>>3678029well you're not alone, literally every noob photographer gets confused by equivalence.
here's what its saying. Suppose you take a lens with a 50mm focal length and put it on a full-frame camera (no crop). You take a picture. Now, without changing the focal length of the lens, or the position of the camera or the subject, you switch to a crop sensor camera and take a picture. If you look at them side by side, the one with the smaller sensor will appear tighter, more "zoomed in". Obviously, because all that's going on is you're cropping out the central part of the image, that's why its called crop.
Now the crop factor number is saying, if you used a full-frame camera, what focal length lens would you have to put on it to get a picture with the same narrower field of view as the camera with the crop sensor? For APS-C the number is 1.5 (or 1.6 for Canon because they just had to be special) which means that you would need a 75mm lens (50*1.5) to get the same field of view. In other words, the 50mm lens on the crop sensor camera is "equivalent" in field of view to a 75mm lens on a 135 camera, so people say its 75mm-equivalent.
People do this because focal length is much more convenient when talking about field of view than, say, specifying it in degrees. Even though field of view depends on both focal length and format size, usually the format size is pretty constant and focal length is what you're changing when you decide how to frame an image.
