Moon & Planet Size on Camera Sensor Calculator

How it works

Size on sensor (mm) = angular size (″) ÷ 206,265 × focal length (mm); pixels = mm × 1,000 ÷ pixel size (µm); focal length to fill = 0.9 × sensor height × 206,265 ÷ angular size; drift = 15″ per second of time

The Moon is only about half a degree across, so even a 300 mm lens renders it a modest 2.6 mm disc — a few hundred pixels on a typical sensor, which is why casual lunar photos look small. The size scales directly with focal length: multiply the angular size in radians by the focal length to get millimetres on the sensor, and divide by pixel size for pixels. Filling an APS-C frame needs about 1,700 mm; a full-frame sensor about 2,600 mm, usually reached with a telescope or a teleconverter. Planets are 40–60 times smaller still, which is why planetary imaging uses long effective focal lengths (f/20–f/30 with Barlows) and high-speed video rather than single frames. The sky drifts 15 arc-seconds a second, so untracked exposures must be short enough that the drift stays under a pixel — easy for the bright Moon, the limiting factor for dim targets.

Worked example

At 600 mm on a 3.9 µm APS-C sensor the average Moon is 5.4 mm or about 1,390 pixels across — 35% of the frame height (roughly 1.5 MP on the disc). About 1,560 mm would fill 90% of the frame; the image scale is 1.34″/px, so untracked exposures under about 1/11 s keep drift below a pixel.

Frequently asked questions