Choosing lenses for your new high-resolution digital SLR:
If you’re a critical shooter who demands the ultimate image quality that your shiny new 12 or 14 MP DSLR offers, having lenses that deliver superior image quality is essential.
by Jason Schneider |
If you take a look at today’s digital SLR market, you could easily conclude that camera makers are in a ferocious megapixel race to produce cameras that equal or surpass their competitors’ performance specs. Every new model seems to deliver more megapixels than the last, and there are now a slew of 10, 12, and 14-megapixel models squarely aimed at the red hot consumer DSLR market, not to mention more expensive pro models that deliver 20 megapixels and more. Like the horsepower rating of cars, which supposedly equates to faster acceleration and a higher top speed, more megapixels generally equates to higher resolution, the technical term that establishes the baseline for sharpness and image quality. And while it’s true that larger pixels are inherently more efficient at gathering light (i.e. they have a higher signal-to-noise ratio or SNR and capture more information), the designers of image-processing firmware and software have devised ingenious methods of enhancing the efficiency of smaller, more densely packed pixels. In other words, you really do get most, if not all, the benefits of having more pixels in the latest DSLR models. Calculating image quality: It’s area that counts! It is important to understand that a 10- or 12MP camera does not deliver twice the image quality of a 5- or 6MP model, because the quality calculation is based on the area (that is, pixel density). So, in order to double the image quality of a 5MP camera you would need to shoot with a 20MP (5x4) camera. Of course, you don’t need to double the image quality to be able to see a noticeable difference when making a big print—if you closely examined matched 16x20-inch portraits made at maximum quality from a 8MP and 14MP cameras, the 14MP print would show more detail in the eyes, skin texture and tiny facial hairs. This, of course, assumes that the lens is capable of resolving all the fine detail that the sensor is capable of capturing. If it can’t, you don’t get all the benefits of all those extra pixels. In short, as megapixels increase, selecting high quality lenses for your DSLR becomes more critical than ever. A pro or serious shooter who makes really large prints with a DSLR that uses a CCD or CMOS sensor with an 10- 12-, or higher megapixel rating will be able to see lens quality differences quite clearly, but when was the last time you made a 16x20 print? Even DSLR shooters seldom make prints larger than 11x14, a size that’s well within the capabilities of an 8MP DSLR and prints of this size account for far less than 1% of pictures taken. Nevertheless, having a combination of more megapixels and a lens of superior quality can be mighty useful when you crop an image and then make a large print. Then there is the psychological factor—the idea that if you want to you can make, say, an exquisite poster-size print of that great shot. If you’re a critical user who demands the ultimate image quality that your shiny new 10MP of 14MP DSLR can provide, having lenses that deliver superior image quality is essential. How many megapixels do you need? Crunch the numbers! So, how many megapixels do you actually need for a given application? Plug in your maximum print size and crunch the numbers! The rule of thumb for determining prints of “photographic quality” is to take the size of the image file at the maximum quality rating (JPEG or RAW) and divide the numbers by 200. These pixel numbers are available in the camera specifications posted online at camera company websites and also listed in the camera’s instruction manual. Example: The Pentax K100D, a 6.1-megapixel digital SLR, delivers a RAW or JPEG file of 3008x2008 pixels at the highest quality setting. Dividing both these numbers by 200 gives a print size of roughly 15x10, which corresponds closely to a standard print size of 11x14. This conversion factor is very severe, and many users would be quite satisfied with a 14x17, 13x19 or even a 16x20 print turned out by this 6MP camera at its hi-res setting. For comparison, the 10MP Canon Rebel XTi delivers a hi-res file of 3888x2582 pixels, which works out to maximum print size of 19x13 inches, the largest size you can get with most consumer printers. