I Want to Destroy a Myth

I started kicking the tires on digital cameras in 1996 soon after I retired. There was new terminology to understand, but the principles of photography that I had learned from many years of using a 35 mm SLR still applied. One new thing that I learned about was the crop factor. So far the sensor in digital cameras is smaller than the frame produced by a 35 mm camera. My Nikon D70 has a crop factor of 1.5. I read reviews of the camera over and over again that said the 18-70 mm lens that comes with the D70 kit is equivalent to a 27-105 mm lens in 35mm terms. These numbers were obtained by multiplying the lens specifications by 1.5, the crop factor. Well folks this just ain’t so. The lens has a range of 18-70 mm period.

Actually the D70 produces a picture that is roughly 2/3 the size of a 35 mm frame. 2/3 is the reciprocal of the 1.5 crop factor. Where the confusion on lens equivalents comes in is the angle of view of the lens on the digital camera is reduced to that of the longer lens, but the perspective of the lens remains the same as when the lens is on a 35 mm camera. Perspective refers to how lines converge when viewed through a lens. When I started out with SLR photography back in the 1970’s, most SLR’s came with what was called a normal lens which was a 50 mm lens. Normal meant that the perspective of the lens was approximately the same as what the human eye sees. Wide angle lenses produce a markedly different prospective. If you try to take a portrait of a subject and move in close enough to fill the frame with a wide angle lens, you will end up with a face with very distorted features. To fill the frame with a normal lens you can get a satisfactory portrait, but you have to move very close to your subject. A telephoto lens in the 100-135 mm range allows you to stand back a comfortable distance from the subject, fill the frame and maintain correct perspective of the facial features.

Perspective is a difficult term to explain. I regularly check the Digital Photography Review website. It is a good source for keeping up with digital photography news and user forums. It also has a glossary of photographic terms. This article on Perspective does a very good job of defining and illustrating the term.

When I look at the exif data for my pictures in the Nikon View software supplied by Nikon, the focal length of the lens is shown as the actual focal length of the lens as it should be. I am using Smugmug for my online photo sharing. When I check the exif data in Smugmug, they are perpetuating the 35 mm equivalent nonsense. I think I should send them an email and ask them to drop the 35 mm equivalent information.