The definition of focal length
is the distance from the center of the lens element to the film (or sensor) surface. This is why a 20mm lens is a little short lens and a 500mm is so long it requires some type of support to hold it steady. A zoom lens has elements that move closer to, or farther from the film surface.
Focal length and magnification are related by the simple equation:
Focal Length X Object size
--------------------------------- = Image Size
Image size is the actual size of the object at the negative or sensor surface. If you wanted to take a picture of an object that is 5 feet tall, using a 50mm lens, from a distance of 10 feet. The actual size at the negative or sensor surface would be: (convert everything to mm)
(50mm) X (1524mm)
---------------------------- = 25mm
If you change the lens to a 100mm and take another picture from the same location the object size at the film surface becomes 50mm, or 2X. Note that by convention all magnification factors (for the 35mm format) are relative to a 50mm lens. Therefore, a 200mm is said to be 4X, and a 400mm lens is said to be 8X.
Here is a real important point that seems to confuse a lot of people the above equation says nothing about the film or sensor size it gives the image size at the film surface. Or stated another way, a 50mm lens produces the same image size regardless of the film or sensor size. In the above example a 50mm lens will produce a 25mm image at the film surface of an 8 X 10 view camera, a 24mm X 36mm 35mm camera, or an 8mm digital sensor. How much of this image is captured on film or sensor depends on the size of the film or sensor.
Cropping and Enlarging:
This is what one does when using a lens designed for 35mm format on a digital camera. Using the example above if the sensor size is say 16mm X 24mm then the most the sensor can record is 16mm of the 25mm image. If the image is then enlarged to 4 X 6 the resulting photograph has a larger image of the object than a 35mm enlarged to 4 X 6. This is not because of greater magnification by the lens it is because of greater enlargement during printing. However, the end result is the same. The digital image from a 50mm lens enlarged to 4 X 6 will be the same size as a 35mm image using a 75mm lens enlarged to 4 X 6.
FAQ written by stev32k, added and modified by editor.