Lens Diffraction Calculator

No lens focuses a point to a point. Light passing a circular aperture spreads into a diffraction pattern whose central maximum, the Airy disc, has a diameter of 2.44 times the wavelength times the f-number. For green light at 550 nm this gives about 6.7 micrometres at f/5.6, 13.4 at f/11 and 26.8 at f/22, a size that depends only on aperture and wavelength and not at all on focal length or sensor format. Whether that blur is visible depends on the sampling grid beneath it. Pixel pitch is the sensor width divided by the horizontal pixel count, roughly 6.0 micrometres for a 24 MP full-frame sensor, 4.4 for a 45 MP full-frame, 3.9 for a 24 MP APS-C and about 3.3 for a 20 MP Micro Four Thirds. Taking the Nyquist criterion of two samples per resolved feature, diffraction begins to cost resolution once the Airy disc exceeds twice the pixel pitch, which is the diffraction-limited f-number this calculator reports. On that threshold a 24 MP full-frame body is limited near f/9, a 45 MP body near f/6.5 and a high-resolution APS-C body near f/6. These are soft onsets, not cliffs: one stop past the limit typically costs on the order of 10 to 20 percent of linear resolution, two stops considerably more, and the loss is irreversible because the detail is never recorded. The model also assumes a diffraction-limited lens, whereas real lenses are aberration-limited wide open, so the practical sweet spot sits near f/5.6 to f/8 where the two effects cross. Wavelength shifts the numbers by about plus or minus 18 percent between 450 and 650 nm, and stopping down past the limit is often still correct when depth of field matters more than peak per-pixel sharpness.

The physics was worked out well before photography needed it. George Biddell Airy, then Astronomer Royal, published his analysis of the diffraction pattern formed by a telescope object glass with a circular aperture in 1835, and the bright central disc has carried his name since. Lord Rayleigh later formulated the resolution criterion that two point sources are just separable when the maximum of one falls on the first minimum of the other, giving observational astronomy its standard measure of a telescopes limiting resolution. Ernst Abbe, working with Carl Zeiss in Jena in the 1870s, derived the parallel diffraction limit for the microscope, which is why numerical aperture rather than magnification governs microscope performance. For most of the film era the limit was academic for general photography. Emulsion grain, film flatness and lens aberrations dominated the error budget, and large-format photographers routinely used f/45 or f/64 without concern; the West Coast photographers who named their circle Group f/64 in 1932 chose the aperture precisely for its depth of field. Digital capture changed the accounting. Because a sensor is a regular sampling grid, its pixel pitch provides a hard reference against which the Airy disc can be measured, and Harry Nyquists sampling theory supplies the two-samples-per-feature rule. As pixel counts climbed through the 2000s and pitches fell below five micrometres, diffraction became a routine practical constraint, and calculators expressing the limit as an f-number entered common use among landscape and macro photographers.