Focal Length

Technically speaking, the focal length of a lens is a measurement in millimetres from the camera’s sensor (or focal) plane to the optical centre of the lens. It is the focal length (combined with the size of the sensor, which we’ll get to in a moment) that determines the angle (or field) of view of a lens. The lower the focal length figure, the ‘wider’ a lens will be be. This means that more of the scene in front of the camera is captured. As focal length increases, the angle of view narrows and the image is increasingly magnified. 

That is not the end of the story though. The size of the sensor inside your camera also has an effect on the angle of view of a lens. The smaller the sensor, the smaller the angle of view of a lens at a given focal length. This effect can be seen with any lens, but a good example is the 24mm focal. On a full-frame camera this is considered to be the go-to lens for landscape photography as it is usefully wide-angle, with an angle of view of 84°. However, fit the same lens to an APS-C camera and the angle of view narrows, down to 61°. This makes the lens far less wide-angle, and closer to a 35mm lens on a full-frame camera. 

The method to work out what the equivalent lens would be is to divide the focal length of a lens designed for full-frame cameras by 1.5. Do this and you can quickly see that a 16mm lens is needed to achieve the same angle of view on an APS-C camera as a 24mm lens fitted to a full-frame camera. (The sensor in a Micro Four Thirds camera is half the size of a sensor in a full-camera. You therefore have to divide the focal length by 2. So you would need a 12mm lens on a Micro Four Thirds to match a 24mm lens.)