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 = 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. 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. However, it is not clear to what extent this definition is used.
Read more about this topic: 35 Mm Equivalent Focal Length