Zoom Lens
A lens whose focal length can be varied continuously across a range while holding focus.
Formula / rule
Zoom ratio = f_max / f_min
Measured in
mm
In depth
A zoom moves internal groups relative to one another to change magnification, which requires the designer to balance aberrations across the whole range rather than at one point. Constant-aperture zooms hold the same maximum f-number throughout by enlarging the front group; variable-aperture zooms let it drift, which is cheaper and lighter but costs light at the long end.
Examples in practice
A 70-200 mm f/2.8 holds f/2.8 across its range; a 70-300 mm f/4-5.6 loses a stop and a third by 300 mm.