Which lens do you need for your Canon SLR camera?

Lenses for full-frame cameras have an EF mount. These lenses use the full size of the sensor. Because these lenses are made for a larger full-frame sensor, they also fit on a smaller APS-C sensor. You can recognize Canon lenses for full-frame SLR cameras by the red dot on the lens. Both Canon full-frame and APS-C cameras have a red dot. This is the point where you attach the red dot of the lens on the camera.

EF-S lenses are suitable for Canon APS-C cameras. You can recognize these lenses by the white square. Because a Canon APS-C sensor has a 1.6 crop factor, you have to multiply the focal length of the lens by that number. So a 50mm lens functions as an 80mm lens in this case. That's useful to photograph athletes or wild animals, because you can zoom in further. Lenses with an EF-S mount aren't suitable for full-frame sensors, because they're larger than the APS-C sensors the lenses are made for.