We mentioned crop factor a few days ago in our lens compatibility post series, and wanted to give an actual visual reference for what is happening so that you can better understand it.

The red box in the image above indicates what you would see on a full frame camera, and the blue box indicates what you would see with the same lens on an APS-C camera. (Full frame and APS-C refers to the sensor size. (If you didn't read our posts on lens compatibility which covers full frame and APS-C, you can refer back here.)

For our test shots below, we used the same lens on both cameras- a Canon EF 50mm F1.8 II.
We also stood and center-focused on the exact same spots in both images.
Image 1, above: shot with a Canon 5D (full frame camera)
Image 2, above: shot with a Canon 40D (APS-C camera)

You can immediately notice a difference in the area that is covered in each shot. There was no zooming or edit cropping done to change the image area. These images are straight from the camera and what you would see through the viewfinder. So as mentioned before, the slightly wide angle of view with the lens changes to a slightly telephoto angle of view with the same lens, just with a switch in sensor size.

This is something to keep in mind when purchasing a non-digital designated lens to use on a digital SLR body that has the smaller APS-C size sensor. In this case (and using Canon as an example), the 50mm EF lens no longer acts as a true 50mm lens on the 40D. If you want to obtain a true 50mm and own one of the APS-C size digital SLRs, you would want to purchase a digital EF-S lens that is in the 50mm range, or an EF lens that is a smaller mm, or wider angle lens to start with.