There’s one important thing to remember about focal lengths: They don’t act the same as they did on 35mm film cameras if you have a sensor that is smaller than a 35mm frame of film. If you have a camera with a smaller sensor, your camera doesn’t capture as much of the scene in front of you as a 35mm film camera.
In essence, the focal length crops to a smaller area of the scene, which is the same as zooming in. This is great when you’re photographing wildlife. You can get closer to your subject without having to break the bank on an expensive telephoto lens. However, when you shoot landscapes, you’re at a disadvantage if your camera has a sensor that is smaller than a frame of 35mm film.
A full-frame sensor has dimensions of 36mm x 24mm. If your sensor is smaller than that, you need to calculate your focal length multiplier and apply it to the focal length of the lens you’re using to get the 35mm equivalent focal length. You may also see the focal length multiplier referred to as the crop factor.
If your camera’s sensor is smaller than a frame of 35mm film (36mm x 24mm), the sensor records only part of what the lens sees. The result is that the lens acts like a longer focal length would on a full-frame sensor. The focal length multiplier depends on the size of your camera’s sensor in relation to a full-frame sensor. The focal length multiplier generally falls between 1.3 and 2.0.
If you slap a lens with a 50mm focal length on a camera with a focal length multiplier of 1.6, the resulting 35mm equivalent is 80mm (50 x 1.6). If you put the same lens on a camera with a focal length multiplier of 1.5, you end up with a 35mm equivalent of 75mm (50 x 1.5).
Knowing your camera’s focal length multiplier is important when choosing accessory lenses. Most Canon cameras that don’t have full-frame sensors, have a focal length multiplier of 1.6, with the exception of the Canon EOS-1D Mark IV. That model has a focal length multiplier of 1.3. Nikon cameras without full-frame sensors have a focal length multiplier of 1.5.
If you can’t find the focal length multiplier for your camera, you can easily calculate it. For example, the sensor on a Canon EOS 7D is 22.3mm x 14.9mm. To find the focal length multiplier for the camera, divide the width or height of a 35mm frame of film by the width or height of your camera sensor.
In the case of the EOS 7D, 36 divided by 22.3 equals 1.614, which rounded off is 1.6. The focal length multiplier for that camera is 1.6.