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In the not-too-distant past, digital SLRs (DSLRs) were expensive professional tools selling for $5000 and up. About 5 years ago, even the so called “prosumer” 6-megapixel DSLRs purchased by early adopters and serious enthusiasts were selling at prices ranging from $1,500 to $2000, putting them out of the range of most average consumers. Today the situation is vastly different. The DSLR is now firmly established as a mass-market item with broad appeal. Most entry-level models are priced at under $1,000, and a few have street prices of around $600. No wonder industry gurus are predicting that DSLR sales will continue to increase in 2011 despite the challenging economy.

One of the main attractions of a digital single-lens reflex (DSLR) camera is, of course, the ability to use interchangeable lenses, but which lenses should you choose from the hundreds, if not thousands, of lenses on the market? Can you use your old 35mm SLR lenses on your new hi-res digital masterpiece? Which focal-length ranges will give you the maximum picture-taking bang for your buck? Should you favor certain focal lengths based on your style of shooting? Do single-focal-length lenses have any advantages over zooms? Do you need to buy special “digital lenses” or a separate macro lens if your zoom has a macro setting? Read on, and you’ll find concise, accurate answers to these and many other DSLR-related lens questions.

Before we get into picking lenses, it helps to understand a few technical things about your shiny new DSLR. Unless you’re shooting with a $7000 high-end pro SLR like a Canon EOS 1D Mark III or Nikon D3X (which have a full-frame 24x36mm 20+-megapixel image sensors) a high-end enthusiast-aimed model like the Canon EOS 5D Mark II or Nikon D700 (which also have full-frame sensors) or an Olympus E-series DSLR (which uses a smaller Four Thirds-format CCD), you can safely assume that your consumer DSLR uses a CCD or CMOS image sensor that measures about 15.6x22.3mm, often referred to APS-C format or as a crop-sensor camera. This format has the same 2:3 aspect ratio as 35mm—when positioned horizontally it’s 1-1/2 times as wide as it is high and will therefore fit a 4x6-inch print perfectly.

What’s more important in terms of selecting lenses is that the diagonal dimension of the 35mm format (43.3mm) is 1.5 to 1.6 times as large as the diagonal of your consumer DSLR’s format, which measures between 27 and 28mm. The diagonal is important because it represents the diameter of the image circle the lens must provide to produce a sharp image over the entire frame. In practical terms this means that you have to multiply the focal length of any given lens you use on your DSLR by about 1.55 to calculate its “35mm equivalent focal length.” Examples: A 35mm lens that works as a moderate wide-angle on your 35mm SLR actually works like a 55.5mm longish normal lens on a DSLR with a 1.5X focal length multiplication factor, and a 50mm normal lens is functionally equivalent to an 80mm medium telephoto on a DSLR with a 1.6X multiplication factor. By the way, you don’t have to calculate these factors for yourself—they’re always included in your DSLRs instruction manual or your lens manual under “specifications.”

Most people who buy a new DSLR tend to get it with the standard “normal zoom” lens. Typically it’s an 18-55mm. It’s also of moderate speed, usually around f/3.5 at the wide-angle setting and f/5.6 at the telephoto end. No doubt these are useful general-purpose lenses for getting started, but they can’t do everything. They don’t provide the ultra-wide-angle settings you may need to record scenic vistas or get the whole family into a holiday shot in a small dining room. At their telephoto end, they don’t provide ideal focal lengths for portraiture (which really begins at effective focal lengths ranging from 90-105mm), nor do they let you zoom long enough to capture most sports action or wildlife subjects. And of course, they don’t provide fast apertures in the f/2.8 range that you may need when shooting non-flash pictures in low light at lower ISO settings, and they don’t provide the shallow depth of field you may want for dramatic pictorial effects.

All-In-One Zooms

Okay, now that you’ve got a handle on some of the parameters, which focal lengths and lens types should you go for? Well, obviously much depends on what you’ve already got in your optical arsenal, but on the all-in-one side, an 18-200mm or a unique new lens like the new Tamron 18-270mm Di II VC PZD 15X zoom that’s optimized for the APS digital format is an excellent choice, since it provides the equivalent of a 28-300mm, or 28-419mm respectively! This has the tremendous advantage of allowing you to leave the lens on the camera as a “universal lens” thereby minimizing the possibility of dust and debris getting on your image sensor when you change lenses—this can result in spots on your pictures. With an all-in-one zoom, you always have both wide angle and telephoto capabilities at your beck and call, eliminating the need to carry to lenses, or miss a shot by not having the right lens on hand.

