December 14 2007
Most Digital SLR camera lenses have a crop factor due to a smaller size electronic sensor compared to 35mm film cameras, producing an apparent increase in focal length.
The crop factor is the ratio of the sensor size, often referred to as the APS size, to that of 35mm film, for most DSLRs about 1.5; although for Olympus with a smaller sensor, it is two.
This produces an apparent magnification effect where the resultant image appears to have been taken with a longer focal length lens. A camera with a crop factor of 1.5 using a 200mm lens produces the effect of a 300mm lens.
Focal length is the distance from the lens to the plane where the image is recorded, either the film strip or digital sensor. This distance is the same for film and digital SLRs so the image is not really 1.5 times bigger as first appears.
What is different is the angle of view. As the digital sensor is in the same place in the camera body as the film and is smaller, it captures less of the image coming through the lens. Using less of the image is similarly to cropping it in a digital or chemical darkroom. Hence the name crop factor, while sometimes wrongly referred to as magnification factor.
The normal 200mm lens in the example above is still only a normal 200mm lens on a DSLR, but with “blinkers” as only part of its image is used. It appears bigger because when printed on standard print sizes because it is enlarged more than a35mm film negative to fit the same paper size.
As manufacturers continually pack more pixels into the same size sensor, the issue of image quality is swinging the way of the digital camera over its film cousin.
There is one quality by product of the crop factor where only part of the image is used. The weak spot of most lenses as far as distortion goes is around the outer edges. Most DSLRs crop this out leaving the sweet spot in the middle of the lens. When composing with the DSLR the worst bits of the lens are cropped out in the original composition.
Not so Wide Angle
The real downside is for situations where a wide angle of view is required; say for a sweeping landscape or a large group of people. A favorite focal length for a film SLR wide is 28mm. However, the crop factor gives it the reduced angle of view of a 42mm focal length lens.
Now a much shorter focal length is required to give the DSLR that wide view, about 18mm. This is the reason why the standard zoom lenses sold with DSLRs starting their spread of focal lengths at 18mm.
The loss of viewing angle accounts for the push by some photographers for full frame digital cameras with sensors the same size as the old 35mm film. This enables their shorter focal length lenses to have their full angle of view.
The new lenses built specifically for normal DSLRs still have the same focal length. However, the manufacturers have reduced the diameter of the lens elements to save on cost, weight, and size. The image they produce is only big enough for the small size digital sensor. On a full size camera, the corners of their images appear rounded.
This is the same picture, first at full size, and then cropped with a 1.5 crop factor showing the narrower angle of view of the DSLR and the true size of the image. Then enlarging the cropped image to match the size of the original makes it look as if it came from a longer focal length lens.