How Close Can I Get?
When the content of the shot is key, shooting with a full-frame sensor lets you get much closer to your subject than shooting with the same lens on a camera with an APS-C size sensor.
For the second setup, I taped a blue rectangle with a 3:2 aspect ratio on the wall. (Both full-frame and APS-C size sensors shoot in the 3:2 aspect ratio.) I mounted the 50 mm lens on the Canon 5D and positioned the tripod so that the rectangle filled the frame, a distance of nine feet from the wall. I switched the lens to the 20D body and positioned the tripod so that again the rectangle filled the frame. The distance was 14.4 feet from the wall. Figure 8 shows the comparison.
Figure 8 The tripods represent the relative positions of the Canon 5D and 20D, using the same lens, to fill the frame with the blue rectangle.