Let's say you're looking at the Brooklyn Bridge through a 35mm film SLR camera.
You can see the entire bridge through the viewfinder.
Then, you take the lens from the 35mm film camera and place it on your digital SLR camera.
You look at the bridge through your digital SLR.
Now, you can't see the entire bridge.
The sensor is smaller in the digital camera, than the frame of film in the film camera.
Because the sensor is smaller, it "sees" less.
The lens appears to be more telephoto.
On Nikon digital SLRs, lenses are 1.5 times more telephoto, compared to a film camera.
On Canon digital SLRs, lenses are 1.6 times more telephoto, compared to a film camera.
1.5 and 1.6 are the lens factors.
You multiply the mm's of your film lens by one of these lens factors to get the focal length of the lens on most digital SLRs.
A few cameras have full-frame sensors.
They're the same size as 35mm film.
Therefore, the focal length of a film lens is the same of these full-frame cameras.
Let's say you have a 50mm lens from a film camera.
If you put the lens on your digital SLR camera, the focal length of the lens is no longer 50mm.
The 50mm lens becomes a 75mm lens (Nikon) or 80mm (Canon).
The lens becomes more telephoto because the smaller sensor doesn't "see" as much of the scene as does film.
The illustration below shows the circular image cast by the 50mm lens.
Lenses are circular, so they caste circular images.
Next, we'll look at how much of the scene the film and sensor can see.
This is what the film can see.
The film is bigger, so it sees more of the scene.
This is what the smaller sensor can see.
The sensor is smaller than film, so the sensor doesn't see as much of the scene.
This is what you see in the viewfinder of the film camera.
You can see more of the waterfall because the film is bigger.
This is what you see in the viewfinder of the digital camera.
You don't see as much of the waterfall because the sensor is smaller.