In photography, the 35 mm equivalent focal length is a measure that indicates the angle of view of a particular combination of a camera lens and film or sensor size. The term is useful because most photographers experienced with interchangeable lenses are most familiar with the 35 mm film format.
On any 35 mm film camera, a 28 mm lens is a wide-angle lens, and a 200 mm lens is a long-focus lens. However, now that digital cameras have mostly replaced 35 mm cameras, there is no uniform 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 as film size was. The 35 mm equivalent 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 is based on equal diagonal angle of view.[1] Alternatively, it may sometimes be based on 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, which have different diagonal-to-width ratios, these two definitions are often not equivalent.
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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 = 45.0 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 |
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 of 1.5) than the actual sensor diagonal.[2] This is because these sensor size specifications refer to the size of a camera tube, while the usable sensor size is about 2/3 of the size of the tube. Tubes are not used on digital cameras, but the same specifications are used.
Apart from the width- and diagonal-based 35 mm equivalent focal length definitions, there is a third definition: EFL = 50 f /d mm.[1] However, it is not clear to what extent this definition is used.