Technique tip: Zoom ratios
Zoom lenses are often described by the ratio between their longest and
shortest focal lengths. The ratio reveals the change in magnification,
but not the actual focal lengths covered.
• For example, each of the following is a 2:1 zoom lens: 25-50mm,
35-70mm and 100-200mm. In each case the longest focal length is two
times the shortest.
• Magnification is directly related to the zoom ratio. The longest focal
length of a 5:1 zoom lens magnifies the image five times more than the
shortest focal length.
• Most zoom lenses for 35mm cameras have a 2:1 or a 3:1 ratio. As the
ratio increases, so do the length, weight and price.
• Aperture values change with the focal length. If a 35-70mm (2:1) zoom
has a maximum aperture of f/2.8 at 35mm, the maximum aperture at 70mm is
f/4. The value of other f-numbers changes accordingly.
• Because aperture values change, it is easiest to use a zoom lens with
an aperture preferred mode of automatic exposure control. Then, as you
zoom the lens the camera will automatically adjust the shutter speed to
the aperture value change, whatever it may be.
More about zoom lenses
Focal Length Range of Zoom Lenses
Tips about Zoom Ratio
Standard or Macro Focusing
Zoom Lens Consideration
Tips on Handling Zoom Lens
Advantage of Zoom Lens vs Unifocal Lens
Disadvantage of Zoom Lens vs Unifocal Lens