How to Convert Focal Length to Magnification

Written by robert korpella
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How to Convert Focal Length to Magnification
Detachable lenses have a focal length to describe how images are captured, while point-and-shoot cameras use magnification or zoom. (digital camera age image by Steve Brase from Fotolia.com)

Point-and-shoot cameras have fixed lenses that can zoom in on a subject. Magnification is the measurement used to describe the zoom feature and the results are displayed as 2x, 3x, 10x or more in magnification. SLR (single lens reflex) and digital SLR (DSLR) cameras have changeable lenses, each with a different focal length. The term "focal length" refers to the distance between the lens and either 35 millimetre film (on SLR cameras) or the sensor (on DSLR cameras) when the lens is set at infinity.

Skill level:
Easy

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Things you need

  • Maximum focal length of the lens
  • Brand name of the SLR or DSLR camera

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Instructions

  1. 1

    Find the focal length of the lens. It is stamped on the body and is expressed in millimetres (mm). For example, the lens might be labelled "50mm." Some lenses have a range, like "75mm to 300mm." When given a range, use the highest of the two numbers.

  2. 2

    Divide the focal length by 50. This number is used because a 50mm lens captures an image the same size as 35mm film, so it has a magnification of 1, or very close to 1. A 75mm to 300mm telephoto lens would then result in a quotient of 6 (300 divided by 50).

  3. 3

    Multiply the quotient by a constant for each brand of SLR or DSLR camera. The constant compensates for the size of the sensor, which is usually smaller than the size of 35mm film. Canon, Nikon, Pentax and Sony have a constant of 1.5. Olympus and Panasonic have a constant of 2. The 75mm to 300mm telephoto lens would have a magnification of 12x for Olympus and Panasonic cameras (the quotient of 6 multiplied by a constant of 2) and 9x for the other four brands (the quotient of 6 multiplied by a constant of 1.5).

Tips and warnings

  • Some larger DSLR cameras have a full-frame sensor, which means it is the same size as 35mm film. A Canon 5D is one example. On these cameras, no constant is necessary for the calculations since there is no need to compensate for a sensor that's smaller than film. For these cameras, the 75mm to 300mm lens would have a magnification of 6x (300 divided by 50).

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