17. VARIABLE APERTURE VS. CONSTANT APERTURE LENSES
THE MAXIMUM APERTURE of a lens is the widest that the lens can open, and it is included in the name of the lens. For example, a 50mm f/1.8 lens has a 50mm focal length and can open to f/1.8, whereas an 85mm f/1.4 lens has an 85mm focal length and can open to f/1.4 (Figure 17.1). You can divide all lenses into two different classes: those that maintain the same maximum aperture regardless of the focal length (constant aperture) and those whose maximum aperture changes depending on the focal length (variable aperture).
A prime lens has a single focal length, while a zoom lens has a range of focal ...
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