Beecher's Handouts /
Focal Length / 17.3 - Lens Factor
Let's say your looking at the Brooklyn bridge through a 35mm
film SLR.
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.
Now, you can't see the entire bridge.
Why?
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.
A 100mm focal length, on a film camera, becomes a 150mm (Nikon)
or 160mm (Canon) lens on a digital camera.
| 100mm |
1.5 x
100mm = 150mm |
1.6 x
100mm = 160mm |
The 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 a digital SLR.
Full Frame Cameras
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.
An Example
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 (50mm to 75mm) 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.
The Scene
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.
What the Film Sees
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.
What the Sensor Sees
This is what you see in the viewfinder of the film camera.
You can see more of the waterfall ecause the film is bigger.
Film Camera
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.
Digital Camera
