How to convert optical zoom to mm

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How to convert optical zoom to mm
The focal length of any lens measures its angle of view. (Portable digital photo camera. Isolated on white. image by diter from

Traditionally, the lens size for an SLR film camera is measured in millimetres. The focal length represents the distance from the centre of the lens to its focal point when the lens is set to focus at infinity. The smaller the focal length, the wider the picture's frame.

Because digital cameras have differing image sensors and built-in lenses, manufacturers measure focal length with what's called a multiplier. A camera with a 3x optical zoom its longest focal length is three times the minimum focal length.

Using a little math, you can convert a multiplier to the traditional millimetre measurement.

Skill level:

Things you need

  • calculator

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  1. 1

    Look at the lens of the digital camera and find the measurement printed on the outside. For example, the Canon PowerShot A720 IS lens is marked as 5.8-34.8mm. Use 34.8.

  2. 2

    Note the camera's zoom multiplier. Use the largest number, whether optical or digital. The A720's multiplier is 6x.

  3. 3

    Multiply the number from Step 1 by the number from Step 2. Using the examples, the calculation would be 34.8 x 6 = 208.8. That means the Canon's lens has a focal length of 35mm to 208mm.

Tips and warnings

  • If the lens size isn't printed on the exterior of the camera, find it in the user's manual. The camera's specifications are usually found in the back of the manual.

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