The Crop Factor Film Camera Lenses on Digital Cameras

Focal lengths for digital cameras are different from what we became accustomed to with conventional 35mm film cameras. For example, the "normal" focal length for a digital point-and-shoot—normal being the angle or field of view of the human eye— is much shorter than the traditional 50mm lens that was considered normal for 35mm film cameras.

And when used with digital SLRs, the focal length of a film camera's lens will vary depending on the size of an individual digital camera's sensor. This is known as the multiplier or crop factor. The sensors on most digital SLRs—some of the more expensive ones are the exceptions—are smaller than a 35mm film frame. A digital SLR's sensor image might measure approximately 24mm x 16mm while 35mm film measures 36mm x 24mm. So a 35mm camera lens used with a DSLR will give a narrower field of view than on the film camera. While the subject image size will be the same on both cameras, the scene will be cropped on the digital camera, giving the impression that the subject has been magnified.

As an example, if you were photographing a baseball pitcher with a 300mm lens on a digital SLR camera with a sensor smaller than 35mm film, you might almost fill the entire frame with the subject. Using the same lens from the same distance, on a conventional 35mm film SLR camera there would be more space around the principal subject but the actual size of the pitcher in both images would be the same. This is simply because the digital sensor is smaller than the 35mm film frame and the subject fills more of the picture area. The photograph of the snowmobile rider on the previous page illustrates the point.

When selecting lenses for a digital camera keep this in mind and be aware that a 300mm lens, for instance, will give an angle of view about the equivalent of a 450mm lens on a film SLR camera. The crop factor varies with different makes of cameras but is in the 1.5X to 1.6X range. Actually, many photographers like this effect with longer lenses as it gives a tighter shot of the principal subject. Also, with wider lenses, less of the scene will be included. For example, a 28mm film camera lens on a digital SLR will become equivalent to about a 42mm lens.

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