Another topic that frequently seems to confuse people, and for which they appear unable to engage in even a small amount of self education.
The focal length of a lens is the distance between its optical center and the point at which parallel rays of light are brought to a point, or focused. It is an optical characteristic of the lens and never changes regardless of the size or type of imaging sensor upon which the image is focused (film, digital). This isn’t a matter of interpretation or opinion, it’s physics.
Lenses produce a circular image, no surprise given that lens elements are circular. The brightness of the image falls off towards the edges of the circle when the lens is wide open, the effect diminishes as the aperture is closed. The recording medium, whether it’s film or digital, must fit within the usable portion of the image circle:
Here the largest rectangle that fits within the usable image circle is the film/sensor size – making a large image circle is expensive, and so the lens is designed to create a usable circle only as big as necessary. Note that in reality it’s not a crisply defined circle – more a case of a bright center that falls off to very dim the further out from the center you go.
If the same lens projects the same image circle onto a smaller sensor, all that happens is that a smaller section from the image circle is used – the lens hasn’t changed, we’ve merely cropped the image being projected by the lens and used only a portion:
From the point of view of the camera, the field of view has changed – the angle which the picture occupies within the scene.
It’s the same lens, at the same distance, but a smaller sensor means that only a portion of the original image is captured, yielding a narrower field of view.
Photographers need an easy way to describe what’s happening here, in terms of what they see. The most accurate way would be to talk in terms of “field of view”; a 50mm lens on a 35mm sensor has a 39 degree angle of view, the same lens on a “1.6 crop” sensor has a field of view of 25 degrees. To describe this we talk about the 35mm equivalent focal length: to get a 25 degree horizontal field of view on a 35mm sensor you’d need an 80mm lens – so a 50mm lens on a 1.6 crop sensor produces an image which is considered equivalent to an 80mm lens on a full frame 35mm sensor.