Focal length comparison: field of view simulator

The focal length, given in millimetres, is the distance from the optical centre of a lens (more precisely its rear principal point) to the sensor when focused at infinity. Together with the sensor size it sets the angle of view: short focal lengths (wide angle) capture a large part of the scene, long focal lengths (telephoto) magnify a small part of it. On full frame, a focal length of around 50 mm counts as normal.

The angle of view describes how much of a scene a lens captures on a given sensor, in degrees. It is usually stated diagonally; the horizontal and vertical angles are smaller. Short focal lengths give a wide angle of view, long focal lengths a narrow one.

No. A 50 mm lens stays a 50 mm lens. An APS-C sensor only records a smaller part of the image, so the angle of view matches a 75 mm lens on full frame (80 mm with Canon's crop factor of 1.6).

APS-C sensors from Sony, Nikon, Fujifilm and Pentax have a crop factor of about 1.5, Canon APS-C about 1.6. Micro Four Thirds has a crop factor of 2, 1-inch sensors about 2.7 and 44 × 33 mm medium format about 0.79. Multiply the focal length by the crop factor to get the 35mm equivalent focal length.

For exposure, no: f/2.8 gives the same brightness on every sensor. For depth of field, yes: at the same angle of view, multiply the f-number by the crop factor to compare. f/2.8 on Micro Four Thirds gives about the depth of field of f/5.6 on full frame. The lens simulator shows the effect for any lens and camera.

No. Perspective, meaning how large near and distant objects appear relative to each other, depends only on where the camera stands. A longer focal length from the same spot shows a smaller part of exactly the same perspective. Move the person in the simulator closer or further away to see the effect of the distance.

Frame width = distance × sensor width ÷ focal length. A 24 mm lens on full frame covers a width of 7.5 metres at a distance of 5 metres, a 200 mm lens only 0.9 metres.

On full frame, 85 mm is the classic portrait focal length, and anything from about 50 to 135 mm works well: you stand far enough away that faces look natural, and the background is easy to blur. On APS-C, about 50 to 56 mm gives a similar frame, on Micro Four Thirds about 42 to 45 mm. For landscapes, wide angles from about 14 to 35 mm are typical, while a telephoto lens picks out details far away.

A simple adapter without glass only bridges the distance to the sensor, so focal length and aperture stay the same. Adapters with optics are different: a focal reducer (speed booster) shortens the focal length, typically by a factor of about 0.71, and a teleconverter lengthens it by 1.4× or 2×, at the cost of one or two stops of light.

The terms are often used interchangeably. Strictly, the angle of view is the angle in degrees that a lens covers on a sensor, while the field of view is the area you see at a certain distance, for example 7.5 metres wide at 5 metres with 24 mm on full frame. The simulator shows both: the angle in the panel and the frame at the person in the scene.

Manufacturers usually state the diagonal angle and round it. Sensor sizes vary slightly between camera models, and some lenses are corrected digitally, which crops the image a little. Fisheye lenses do not follow the formula for rectilinear lenses at all.