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Field of View Crop
In Digital SLRs
by Jim Doty, Jr.

The sensors in most digital SLRs are smaller than 35mm film. When a lens that was originally designed for a 35mm film camera is put on a digital camera body, the field of view is different. The image created looks like the focal length of the lens has been magnified so this is sometimes called the "magnification" factor.  A 100mm lens on a digital camera with a 1.6x magnification factor produces an image that looks like a 160mm lens on a 35mm film camera.

In actuality, the focal length of the lens has not been optically magnified at all. The smaller digital sensor just crops more deeply into the image circle that is projected by the lens. It is more accurate to refer to this as a "field of view" (FOV) crop. The field of view crop for most digital SLRs ranges from 1.3x to 2x, depending on the camera brand and model.

All lenses project a circular image but the film only "sees" and records part of this image. Film cameras have a rectangular or square window to the film plane so you end up with rectangular or square slides or negatives.  Lenses must be designed with a large enough image circle to cover the film format. You can't use most 35mm lenses on a large format camera because the image circle isn't large enough.


Illustration A

In Illustration A, the image circle is being projected by a 400mm lens that is designed to "cover" the film window of a 35mm camera body. The larger rectangle represents the 35mm film window and the resulting photo on film. If you put the same 400mm lens on a digital camera with a "full frame" sensor (like the Canon 1Ds and 1Ds II), the image would be the same as on a 35mm film camera.

When you put the same 400mm lens on a digital SLR with a field of view crop, the image circle projected by the lens is exactly the same size, BUT the sensor records a smaller area of the image. In the illustration above, the smaller rectangle represents a digital sensor with a 1.6x FOV crop. Since the  digital sensor records a smaller portion of the same image circle, the caribou looks closer. It looks like you took the photo with a 640mm lens (Illustration B). This is what gives rise to the phrase magnification factor.

Illustration B

400mm lens on a 35mm film SLR

400mm lens on a D-SLR with a 1.6x FOV Crop

All of this is good news for wildlife photographers or any one else using long lenses since all of their telephoto lenses "act" like they are longer due to the field of view crop. It is like having an instant teleconverter but without any loss of image quality or loss of light. And you don't have to spend megabucks for a bigger, longer, and heavier lens.

It is bad news for photographers when they use wide angle lenses because the field of view crop throws away much of the image. A 28mm wide angle lens on a digital SLR with a 1.6X FOV crop has the same field of view as a 45mm lens on a 35mm film camera. The wide angle lens now acts like a normal lens. Manufacturers are coming out with a variety of wider angle lenses for use on digital cameras. Some of these are "digital only" lenses.

Digital lenses are being produced for digital cameras with a field of view crop. As you can see from Illustration A, the image circle doesn't have to be nearly as big to cover the digital sensor so these digital lenses can be much smaller in size. "Digital only" lenses are optical lenses like any other lens, they just have a smaller image circle.

There are two reasons "digital only" lenses won't work on film SLRs. One is that the image circle won't cover the 35mm film format. The second is you could damage the lens and/or the camera.

The newer digital SLRs have a smaller mirror so some of the new digital lenses protrude farther into the mirror box. If you could mount some of these lenses on a film camera body (or even an older model digital SLR), you would risk damage from the larger mirror striking the back of the lens. To prevent this from happening, these lenses are designed so they won't  mount on the cameras they could damage.  In some cases, there are do-it-yourself modifications to get some of the newer lenses to fit on some older digital cameras. The most notable example is to saw off part of the rear mount of a Canon EF-S 10-22mm lens so it will fit on a Canon 10D body. You do this at your own risk.

Why would someone do this? Because the digital only EF-S 10-22mm lens won't mount on a Canon 10D, it is only for the Canon Digital Rebel and the Canon 20D. Canon 10D owners want the truly wide angle of view that this lens provides (the equivalent of 16-35mm in 35mm terms). Since the mirror barely clears the back of the lens, some 10D owners modify the lens to fit. As I said, you do this at your own risk.

If you have or will soon get a digital SLR with a FOV crop, enjoy the larger images from your telephotos lenses which have all grown longer. And save your money for one of the new wider angle lenses so you can once again have a truly wide angle field of view.

April 10, 2005

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