How to Convert Focal Length to Magnification

slr camera image by Piotr Wardy...„ski from Fotolia.com

When budding photographers switch from a point-and-shoot camera to an SLR, they often want to know what the magnification, or zoom, of a lens is, as they are used to seeing figures like 3x or 6x zoom when buying compact digital cameras. However, interchangeable lenses don't list zoom or magnification, just focal length. To know what the magnification is, you have to understand focal length and how to convert it. You also have to know the size of your camera's sensor.

Instructions

Difficulty: Moderate

Step 1

Check the focal length of the lens. Focal length measures the distance of the lens to the distance of the film, or digital sensor, when the lens is focused on infinity.

Step 2

Divide the maximum focal length of the lens by 50. A 50mm lens is a 1x magnification with 35mm film because it takes an image that is the same size as 35mm film (24 x 36mm).

Step 3

Multiply the number by 1.5 for Nikon, Canon, Pentax and Sony SLRs; and by 2 for Olympus and Panasonic SLRs. The image sensors of digital SLRs are usually smaller than 35mm film, so they have a "cropped magnification factor" that needs to be added to the magnification number of the focal length.

Tips & Warnings

Some SLRs have "full frame sensors," meaning the sensor is the same size as 35mm film. Some models with full frame sensors are the Nikon D700, D3 and D3x, Canon 5D and 1Ds, and Sony A900. With these cameras, magnification is derived by taking the maximum focal length of the lens and dividing by 50.

Zoom is different than magnification. Zoom is simply a measure of the difference between the widest angle of a zoom lens and the largest telephoto value. For instance, a 5x zoom lens would be a lens with a 28mm to 140mm.

Article Written By Candace Horgan

Candace Horgan has worked as a freelance journalist for more than 12 years. Her work has appeared in various print and online publications, including the "Denver Post" and "Mix." Horgan holds a Bachelor of Arts in English and history.

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