If you use a “crop factor” camera, one with a smaller sensor than a 35mm negative, you need to use smaller lens focal lengths to achieve the traditional effect a lens gives you with a 35mm sensor or negative. Like so:

So to get a “standard 50mm picture”, use a 35mm lens if you have a Digital Rebel, say (an “APS-C sensor” camera, which is the common size). And so on. You may want to copy down this table!