An f-stop (or f-number) is the ratio of a lens's focal length to the diameter of its aperture opening. It controls how much light enters the camera. A lower f-stop number (like f/1.4) means a wider aperture, allowing more light in and creating a shallower depth of field. A higher f-stop (like f/16) means a narrower aperture, less light, and a deeper depth of field. The f-stop scale is logarithmic: each full stop (f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16 → f/22) halves the amount of light reaching the sensor.

Aperture directly controls depth of field (DOF). A wide aperture (small f-number, e.g., f/1.4) produces a shallow depth of field — only a thin slice of the scene appears sharp, which is ideal for portraits and isolating subjects. A narrow aperture (large f-number, e.g., f/16) produces a deep depth of field — more of the scene from foreground to background appears sharp, which is ideal for landscapes. The relationship is: larger aperture = shallower DOF; smaller aperture = deeper DOF. DOF is also affected by focal length, focus distance, and sensor size.

The difference between f/2.8 and f/5.6 is exactly 2 full stops. This means f/2.8 lets in 4× more light than f/5.6 (each stop doubles or halves the light). In practice: if your correct exposure at f/5.6 requires a shutter speed of 1/60s, switching to f/2.8 would require a shutter speed of 1/250s (or lowering ISO by 2 stops) to maintain the same exposure. The wider f/2.8 aperture will also produce a noticeably shallower depth of field.

Hyperfocal distance is the closest focus distance at which everything from half that distance to infinity appears acceptably sharp. It's calculated using the formula: H = f² / (N × c), where f is focal length (mm), N is the f-stop number, and c is the circle of confusion (mm, based on sensor size). When you focus at the hyperfocal distance, your depth of field extends from H/2 to infinity, maximizing the sharp area in your image — a technique widely used in landscape photography.

F-stop numbers are based on powers of √2 (approximately 1.414). Each full stop multiplies the f-number by √2, which halves the aperture area and therefore halves the light. The sequence goes: f/1.0 → f/1.4 (×√2) → f/2.0 (×√2) → f/2.8 (×√2) → f/4.0 → f/5.6 → f/8 → f/11 → f/16 → f/22. This geometric progression ensures each full stop represents a doubling or halving of light. The "f/" notation indicates it's a ratio: aperture diameter = focal length ÷ f-number.

For landscape photography, f/8 to f/11 is typically considered the "sweet spot." These apertures provide a deep depth of field to keep both foreground and background sharp, while minimizing diffraction (which softens images at very small apertures like f/22). Many landscape photographers also use the hyperfocal distance technique: set the aperture to f/8 or f/11, calculate the hyperfocal distance using a calculator like this one, and focus at that distance to maximize sharpness from near to far.

Sensor size affects depth of field through the circle of confusion (CoC). Larger sensors use a larger CoC (e.g., Full Frame: 0.029mm), resulting in shallower depth of field at the same aperture and equivalent focal length. Smaller sensors (e.g., M4/3: 0.015mm) produce deeper depth of field. This is why smartphones with tiny sensors have nearly everything in focus, while full-frame cameras can achieve beautifully blurred backgrounds. Use the sensor selector in our DOF calculator to see how different formats affect your results.

Diffraction is an optical effect that causes light waves to bend when passing through a small aperture, reducing image sharpness. It becomes noticeable at very small apertures (typically f/16 and smaller on full-frame, f/11 on APS-C). While stopping down increases depth of field, diffraction eventually counteracts the sharpness benefit. The "diffraction-limited aperture" varies by sensor: ~f/11-f/16 for full frame, ~f/8-f/11 for APS-C. This is why most lenses are sharpest around f/5.6-f/8 — balancing depth of field and diffraction.