Photo Credit: Michel Tcherevkoff

Step 1: Let your pictures be your guide.

The first step is to figure out what kind of photographs you’d like to take, and then research the lenses that suit those shots. Generally, lenses fall into four categories: wide-angle, standard, telephoto and super-telephoto. The exact range varies (especially with digital cameras), but wide-angle lenses offer the widest “field of view,” or amount of the subject that’s in the frame, while super-telephotos only capture a small, magnified area of the image.
Years ago, zoom lenses—those that change focal lengths with the twist of a dial—were inferior to single-focal-length lenses, called “primes.” While a good prime is still likely to produce better image quality than a zoom, the difference is much less than with earlier zooms. Many photographers start off with a zoom that goes from wide-angle to telephoto, and they add lenses from there.
    There are also lenses for specialty photography, such as macro lenses and tilt-shift lenses. But if you don’t intend to take pictures of the wings of a bee (macro lens) or of tall buildings (tilt-shift lens), you won’t need lenses like these. For the new shooter, having a lens with a nice zoom range and clear image quality is more important than having a “fast” aperture and fancy lens coatings. That’s why most digital camera systems priced less than $1,000 come in a version with a “kit” lens. These starter lenses, often with inexpensive zooms, are good values for the new photographer, and are great tools to start shooting with. Don’t splurge on a better lens until you’ve played with one of these starter lenses and gotten a feel for the type of photography you like.

Step 2: Learn how a lens operates—and how that affects the price.

While today’s lenses are complicated structures of glass, electronics and high-impact materials, at their heart they’re basically tubes that refract light, pass it through to the sensor or to the film, and thus turn it into an image. Inside the tubes are a number of glass elements that move the light to the focal point. In a very simple lens, the distance between the rear element and the recording surface is recorded in millimeters. The higher the number, the longer the distance between rear element and recording surface, and the “longer” the lens is said to be. For example, a 20mm lens would provide a wider angle of view than a 200mm lens.
    In practice, the internal glass optics help reduce the distance between the back of the lens and the camera’s recording plane, allowing cameras to be more compact. In zoom lenses, the distance between the internal elements moves to create different effective focal lengths. Each piece of glass affects image quality, so lenses that use better-quality internal elements are more expensive than ones that use lower-quality glass. (That’s the reason for many of the additional letters in a lens’s name; they indicate the quality of the internal construction. There is no consistent “code” to the letters. Sometimes companies seem to make them up at random.)   
    The maximum aperture of the lens, or the maximum amount of light the lens lets in, also affects the price. Letting in more light requires a more sophisticated design, which increases both the cost and the size of the lens. For example, an 85mm f/2.8 is less expansive than an 85mm f/1.2. In zoom lenses it’s harder to keep the f-stop the same across the whole zoom range, so one often sees lenses described as f/2.8-4.5, which means that at the widest setting the lens has a maximum aperture of f/2.8 while at its most telephoto setting the maximum aperture is f/4.5. For most consumer photographers, this isn’t an issue, but professional photographers want to have the maximum level of control over depth-of-field, so they will pay extra to have a lens with a wider f-stop across the zoom range.

Step 3: Find the lens that's right for you.

You’ve researched which types of lenses are appropriate for the pictures you’d like to take, and you’ve learned how certain features affect price. Now it’s time to make some decisions: Decide on focal length. Do you like wide angles or tight close-ups? If you want to shoot using a variety of focal lengths and only plan to have one lens, look for a lens with a wide zoom range. Next, decide if it’s worth spending the money on a lens with a wide aperture. The tradeoff is between depth-of-field and cost; more money buys a lens that lets in more light, and thus gives you more control over how much of the image is in focus and where the out-of-focus area begins.
    Once you’ve narrowed your choices, compare prices. Keep in mind that higher priced lenses will have tighter quality controls, but might not pair well with certain shooting styles. For example, the price difference between a 70-210mm f/2.8 and a 70-210mm f/4.5-5.6 could be thousands of dollars, and the more expensive lens will also be bigger and heavier.
Bonus features like image stabilization are sometimes worth the extra money. Image stabilization uses internal gyroscopes to reduce the blurriness associated with moving a camera while taking a photograph in low light. This feature is easier to add to a lens than it is to manufacture the same optics with a larger f-stop, so many consumer and prosumer lenses are showing up with both a variable f-stop and image stabilization, allowing a photographer to hand-hold a shot even when light drops considerably. Check if one of your choices has a vibration reduction system, often bearing the letters VR or IS. A company will often remove the non-image stabilized lens from production when the IS version is available, only adding about $100 to the price.
    If your camera uses a digital sensor that’s smaller than a full-35mm frame, (the Nikon line or the Canon 30D, for example) you might also consider buying a lens designed specifically for your sensor size. Why? The optics in most of today’s lenses were designed for the image area of film, which is larger than many consumer digital camera sensors. Because the lens is designed to focus correctly on the surface of the recording material it’s designed for, the digital-sensor-only lenses focus all of the light directly on the CCD or CMOS sensor, without any wasted imaging area or degraded image quality. Because they don’t need to cover as much imaging space, the lens can be smaller, lighter and more compact. The downside to digital-specific lenses is that they won’t work properly with full-frame sensors. Digital-specific lenses are available from all the major manufacturers, so you simply pick the model that’s right for your camera.