Bokeh Calculator — Background Blur Disc Size

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.

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).

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.

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).