Kim Komando Q&A: "Buying DLSR camera lenses"

QUESTION: My wife bought me a digital SLR for Christmas. She doesn't know much about lenses, so she didn't buy one. I'm trying to decide what lenses I should buy. I want a general-purpose zoom lens. I don't need a really powerful zoom at this point, though. I'm also thinking about buying a wide-angle lens. Can you help me out?
Steve in Joliet, IL, listening on WLS 890 AM

ANSWER: Digital SLRs are becoming increasingly popular. So I bet more and more people will face this question. And it can be a confusing situation. With a 35mm film camera, choosing a lens is fairly simple. A 35mm lens captures images as you see them. Shorter lenses take wide-angle pictures. For example, 28mm, 24mm and 20mm are common focal lengths for wide-angle lenses.

Lenses with focal lengths greater than 35mm are telephoto lenses. They allow you to take close-up photos without getting closer to your subject.

Unfortunately, choosing lenses for a digital SLR is more complicated. That's because the sensor that captures images is usually smaller than a frame of 35mm film. Lenses, on the other hand, are still labeled for 35mm film cameras. That is, unless you buy a digital-specific lens. I see few of these advertised.

Because digital sensors are smaller than film frames, images are magnified. This is called field of view crop.

Field of view crop is also called the focal length multiplier or multiplication factor. Common values for field of view crop range from 1.3 to 2. You multiply a lens' focal length by the multiplier to determine its 35mm equivalent.

Finding the focal length multiplier can be difficult. For my camera, I found it on a camera review site on the Internet.

Say, for example, your camera sensor's field of view crop is 1.5. A 50mm lens will give you a focal length of 75mm.

With a digital SLR, you'll probably want lenses with shorter focal lengths. This is particularly true when it comes to wide-angle lenses. On a digital, 28mm and 24mm lenses may not produce wide-angle photos.

When you look at lenses, you'll see two different types: fixed-length and zoom. Don't confuse zoom lenses with telephoto lenses. Zoom simply means that the barrel of the lens extends to change the focal length. You'll find both wide-angle and telephoto zoom lenses.

Which is better? A zoom lens will give you more versatility; it is also cheaper than buying many different lenses. But some people believe zoom results are less crisp.

If you're just starting out, I'd recommend a standard zoom lens. About 28-105mm is good for beginners. This allows you to take pictures from a variety of perspectives. Of course, you'll probably miss out on wide-angle shots.

If you like to take landscapes, pick up a wide-angle lens. A focal length shorter than 16mm will give you the fisheye effect, though. This distorts the image, causing it to curve around the edges.

When you pick out a lens, you'll also see f-numbers. They refer to aperture, or how wide the lens opens. Aperture affects shutter speeds and field of view.

F-numbers are confusing; they're really a fraction, but rarely expressed that way. You'll see lenses labeled f/2.8-4, for example. This means it has aperture openings ranging from 1/2.8 to 4.

A large f-number gives you a wide depth of field. Shutter speeds are slower, as the aperture doesn't open very far. Small f-numbers give you a shallow field of view and fast shutter times.

Ideally, the lens should have a wide range when it comes to f-numbers. But it is more important to have a lens with a small f-number (for example 1/1.4 as opposed to 1/2.8). The smallest f-number is often referred to as the lens' speed. That's because it will dictate the fastest shutter speed you can get with the lens.

And finally, make sure the lens will fit your camera. Manufacturers such as Tamron and Sigma make lenses that fit other manufacturers' cameras. If you buy a third-party lens, make sure it is compatible.

I would spend a little more for a lens made by the camera manufacturer. You shouldn't have compatibility issues, and all the features should work. And, if you have problems, you only have to deal with one company.

If you're thinking about upgrading to a digital SLR, check out my buying guide.

You may be able to use lenses from your old film SLR. I have a tip that covers what you need to know.

Still using a point-and-shoot camera? You can also add lenses to it.