds more. A "slow" lens tops out around f/4 or f/5.6. Fast lenses cost more and weigh more, which is part of the what lens should you buy decision.
The f-number scale
The standard stops are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. Each step halves the light of the one before it. That matters because it connects directly to the exposure triangle: close down one stop of aperture and you can open up one stop of shutter speed to match.
What aperture should I use for portraits?
A wide aperture like f/1.8 to f/2.8 blurs the background and makes your subject stand out. Just watch focus: at f/1.8 the sharp zone is thin, so focus on the eyes.
What aperture is sharpest?
Most lenses are sharpest a couple of stops down from wide open, often around f/5.6 to f/8. Very narrow apertures like f/16 lose a little sharpness to diffraction.
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Read guideResearched, not personally tested: picks come from specs, verified-owner reviews, and expert sources, scored into the Aperture Score. As an Amazon Associate I earn from qualifying purchases. We may earn a commission from links here, at no extra cost to you. How we research →