How to Compute the Zoom Lens Factor

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The introduction of digital cameras created some confusion with how to interpret lens focal lengths. While most photographers were accustomed to comparing lenses in relation to 35 mm film---so named because the film itself is 35 mm wide---digital sensors came in a variety of widths and could not be easily compared. As a result, photographers and camera manufactures created a "crop factor" metric, which could be used to convert lens focal lengths on digital cameras to their 35 mm equivalent. Calculating your zoom lens factor will yield three numbers: your lens's 35 mm equivalent, its zoom range and its magnification factor.

35 mm Equivalent

  • Find out your camera's crop factor. On digital cameras, this is usually a number between 1.3 and 2.0. "Full frame" cameras will feature a crop factor of 1.0. The most common crop factor for dSLRs (digital Single Lens Reflex) is 1.6. You can find this information in your camera's manual or on your camera manufacturer's website.

  • Determine the focal length range for your lens. Prime lenses will have only one number---expressed in mm---while zoom lenses will have two.

  • Multiply these numbers by your camera's crop factor. These new numbers will be the lens's 35 mm equivalent. For instance, if you are using a zoom lens with a range of 70 to 200 mm with a camera with a crop factor of 1.6, your photograph will be equivalent to what can be captured on a 35 mm film camera with a 112 to 320 mm lens.

Zoom Range

  • Recall the focal length range of your lens. A prime lens with only one number does not have a zoom range.

  • Divide the maximum focal length of your lens by the minimum focal length. This will give you your lens's zoom range. For instance, a 70 to 200 mm zoom lens will have a zoom range of 200/70, or roughly 3. This means the 70 to 200 mm lens can also be accurately described as a 3x zoom lens.

  • Reverse the steps to find the 35 mm equivalent range for point-and-shoot cameras, which are typically advertised according to their zoom range. You must know the camera's minimum focal length---its wide-angle number expressed in mm---or maximum focal length---its telephoto number expressed in mm.

Magnification Factor

  • Recall your camera's crop factor and your lens's maximum focal length.

  • Multiply the crop factor by the maximum focal length. For instance, if your lens's maximum focal length is 200 mm and your camera has a crop factor of 1.6, your adjusted maximum focal length is 320 mm.

  • Divide this number by 50 to calculate the lens's magnification factor. For instance, a 70 to 200 mm lens's magnification factor will be 320/50, or roughly 6. This means that at its maximum focal length, this lens will magnify an image 6x when compared to the human eye.

Tips & Warnings

  • While zoom range is a common metric for point and shoot cameras, it does not give any information about the camera's magnification potential. For example, while 300 mm lens will have much higher telephoto capabilities than a 200 mm lens, the zoom range for a 200 to 300 mm lens will be only 1.5x, while a 50 to 200 mm lens will be 4x. Always look at the maximum telephoto distance before making a purchase decision.

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