35mm Equivalent Focal Length

Digital Changes Relationship Between Focal Length and Angle of View

35 mm equivalent focal length is the focal length of a thin lens which forms images that most nearly duplicate those of a given compound lens, thick lens, or system of lenses. This term originates from the time when the overwhelming majority of photography was created with 35 mm film.

Full Frame vs APS-C
The resulting images from 50 mm and 70 mm lenses for different sensor sizes; 36x24 mm (red) and 24x18 mm (blue)
On any 35 mm film camera, a 28 mm lens is a wide-angle lens, and a 200 mm lens is a telephoto lens. However, now that digital cameras have mostly replaced 35 mm cameras, there is no direct relation between the focal length of a lens and the angle of view, since the size of the camera sensor also determines angle of view, and sensor size is not standardized like film size was. The 35 mm effective focal length of a particular lens-sensor combination is the focal length that one would need for a 35 mm film camera to obtain the same angle of view.

Most commonly, the 35 mm equivalent focal length refers to the diagonal angle of view. Alternatively, it may also refer to the horizontal angle of view. Since 35 mm film is normally used for images with an aspect ratio (width-to-height ratio) of 3:2, while many digital cameras have a 4:3 aspect ratio, these two definitions are not equivalent.

Conversions

A standard 35 mm film image is 36 mm wide by 24 mm tall (35 mm refers to the height of the film including the perforations for film transport), and the diagonal is 43.3 mm. This leads to the following conversion formulas for a lens with a true focal length f:
Image size diagonal-based EFL width-based EFL
4:3 (sensor width w) f35 = 34.6 f /w mm f35 = 36.0 f /w mm
4:3 (sensor diagonal d) f35 = 43.3 f /d mm f35 = 48.0 f /d mm
3:2 (sensor width w) f35 = 36.0 f /w mm f35 = 36.0 f /w mm
3:2 (sensor diagonal d) f35 = 43.3 f /d mm f35 = 43.3 f /d mm
Technical Phototography Article><a href=For historical reasons, sensor size specifications such as 1/2.5" do not match the actual sensor size, but are a bit larger (typically about a factor 1.5) than the actual sensor diagonal.

Apart from the width- and diagonal-based 35-mm effective focal length definitions, there is a third definition: EFL = 50 f /d mm. However, it is not clear to what extent this definition is used.

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