What is focal length?
Rather than referring to the physical length of the lens, focal length refers to the distance between the lens’s optical centre and the camera sensor, when focused at infinity.
This means, that for a camera with a full frame sensor (36x24mm sensor size) would see a perspective similar to the human eye when mounted with a 50mm (‘standard’) lens, but a cropped sensor camera with a 1.5x focal length conversion would see the equivalent of a 75mm lens. A more detailed, yet understandable explanation can be found in my article about sensor sizes.
A ‘cropped sensor’ camera does not have a sensor that is actually cropped, but due to the smaller sensor, the lens is exposed to significantly less of the image, and therefore, the image appears ‘cropped’ in comparison to one taken with a full-frame camera.
A lens with an angle of view greater than 50mm is a telephoto lens, less than 50mm gives a wide angle lens – so called simply because it has a wide angle of view – and a 50mm lens, as said before, is considered ‘standard’.
Angle of View
Angle of view is simply the amount of a scene that a lens can take in. A fish eye lens can have a very wide angle of view, such as 180 degrees, which allows it to see all of the scene in front, and to the side, of it. In contrast, a 200mm lens would see only 12.3 degrees of the view in front of it, meaning that you can fill your frame with more distant objects.
Effective Focal Length
When viewing a lenses specifications, you may see ’35mm equivalent focal length’. This measurement is given for a ‘standard’ camera. So, the lens will be 35mm when used on a full frame camera, but if used on a micro four thirds camera, where the sensor is about half the size of a FF sensor, the subject will appear about twice as big.
So, on a camera like the Canon EOS 650D, a 75-300mm lens will appear to be 120-480mm (1.6x crop factor). The perspective and image magnification remains the same, but the angle of view changes so the lens is still a 75-300mm lens, but produces the effect of a 120-480mm lens on a 1.6x crop factor camera.