Frame SizeA full-frame digital SLR is fitted with an image sensor that is the same size as a 35 mm (36x24 mm) film frame. These sensors are currently expensive and only found on the professional-level cameras. Most SLR lenses were created for 35 mm film, when a smaller sensor is used (as in the Nikon D40, D90, Canon Rebels and 50D), less area of the lens is used.

Though there are other subtle differences, one simple way to compensate for this conversion is to take a few steps back when using anything less than a full frame sensor.