Demystifying lenses

When a good friend of mine recently purchased an inexpensive digital SLR, I knew that something fundamental in the fabric of space and time had changed: This is the guy who always used a point-and-shoot camera and never would have considered a film SLR.

So what has changed? To be honest, I'm not sure. Perhaps it's that digital SLRs are a lot easier to use and often require less effort to take better pictures than their film cousins. Whatever the explanation, a lot of people are making the switch to digital SLRs these days.

But no matter how easy-to-use digital SLRs become, some things won't change much. Take lenses, for example: I get tons of questions about how to purchase and use the myriad lenses available for today's digital SLRs. So this week I thought I'd answer the top questions I get about interchangeable lenses.

What does "focal length" measure?

The technical answer is that the focal length is the distance from the lens to the point at which light passing through the lens is focused, measured in millimeters. In more practical (and understandable) terms, the focal length tells you the magnifying power of the lens. A small focal length of up to about 35mm is considered wide angle; focal lengths between 35mm and 70mm are considered normal, because this range approximates what the human eye sees; and anything beyond 80mm gets into telephoto territory.

What is the difference between a prime and a zoom lens?

You might hear the term prime bandied about when discussing camera lenses. A prime lens is simply any lens that only has a single focal length, whereas a zoom lens has range of focal lengths, such as 12-24mm, 70-300mm, or 18-200mm.

Zoom lenses are obviously more convenient to use, but there are engineering trade-offs involved in a lens that can move through a wide range of focal lengths. Prime lenses perform better--and they are less expensive.

Serious photographers tend to carry a few prime lenses in common focal ranges, but the rest of us get by with one or two zoom lenses that cover the whole gamut.

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Dave Johnson

PC World (US online)

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