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Focal Length Facts—06/28/10

A beginner’s guide to focal lengths and how they affect photographs

This Article Features Photo Zoom

In the good old days there wasn’t much to know about lens focal lengths. A 100mm lens was a 100mm lens; the only thing you needed to know was whether that lens was a telephoto (as it would be on a 35mm film camera), a normal lens (like on a medium-format camera) or a wide angle (as it would be on a 4x5 view camera). Most people quickly learned what the focal lengths represented for their particular camera format. But these days, with so many digital camera sensor sizes and other lens peculiarities, the millimeter measurement of a lens’ focal length tells only part of the story. Many other factors go into determining the effects a particular lens will produce—from magnification factor to zoom range and much more. What follows is a lens focal-length primer, with several key points that will help you understand focal lengths as they relate to your camera and to the pictures you use them to make.

1. The focal length of a lens is the measurement of the distance from the center of a lens to the point at which its image is focused. The longer the distance, the longer the lens. The longer the lens, the more telephoto it’s considered. The shorter that distance, the wider the angle of view. The most common measurement of lens focal lengths is in millimeters, although some old-school photographers still refer to large format lenses in inches. (If you’re interested, you roughly can convert inches to millimeters by using a 1:25 ratio. An eight-inch lens approximates a 200mm lens.)

2. A full-frame digital sensor is equivalent in size to a 35mm film frame, making this the standard focal length baseline that today’s lenses are measured against. Smaller formats often have shorter focal lengths (say, a 10mm wide angle that seems unbelievably short) but in “equivalent” terms they’re much more akin to more familiar focal lengths (say, a 17mm lens that is the equivalent to a 28mm lens in 35mm equivalent terms).

3. Lenses have various classifications based on focal length and the field of view they provide. A wide-angle lens provides a much greater field of view, and is generally considered to be any lens 40mm or shorter (again, in full frame equivalent terms). A normal lens—on a full frame DSLR—is the distinction given to any lens that ranges roughly from 40mm to 65mm or so. These lenses are “normal” because they provide an angle of view that approximates that of the human eye. Telephoto lenses on full-frame cameras usually are lenses longer than 70mm, and they range upwards of 300, 600 and even 1000mm. The longer the telephoto, the narrower the angle of view and the greater the magnifying power it provides. That’s why wildlife and sports photographers so often use 600mm and longer telephotos. Most amateur users, though, tend to top out around 300mm lenses for most uses.

4. The effect that a smaller sensor has on a lens of a given focal length is called a crop factor or magnification factor. This is because a smaller sensor produces a similar effect to cropping a larger sensor—effectively magnifying the image. Some photographers object to this narrowing of the angle of view because they’re used to a lens of a certain focal length producing a certain corresponding angle of view. Other photographers actually prefer a crop factor because it has the effect of making a long telephoto lens behave like an even longer telephoto lens. If you photograph sports or wildlife, a 400mm lens placed on a camera with a 1.5 magnification factor would behave more like a 600mm lens. That’s a heck of a telephoto bonus.

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