Canon Digital Camera

Camera Works: crop factors, 35 mm equivalents, and digital lenses

When you're shopping for digital cameras today, you'll see that they advertise lens focal lengths with numbers in "35 mm equivalent." Other specs might talk about a "crop factor" or "focal length multiplier" of 1.5 or 1.6 or 2.0 or more. What do those mean, and why didn't we hear about them back in the film days? Learning about 35 mm film will help us find out.

It's amazing how long 35 mm film (known as 135 film for still cameras) has been around. William Dickson, Thomas Edison, and George Eastman established its dimensions and specifications, right down to the distance between the sprocket holes, back in 1892—but that was for movie film. A number of still camera makers had the clever idea of using the same film stock for still pictures in the early 20th century, and it really took off among professional and enthusiast photographers in the 1920s, when Leica brought out its first tough little rangefinder cameras and excellent lenses.

What is a full frame?

35 mm film gets its name honestly: the strips are 35 mm (about 1.4 inches) across, including the sprocket borders. In movie cameras it runs vertically, and the individual movie frames are 22 mm wide (leaving 13 mm for the margins and sprockets) by 16 mm high. But still cameras like the Leica M series (introduced in 1954) and Nikon F SLRs (first appearing in 1959) ran the film horizontally, as well as doubling the size of the image, making it 24 mm high by 36 mm wide. Today we call a film frame or digital sensor of that size "full frame."

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