On a digital SLR, what is crop factor?

Answer 1

Depending on sensor size, the amount of "image that hits the sensor gets cropped with different lenses.
Many APS-Sensor sized cameras have a 1.5 to 1.6 crop size, some have a 1.3 and some have a 1:1 (or full frame) sensor crop factor.

What this means is that if you shoot with a 1.6, your 20mm lens would become a 32mm, so you loose your wide angle, but if you have a 400mm on a full frame, you can get 640mm distance with it on a 1.6.
This also means that your full frame camera will allow you to create a greater bokeh (soft focusing) around your subect at the same aperture levels, due to the fact that you can get shallower depth with its lenses.

Answer 2

For most Nikon's the crop factor is 1.5 and for most Canon's, the crop factor is 1.6.

Because sensors are generally smaller than a full frame image on 35 mm film, the frame gets filled up sooner. Bad explanation... Here's a great explanation, if you are a visual learner. Scroll about 2/3 of the way down the page until you see the picture of the bird with 3 rectangles drawn around him. Read the associated text.

http://www.luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml

Answer 3

The digital image sensor, is smaller than 35mm film. For this reason the same lens 'appears' to magnify more on a digital SLR than a film SLR. So to compare lenses in the case of Canon, you multiply by 1.6, in the case of Nikon 1.5 etc.

Answer 4

The difference in picture you see on the (optical or electronic) view finder, and the (size of the picture) that actually gets recorded in memory. The images do not always match