Onlin [e] Guid [e]

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.

What's the Relevance of f-Numbers on Zoom Lenses?

All camera lenses have a maximum aperture setting--in other words, how big an opening the lens can make to admit light during exposure. The smaller the number, the larger the opening will be.

Engineering compromises mean that many zoom lenses can't open as wide as you might like. My nifty 18-200mm zoom, for example, offers enough wide- and telephoto oomph to cover 90 percent of the photographic situations I usually encounter. But set to wide angle, it has an f-number of f/3.5. When I zoom all the way to 200mm, it degrades to f/5.6. Compare that to some 200mm prime lenses that can open up to f/20, and you can see that there's a lot less light available to shoot pictures with my zoom. That means fast-moving subjects will blur unless I increase the ISO or shoot in the middle of the day when there's plenty of sunlight available.

All things being equal, the lens that offers a bigger aperture (the smallest f-number) is always the better choice--and it will always be more expensive.

Is the Diameter of the Lens Important?

Not especially.

Some people are surprised to find that there isn't a standard diameter among interchangeable camera lenses. My 18-200mm lens has a diameter of 72mm, for instance, while my 80-400mm lens has a 77mm diameter. Generally, telephoto lenses need more glass to be able to collect more light. The size of any given lens is the result of many design decisions, however, and not something to consider in your buying criteria.

Of course, that means you generally can't share the same set of screw-on filters among several lenses. You might want to see if there's a step-up or step-down adapter available for your lens that will let you attach a different-sized filter. Beware of step-down adapters, though, since they can cause vignetting--if the filter is smaller than the lens you're attaching it to, it can produce shadows in the corners of the picture that you'll have to crop away.

What Is Image Stabilization?

Some premium lenses come with built in stabilization that allows you to freeze the action as if you were shooting with a faster shutter speed than you really are. This feature is great for telephoto lenses with mediocre f-numbers in low light, for example, or for shooting pictures without a tripod.

Image-stabilized lenses used to cost a fortune, but these days they're appearing at a relative bargain. But keep in mind that image stabilization isn't the cure for all your blurriness issues. An image-stabilized lens won't be able to freeze the action if the subject itself is moving, for instance. But I am a big believer, and all of my lenses are now image-stabilized.

As an aside, I've noticed that a few new digital SLRs are hawking image-stabilization in the body, claiming that this makes the lenses cheaper since you only have to buy the stabilization technology once and it's automatically available to all your lenses. The jury is still out on this approach; the longer a lens is, the more powerful the image stabilization motor will need to be, and the stabilization built into these new cameras might not be able to keep up.