Depth of Field and Depth of Focus

Depth of field (DOF) is the object-space range around the focus plane that meets a stated sharpness criterion. Depth of focus is the image-side tolerance to moving the sensor plane along the optical axis while remaining within that criterion. DOF concerns object position; depth of focus concerns image-plane position.

An out-of-focus point forms a finite blur spot called the circle of confusion (CoC). It is considered acceptably sharp only when its diameter is below a chosen threshold c. In a paraxial thin-lens model with object distance u > f and focus distance s, hyperfocal distance is approximately H ≈ f²/(N c)+f; near limit Dn ≈ Hs/[H+(s−f)], and far limit Df ≈ Hs/[H−(s−f)] (a zero or negative denominator means the far limit is treated as infinity). DOF is Df−Dn. For close-up or high magnification, use macro equations or a measured MTF target instead.

At the same framing and CoC criterion, stopping down (increasing N) usually increases DOF. A shorter focal length also often increases DOF, but comparisons change if camera distance is adjusted to preserve framing. Focusing farther away generally increases DOF. Sensor format alone does not determine DOF: the chosen CoC threshold, output size, and viewing conditions matter.

In the common hyperfocal approximation, the denominator for the near limit is H+(s−f), while the far-limit denominator is H−(s−f). The latter is smaller, placing the far limit farther from focus; this is the usual basis of the rule “far DOF is greater than near DOF.” Near the hyperfocal distance, the far limit tends to infinity. Macro work, high magnification, asymmetric lenses, or an MTF-defined criterion can depart from this simple rule.

Paraxial image-side depth of focus is often on the order of N·c, with the coefficient depending on the definition, one-sided versus total range, wavelength, and diffraction criterion. Small pixels, high resolution, and fast apertures typically demand tighter sensor flatness, lens tilt, and active-focus control. A useful acceptance test specifies distance, aperture, spatial frequency, and minimum MTF rather than simply requiring “enough DOF.”