Conversion of a Digital Lens Length to an SLR Lens Length
Digital cameras can often use lenses designed for film cameras made by the same manufacturer. The focal length, or magnification, of a lens changes when attached to a digital camera. This is due to the way in which digital cameras record images.
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Focal Length
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Camera lens designs use circular lenses, but the images recorded are all rectangular. This happens because the film or digital sensor is rectangular and the lens projects a circular image with a diameter matching the diagonal length of the sensor. Light must be in focus when it strikes the sensor. The focal length of a lens is the distance behind the lens that the image appears in sharp focus. So a 50mm lens focuses the image 50 mm behind the lens, and a 400mm lens focuses it 400 mm behind the lens.
Film Size
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Traditional celluloid film is 35 mm wide, with a diagonal measurement of 43.3 mm. This was the accepted standard for all small SLR film cameras, and lens focal lengths were calculated on the assumption that every lens needed to produce a circle of light 43.3 mm in diameter so it would cover the entire 35mm film.
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Digital Sensor Size
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Digital sensors are smaller than a sheet of traditional film. Digital SLR cameras may use 13.5 x 18, 15.7 x 23.5 or 24 x 36 mm sensors. Smaller digital cameras use sensors as small as 3.96 x 5.27 mm.
Why Focal Length Changes
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Imagine the image from a film camera lens being focused on a traditional 43.3-mm-wide film. The image width would match the diagonal length of the film and record a photograph showing everything in the image at a 1:1 ratio. Replacing the film with a smaller target will crop the area of the image falling on the sensor and thus record only part of the scene. The amount of change is known as the "crop factor." Although the field of view is smaller, the image will still fill the entire photograph. This gives the impression that the view had been magnified. Digital sensors are all smaller than 35 mm film, so they all produce a magnified version of the original lens focal length. The degree of magnification is related to the crop factor and is called the "focal length multiplier."
Calculating the Difference
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The crop factor is equal to the focal length multiplier, so a traditional 50mm lens used with a digital sensor with a 1.6 crop factor gives the same field of view as an 80mm lens. This is calculated by multiplying the focal length of the traditional lens by the crop factor; 50 x 1.6 = 80. Technically the focal length doesn't change, only the field of view, but the effect is the same.
Common Crop Factors
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A 36.0 x 24.0 mm sensor has a 1.0 crop factor.
A 28.1 x 18.7 mm sensor has a 1.3 crop factor.
A 23.6 x 15.8 mm sensor has a 1.5 to 1.7 crop factor.
A 17.3 x 13.0 mm sensor has a 12.0 crop factor.
Online focal length calculators make the conversion easy for any crop factor and focal length combination.
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References
- Photo Credit lens image by leafy from Fotolia.com
