What determines bokeh strength?

Three main factors: aperture (wider = more blur), focal length (longer = more blur at same framing), and subject-to-background distance (further apart = more blur). An 85mm f/1.4 with 5m between subject and background produces very strong bokeh.

What is the blur disc?

A point light source in the background (like a streetlight) appears as a disc rather than a point when out of focus. The diameter of this disc is the blur disc size. Larger discs mean more background blur. The shape of the disc depends on the aperture blade design (circular = smoother bokeh).

Does sensor size affect bokeh?

Yes. Larger sensors require longer focal lengths for the same framing, which increases blur. A full-frame camera at 85mm f/1.8 produces roughly the same blur as APS-C at 56mm f/1.2 (when framing the same). The entrance pupil diameter (f/N) is the key factor for a given framing.

What makes good vs bad bokeh?

Good bokeh has smooth, even blur discs with soft edges. Bad bokeh has hard edges, onion-ring patterns, or busy nervous backgrounds. Lens design (special elements, rounded aperture blades) affects bokeh quality. More aperture blades (9+) create rounder, smoother bokeh than fewer blades (5-7).

PhotoCalcs

Bokeh Calculator — Background Blur Disc Size

Estimate the background blur disc size for a given lens, aperture and subject-to-background distance.

Focal Length

Focal length of the lens in millimetres

Aperture

Lens aperture (f-stop number)

Subject Distance

Distance from camera to subject

Background Distance

Distance from camera to the background

How We Calculate This

Blur Disc Diameter

d = (f × D × |b − s|) ÷ (b × (s − f))

Where f = focal length, D = entrance pupil (f/N), s = subject distance, b = background distance (all distances in the same units). The result is the diameter of the out-of-focus blur disc as projected on the sensor.

Entrance Pupil

D = focal length ÷ f-number

The entrance pupil diameter sets the maximum bokeh-ball size: a wider physical aperture (larger D) means bigger, smoother blur discs.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.