On Digital SLR’s, when mounted with a lens of given focal length, you would have noticed a considerable change in the Field Of View, when compared to a film camera. This is nothing but the “Crop Factor”.
Let’s consider the prime 50mm lens:
When mounted on a 35mm film camera the focal length of the lens remains the same i.e. 50mm, as the lenses are designed for 35mm films/equivalent sensors.
When mounted on any Digital SLR the focal length remains the same but due to the crop factor the field of view drops considerably, as the sensor sizes of most of the mid-level D-SLR’s are smaller than 35mm.
For Canon EOS 350D the sensor size is 22.2 x 14.8 mm
For Nikon D70S the sensor size is 23.7 x 15.6 mm
So a wide angle lens loses it’s field of view when mounted on a D-SLR (of sensor size less than 35mm)
For instance when a 50mm lens is mounted on a CANON EOS 350D D-SLR, whose sensor size is 22.2 x 14.8mm, the crop factor calculates to
crop factor = 35/(sensor size)
i.e. crop = 35/22.2 = 1.6
so now the field of view, for a 50mm lens, on a D-SLR translates to:
field of view = focal length of the lens X crop factor
i.e. field of view f = 50 x 1.6 = 80mm (i.e field of view is equivalent to a 80mm lens)
wow!! it’s almost become a tele photo lens. Note that the focal length remains the same but the field of view of this lens now becomes the equivalent field of view of a 80mm lens.
If you are a D-SLR (serious amateur level) user and if you are going to buy a lens then don’t forget the crop factor otherwise you end up picking up a 16-35 L series lens for 1,500 USD and when you mount it on your D-SLR you’ll have a field of view equivalent of a 25-56mm lens. So plan your buy keeping the full frame sensor in your mind.
Invest in expensive lenses only if you are going to switch to a full frame sensor in the future.
For Nikon and Fuji D-SLR users the crop factor is approx: 1.5
For Canon D-SLR users the crop factor is approx: 1.6
Through the lens 
Posted by stephen binnie on December 3, 2006 at 1:50 am
This is a most misleading article. To suggest that by putting a 50mm lens from a 35mm camera onto a DSLR changes the focal length of the lens is completely untrue….unless of course you are a magician and have another lens up your sleeve. It is the sensor size and the angle of view that changes. The ‘magnification factor’ is just apparent because the angle of view has changed.
Posted by praveenc on December 4, 2006 at 2:53 am
Thanks Stephen for pointing it out. I used focal length and field of view interchangeably, which, I agree is a blunder.
What I meant was that the field of view of a mounted lens on a DSLR is equivalent of a telephoto lens..to mention that it looses focal length was a mistake.
I’ve made the necessary changes to this post. It now stands corrected
Posted by Steve Lockhart on December 14, 2006 at 5:51 am
Thanks for the crop factor info.
My work is in architectural graphics whereby digital rendered “models” are inserted into shots of the site. Field of view is critical in placing the model accurately.
Info on this site has helped me sort out a few issues.
Posted by Tom Massey on January 26, 2009 at 4:04 am
Thank you for a great explanation. The camera and lens manufacturers have not done a great job of advertising any of this info. Not to say it is not mentioned in the manuals but they do not emphasize it. A lens designed for an APS size chip cannot be used for a 35mm film camera simply because its image circle is not big enough. And, for those in the know, a run on used 24mm lenses to give you the equivelent of a 35mm lens in 35mm format. Very good article. Thanks.