Bokeh, aperture & focal length — explained with diagrams

What is bokeh?

Bokeh (from the Japanese boke, meaning blur or haze) refers to the aesthetic quality of the out-of-focus areas in a photograph. A lens with "good bokeh" renders background lights as smooth, round circles. A lens with "harsh bokeh" renders them as sharp-edged, distracting shapes.

The amount of background blur — the separation between subject and background — is controlled by three things: aperture (f-number), focal length, and subject distance. Understanding all three is what separates photographers who get the shot from those who wonder why their portrait doesn't look like the reference photo.

At f/1.8, the depth of field is shallow — only the subject is sharp. At f/8, most of the scene is in focus.

Aperture: the f-number explained

The f-number (f/1.8, f/2.8, f/8 etc.) describes the size of the aperture opening in the lens. Counterintuitively, a smaller f-number means a larger opening — more light, more background blur. A larger f-number means a smaller opening — less light, sharper background.

When a lens is described as "fast" (f/1.4, f/1.8, f/2.8), it means it lets in a lot of light — useful in low-light conditions and for creating background blur. "Slow" lenses (f/4, f/5.6) are cheaper and smaller but less capable in the dark.

Each full stop doubles or halves the light. The blur drops off rapidly past f/5.6.

Focal length: what the numbers mean

Focal length (measured in mm) controls how much of the scene you capture. A 24mm lens is wide — it sees a lot of the scene. A 200mm lens is telephoto — it sees a narrow slice of the scene, magnified.

Note: on an APS-C camera (the sensor in most entry and mid-range cameras), multiply the focal length by 1.5× to get the "equivalent" view compared to full-frame. A 50mm lens on APS-C behaves like 75mm on full-frame.

All four shots taken from the same position. Longer focal lengths magnify and compress the scene.

The portrait sweet spot

85mm at f/1.8 is the classic portrait focal length for a reason. At this distance, the face fills the frame without the distortion a wide lens introduces (wide-angle lenses exaggerate the nose and push the ears back — unflattering in portraits). The background compresses and blurs smoothly.

On an APS-C camera, a 56mm f/1.4 (equivalent to 85mm full-frame) gives the same effect. Sigma and Fujifilm both make excellent 56mm f/1.4 lenses for APS-C mounts at around €350–380.

Distance: the overlooked variable

The closer you are to your subject (and the further your subject is from the background), the more background blur you'll get — regardless of aperture or focal length.

A practical tip: if you want more bokeh, don't just open the aperture wider — also move your subject further from the background. A subject standing 1 metre in front of a plain wall will always have a sharper background than a subject standing 10 metres in front of the same wall.