Learning Center Choosing Lenses

In the not-too-distant past, digital SLRs (DSLRs) were expensive professional tools selling for $5000 and up. As little as three years ago, even the so called “prosumer” DSLRs purchased by early adopters and serious photo 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. When Canon broke the $1000 price barrier by bringing out the Canon EOS Digital Rebel in 2003, they created, in effect, a new market category, the consumer DSLR, and other manufacturers followed hot on their heels. You can now buy a Pentax*ist DS or a new Nikon D50 for about $900, an 8-megapixel Canon D Rebel XT for under $1000, and the original Canon Rebel D for under $800, all with normal zoom lens. No wonder industry gurus are hailing 2005 as the year of the DSLR.

One of the main attractions of a single-lens reflex 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 digital marvel? 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 advantage 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 answers to these and many other DSLR 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 high-end pro SLR like a Canon EOS 1DS Mark II (which has a 24x36mm CMOS image sensor that’s the same size as a 35mm film frame) 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, or roughly the same size as C format used on APS film cameras. 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 or 1.6 times as large as the diagonal of your 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 format. In practical terms this means that you have to multiply the focal length of any given lens you use on your DSLR by 1.5 or 1.6 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 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 under “specifications.”

Most people who buy a new DSLR get it with the standard “normal zoom” lens. Typically it’s an 18-55mm or 18-45mm that’s the respective equivalent of a 28-85mm or 28-70mm lens on a 35mm SLR. 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 begin at 100-105mm), 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/1.4 to f/2.8 range that you may need when shooting non-flash pictures in low light.

This brings us to question number one, can you press your 35mm SLR lenses into service on your DSLR? The answer is yes, providing your DSLR uses the same lens mount as your 35mm SLR. If your DSLR has a Canon EF, Konica Minolta Maxxum, Pentax K-AF, or Nikon F mount, in the vast majority of cases your 35mm SLR lenses, camera brand or independently made, will work on your camera. However if you have older non-autofocus lenses in Pentax and Nikon mounts, you’ll have to focus them manually, and there may be other restrictions on metering. The good news here is that the 50mm f/1.4 or f/2 normal lens from your 35mm SLR will now be a good high-speed moderate tele, your 28-80mm normal zoom will be equal to a 45-125mm or thereabouts, and your 80-200mm tele zoom will be the equivalent of a whopping 120-300mm long tele zoom on your DSLR. The situation is less rosy on the wide-angle end of the spectrum. Due to the multiplication factor, your 20mm will give you the35mm-equivelent coverage of lens of 30mm or so lens (a focal length already covered by your DSLR’s normal zoom) and even your expensive 14mm ultra-wide will only equal a 21mm on 35mm.

What about the image quality of 35mm lenses on DSLRs? Much ink has been spilled on this subject, but there are three key factors to consider—the angle at which the lens’s light rays strike the sensor, antireflection coatings, and optimum format coverage. Much has been made of the fact that the edge light rays transmitted by many ultra-wide-angle lenses designed for 35mm cameras strike the image sensor at very oblique angles. This was perfectly acceptable when film was the capture medium, but it can cause image degradation at the edges and corners of the field when CCD and CMOS sensors are used. This effect can certainly be shown in side-by-side comparison pictures shot with 35mm and digitally optimized ultra-wide-angle lenses on professional, 24x36mm-format DSLRs, but it is far less pronounced with consumer DSLRs using APS-size image sensors. It is also true that digitally optimized lenses made by camera makers and leading independents (e.g. Tamron Di lenses) use improved coatings designed to reduce flare, which is more likely to affect pictures taken with digital imaging systems. However, the most important lens factor affecting DSLR image quality is optimum format coverage. In general, lenses specifically designed to cover the APS-C format have an inherent image-quality advantage over lenses designed to cover the larger 35mm format because their coverage precisely matches the frame. The bottom line: By all means use your present 35mm SLR lenses on your DSLR, but when buying new lenses for your DSLR, favor those optimized for digital photography. Furthermore, unless you plan to use these new lenses on both your 35mm SLR and your DSLR, you’re better off buying lenses designed to cover your DSLR’s format.

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 telephoto side, an 18-200mm optimized for the APS digital format is an excellent choice, since it provides the equivalent of a 28-300mm in 35mm. This has the advantage of allowing you to leave the lens on the camera as a “universal lens” thereby minimizing the possibility of dust getting on your image sensor when you change lenses—this can result in spots on your pictures. Don’t try to clean your DSLR’s sensor with lens tissue or a lens brush! It should really be cleaned professionally, and that costs $30-50 a pop! The only downside to a long-range zoom lens like an 18-200 is that its aperture is usually fairly small (f/6.3 or so) at the 200mm setting. A nice solution to this dilemma would be getting a 180mmm f/3.5 macro lens, which would provide a long tele setting (270mm in 35mm terms) plus the ability to focus down to 1:1 (life-size). And speaking of fast macro lenses, a 90mm f/2.8 (equal to the classic 135mm tele for 35mm) is another good choice that will also get down to 1:1 and is also great for portraiture. As for the 18-200 or any other long-range zoom, 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 cost you $100 or less is not a great idea. In fact, if you’re in the market for a new DSLR, buy the body only and make the 18-200 your “universal” zoom lens!

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 an 11-18mm zoom lens, the equivalent of a 17-28mm lens in 35mm. While 11-18mm may not sound like a large range, it really is 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 f/4.5-5.6 aperture, so if you need something faster, and still pretty wide, take a look at a 17-35mm f/2.8-4.0, the 35mm equivalent of a 26-53mm zoom, or a 28-75mm with constant f/2.8 aperture, the equivalent of a 42-113mm. The latter zoom is not all that wide or long, but it’s a great general-purpose lens for low-light shooters that’s used by many pros.

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. 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 a 200-500mm zoom on that DSLR of yours, looking through the finder as you zoom, and imagining what the 35mm equivalent of a 300-750mm can do for your pictures. That’s what I did, and I was hooked. When you get right down to it, that’s true joy of owning a DSLR in the first place—it’s all in the lenses!

 

Images taken with a few different Tamron Lenses

AF28-300mm at 200mm

      
AF28-300mm at 28mm        AF28-300mm at 70mm         AF28-300mm at 100mm

SP AF11-18mm at 11mm

SP AF11-18mm at 18mm

AF28-300mm at 300mm

SP AF11-18mm at 11mm

SP AF11-18mm at 11mm

AF28-300mm at 300mm

SP AF11-18mm at 18mm

SP AF90mm 1:1 Macro

SP AF180mm 1:1 Macro

SP AF180mm 1:1 Macro

SP AF180mm 1:1 Macro