What it frames

The Canon EOS 850D with the Canon EF 300mm f/4L IS USM, over a 30 cm bird on a 1 m fence post. Everything is drawn to scale: the solid frame is what the camera takes in, the dashed ones are 24, 50 and 135mm on full frame from the same spot. Its APS-C (Canon) sensor (22.3 x 14.9 mm) has a crop factor of 1.6, so the dashed frames are marked with the focal length that gives the same view on it.

Camera Lens Focal length 300mm, fixed

Distance (m)

Scene

300mm on the Canon EOS 850D frames 1.11 m × 74 cm at 15 m: like 480mm on full frame, 4.3° across.

The subject is 15 m away; things behind it are drawn smaller, as the camera sees them.

Inside the frame: the picture you would get.

The numbers

Frame at 15 m1.11 m × 74 cm
Full-frame equivalent480mm
Angle of view4.3° × 2.8°, 5.1° diagonal
Depth of field at f/414.8 m to 15.2 m (35 cm)
Depth of field at f/814.7 m to 15.4 m (70 cm)
Hyperfocal at f/41259 m
Hyperfocal at f/8630 m
Circle of confusion0.018 mm

Depth of field: what is acceptably sharp, wide open and at f/8, focused on the subject.

How it is worked out

The frame at a distance is distance × sensor size ÷ focal length, across and up; the angle of view is 2 × atan(sensor size ÷ (2 × focal length)). Depth of field uses the thin-lens formulas: the hyperfocal distance H = f² ÷ (N × c) + f, the near limit s(H − f) ÷ (H + s − 2f) and the far limit s(H − f) ÷ (H − s), sharp to infinity from H on, with a circle of confusion c of the sensor diagonal ÷ 1500 and, on a zoom whose aperture changes, the maximum aperture taken linearly between its ends. These are for a rectilinear lens; a fisheye takes in more. Ask for this page with Accept: application/json for the numbers alone.