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.

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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