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"Get a 50mm lens." That is a statement that many photographers will tell you. But do they mean a 50mm lens or the equivalent of a 50mm lens on your camera? Make no mistake, a 50mm lens is a 50mm lens, but sensors have magnification and crop factors depending on your camera model.
Here's an old chart that shows off the different sensor sizes of various cameras. Now, keep in mind that full frame is approximately the size of 35mm film.
APS-C sized sensors are in most DSLR cameras, like the Canon Rebels and the lower-end Sony and Nikon series cameras as well. Because they are smaller than a full-frame sensor, full-frame lenses will have a crop/multiplication factor.
For Canon, this multiplication factor is 1.6x. That means that if you're using a 100mm lens, your multiplication and field of view will be an equivalent of 160mm.
For Nikon, Sony, and Pentax the multiplication factor is 1.5x. So your 100mm lens will have a field of view and equivalent of 150mm.
Four-thirds and Micro four-thirds sensors have a 2x crop. So the 100mm lens will be 200mm.
APS-H sensors are in the Canon 1D series, and have a multiplication factor of 1.3x.
Different lenses are designed to be used with different sensor sizes. By that, I mean that there are lenses designed to be used with full-frame cameras, and lenses designed to be used with smaller sensors. Full-frame lenses can go onto APS-C bodies, and be subject to the crop/multiplication factor. If you put an APS-C lens on a full-frame body, it either won't work, or will only take a photo using a very small portion of the sensor. This is because of the size of the imaging circle around the part of the lens that goes into the body.
In the case that the circle is too small, only a small area of the sensor will be used to capture an image. If the imaging circle is larger than the camera sensor, the sensor will generally only capture the middle area of the lens's field of view—hence the crop factor.
To demonstrate the crop factor, I'm going to use my Canon 5D Mk II, 7D and 35mm F/1.4 L lens.
Above is a photo taken with the Canon 35mm F/1.4 L on a Canon 5D Mk II. The focal length is a true 35mm. When this lens gets mounted on the 5D Mk II, the full-frame sensor is utilizing the entire imaging circle of the lens.
When you put the 35mm F/1.4 L on the Canon 7D, you get a 56mm field of view equivalent, due to the crop factor magnification. When the lens is mounted on the 7D, the cropped APS-C sized sensor is utilizing only the center of the lens circle.
Each of the above photos was taken from the exact same location.