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 four 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. The DSLR is now firmly established as a mass-market item with broad appeal. As a result, you can now buy a 10-megapixel Pentax K200D or a new 10MP Nikon D60 for about $700 complete with 18-55mm zoom lens, a 14-megapixel Canon EOS 50D with 17-85mm USM lens for around $1400, and a 12.2MP Canon Rebel XSi with 18-55mm IS lens for a little over $650. No wonder industry gurus are predicting that DSLR sales will continue to increase in 2009 despite the challenging economy.

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, accurate 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 1D Mark III or Nikon D3X (which have a 24x36mm 24-megapixel CMOS image sensor that’s the same size as a 35mm film frame), a high-end enthusiast-aimed model like the Canon EOS 5D Mark II (which also has a 24x36mm CMOS sensor) or 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 to 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 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 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 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 effective focal lengths ranging from 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 at moderate ISO settings in the ISO 100-400 range.

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 (Note: Sony DSLRs accept Konica Minolta and Sony lenses). 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 telephoto zoom will be the equivalent of a whopping 120-300mm long telephoto zoom on your DSLR. The situation is less rosy on the wide-angle end of the spectrum. Due to the aforementioned multiplication factor, your 20mm will give you the 35mm-equivalent coverage of a 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 your APS-C-format DSLR.

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 and Di-II 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 consumer DSLR’s APS-C 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, 18-250mm or 18-270mm lens optimized for the APS digital format is an excellent choice, since it provides the equivalent of a 28-300mm, 28-388mm, and 28-418mm 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. (By the way, don’t try to clean your DSLR’s sensor with lens tissue or a lens brush! It should either be cleaned professionally, (at $30-$50 a pop) or with one of the specialized cleaning devices used for this purpose.

The only downside to a long-range zoom lens like an 18-250mm is that its aperture is usually fairly small (f/6.3 or so) at the longest setting. A nice solution to this dilemma would be getting a Tamron 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, the Tamron 90mm f/2.8 (equal to the classic 135mm tele for 35mm) is another good choice that will get down to 1:1 and is also great for portraiture. When considering mega-zooms in the 18-200 to 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 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 mega-zoom your “universal” zoom lens. Indeed, acquiring a unique mega-zoom like the Tamron AF 18-270 f/3.5-6.3 Di-II VC LD Aspherical MACRO, will not only provide an incredible 28-418mm equivalent range plus macro to 19.3 inches, but also add effective tri-axial Vibration Compensation (VC) to your Nikon or Canon camera, delivering markedly increased sharpness when shooting handheld at long telephoto settings.

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 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 17-50mm with constant f/2.8, the 35mm equivalent of a 26-75mm 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