Depth of Field and Circle of Confusion

Depth of field and circle of confusion

The circle of confusion (CoC) is the biggest blur spot your eye still reads as sharp. A common way to pin it down is c = d / 1500, with d being the sensor diagonal. That puts full-frame ≈ 0.030 mm, APS-C ≈ 0.020 mm and Micro Four Thirds ≈ 0.015 mm. Once your subject sits well short of the hyperfocal, depth of field is close to DOF ≈ 2·N·c·(s/f)², where N is the aperture, s the subject distance and f the focal length. Run a 50 mm at f/5.6, 3 m, CoC 0.03 mm through that and you land around 1.2 m of DOF.

Applications

Photographers lean on depth-of-field calculations for portrait, landscape and product shoots, and to match gear to whatever sensor format they want to work with. In macro work it underpins focus stacking, where you take overlapping slices of shallow DOF and merge them into one image that is sharp front to back.

FAQ

Why does a smaller sensor seem to have more DOF? A smaller CoC does ask for a tighter focus tolerance, which would pull the other way. But to keep the same field of view you reach for a shorter focal length, and that effect wins out. The (s/f)² term grows and DOF expands with it.

What CoC should I pick? For prints, the manufacturer default for your sensor (d/1500) does the job. If you are pixel-peeping on a 4K monitor, drop to something stricter like d/3000, which matches what you will actually see.

What is hyperfocal distance? It is the nearest point you can focus on while still keeping the far edge of DOF out at infinity. The formula is H = f² / (N·c) + f, and focusing there keeps everything from H/2 to infinity acceptably sharp.