The only real downside to an ultra-long-range zoom lens like an 18-270mm is that its aperture is usually fairly modest (f/6.3 or so) at the longest focal-length setting. However, this is less important these days because the current crop of DSLRs delivers such outstanding performance at ISO settings up to ISO 6400!

When considering all-in-one zooms in the 18-200 or 18-270mm range, do not be concerned that its focal-length range may overlap or duplicate your normal zoom. Restricting your lens choices based on the fact that you already own a standard kit lens that actually cost you $100 or less is not a great idea. In fact, if you’re in the market for a new DSLR, it may be advantageous to buy the body only and make the all-in-one your “universal” zoom lens. Indeed, acquiring a unique all-in-one like the new ultra-compact Tamron AF 18-270 Di-II VC PZD not only delivers an incredible 28-419mm equivalent range plus macro down to 19.3 inches at all focal lengths, but also provides second-generation tri-axial Vibration Compensation (VC) to your Nikon or Canon camera for markedly increased sharpness when shooting handheld at long telephoto settings, and fast, accurate, quiet PZD (piezoelectric) motor for improved AF performance. Note: This lens is also available in Sony mount without VC because Sony DSLRs have in-body image stabilization.

Wide Zooms

What about the wide-angle end of the spectrum? Well, if you want to go really wide, which is a good idea if you shoot a lot of landscapes and interiors, by all means consider the unique Tamron 10-24mm f/3.5-4.5 which covers an unprecedented ultra-wide to semi-wide 16-37mm equivalent. While a 10-24mm may not sound like a large range, it really is quite spectacular in terms of angular coverage as you will quickly see if you put it on your DSLR and zoom it while you look through the viewfinder.

Such lenses are also usually of moderate aperture, so if you need something faster, and still pretty wide, take a look at a Tamron’s 17-50mm with constant f/2.8, the 35mm equivalent of a 26-75mm zoom. It’s a great general-purpose lens for low-light shooters that’s used by many pros.

Fast Zooms

And speaking of fast lenses, the Tamron 90mm f/2.8 (equal to the classic 135mm tele for 35mm) and the remarkable new Tamron 60mm f/2.0 Di II LD (93mm equivalent) are excellent choices that provide a wide open aperture and will also get down to 1:1 macro for close-up photography. These lenses are also great for portraiture.

Telephoto Zooms

Telephoto zoom lenses in the 70-300mm and 200-500mm ranges are also excellent choices for anyone else who requires the extended reach needed to capture distant detail and bring it up close. Lenses in this category also provide somewhat wider apertures at the long telephoto settings, a big plus when shooting in low light, or for creating dramatic pictorial effects when shooting wide open. An excellent example is the recently introduced Tamron SP 70-300mm f/4-5.6 Di VC USD that is used by many on high-end full-frame cameras, and provides an impressive 109-465mm equivalent coverage when used on APS-C-format consumer DSLRs. It also incorporates built-in VC, an advanced image stabilization system that provides up to a 4-stop anti-shake advantage that’s crucial when shooting handheld at long distances. Surprisingly compact for a fast high-performance long telephoto zoom, its minimum length is only 5.6-inches, and it weight in at under 27 ounces. By the way, SP stands for Superior Performance, and this lens delivers. And if you’ve always had a hankering for shooting birds on the wing, wolves in the wild, or baseball from the bleachers, just try putting a really long tele zoom like the Tamron SP AF 200-500mm f/5.6 Di LD IF on that consumer DSLR of yours. Now look through the finder as you zoom, and imagine what the 35mm equivalent of a 300-750mm can do for your pictures! That’s what I did, and I was hooked.

The secret message here is that, while it helps to be technically savvy when choosing lenses for your DSLR, it should also be a fun, hands-on process. You wouldn’t buy a car without taking a test drive, and there’s really no substitute for getting down to your camera dealer and trying these lenses on your DSLR or looking through your buddy’s camera to see what really works best for you. When you get right down to it, that’s true joy of owning a DSLR in the first place—it’s all about the lenses!

Images taken with a few different Tamron Lenses