Subject Isolation Calculator — Background Separation

Strong isolation requires shallow DOF (wide aperture, longer focal length, close distance) AND sufficient subject-to-background distance. A 85mm f/1.4 with 5m between subject and background gives excellent isolation. The background should be far enough to fall well outside the DOF zone.

The Isolation Index is an indicative in-house guide, not a physical unit. It is derived directly from the background blur-disc diameter (the size each out-of-focus background point grows to on the sensor), scaled by 200 and capped at 100. The physically meaningful number is the Background Blur Disc itself: a larger disc means stronger background separation. As a rough guide, a blur disc above about 0.25 mm gives clearly visible separation and above 0.40 mm melts the background away.

Subject-to-background distance often has more practical impact than aperture. An f/4 lens with 10m between subject and background can isolate better than f/1.4 with 1m separation. However, wider apertures always help — the ideal is both wide aperture and good separation.

Yes, in two ways. Longer focal lengths give shallower DOF at the same aperture and framing. They also compress perspective, making backgrounds appear larger but more blurred. For portrait isolation, 85-135mm on full frame is the classic choice.