What does the equivalent focal length of 35mm mean?

In photography, the equivalent focal length of 35mm is used to represent the viewing angle obtained by combining a specific camera lens with a negative or an image sensor. Since most photographers use ring lenses and are familiar with the format of 35mm film, this term is very helpful to them.

On a camera with 35 mm film, the 28 mm lens is a wide-angle lens and the 200 mm lens is a telephoto lens. But now digital cameras have almost completely replaced 35mm negative cameras, and there is no uniform correlation between focal length and viewing angle. Because the camera's sensor size will also affect the viewing angle, and the sensor size is no longer as unique as it was in the negative film era. For a specific combination of lens and sensor, its 35mm equivalent focal length means that on a 35mm negative camera, this focal length can obtain the same viewing angle.

Extended data:

brief introduction

Generally, the equivalent focal length of 35mm is based on the diagonal of viewing angle. And sometimes it may be a horizontal line based on the viewing angle. Because the aspect ratio of 35mm film is 3:2, and the sensor aspect ratio of many digital cameras is 4:3, these two definitions are not completely equal.

The 35mm equivalent focal length is obtained by multiplying the actual focal length by the focal length conversion rate. Typical focal length conversion rates of Nikon and Sony APS-C formats are 1.5 times, Canon APS-C formats are 1.6 times, M4/3 formats are 2 times, and ordinary digital cameras are about 5-6 times. Using 35mm equivalent focal length will ignore the depth of field, and the smaller the sensor, the greater the depth of field. But the equivalent depth of field can also be calculated in the same way according to the focal length conversion rate. For example, a 50mm f/2 lens mounted on a miniature 3/4 system camera can be equivalent to a 100mm f/4 lens mounted on a Quan Huafu SLR.

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