Apple Aperture Digital Photography Fundamentals - Manual (Page 12)

12 Chapter 1 How Digital Cameras Capture Images
Focal Length
An important attribute of a lens, besides its quality, is its focal length. Focal length is
technically defined as the distance from the part of the optical path where the light
rays converge to the point where the light rays passing through the lens are focused
onto the image plane—or the digital image sensor. This distance is usually measured in
millimeters. From a practical point of view, focal length can be thought of as the
amount of magnification of the lens. The longer the focal length, the more the lens
magnifies the scene. In addition to magnification, the focal length determines the
perspective and compression of the scene.
Understanding Lens Multiplication with DSLRs
Most interchangeable lenses were originally created and rated for the 35 mm film
plane of traditional SLRs. If you compare the area of a 35 mm film plane with the area
of most digital image sensors’ image planes, you’ll see that the area of most digital
image sensors is a bit smaller. The focal length of a lens changes when it is put on a
DSLR with a digital image sensor smaller than 35 mm. This smaller image plane
effectively increases the focal length of the lens because more of the image circle
coming out of the lens is cropped. For example, if you put a 100 mm lens on a DSLR
that has a 24 mm digital image sensor, the focal length of the lens is multiplied by a
factor of approximately 1.3. A 100 mm lens with a 1.3x multiplication factor effectively
becomes a 130 mm lens (100 mm multiplied by 1.3).
Another reason to take lens multiplication into account is that shooting wide-angle
images becomes increasingly difficult when using cameras with smaller digital image
sensors. For example, if your digital image sensor is 24 mm, you require a lens with a
focal length less than 24 mm to achieve a wide-angle view. Check your camera
specifications for the size of your digital image sensor.
Camera body
(side view)
Lens
Digital image sensor
Focal length Light