The focal length of a lens doesn't really tell you its actual angle of view because this depends on the sensor size too. But you can use a 'focal factor' to work out what the equivalent focal length will be.
For example, most digital SLRs have an APS-C sized sensor measuring around 24 x 16mm. This is smaller than a full-frame sensor, or a 35mm negative size, which is what most people go by. The difference is 1.6x (a full-frame sensor is 1.6x wider).
So if you use a 50mm lens on an APS-C camera, you multiply this focal factor by 1.6x, which gives you an equivalent focal length of 80mm.
In other words, because the sensor is smaller, a 50mm lens fitted to an APS-C camera actually looks like an 80mm lens.
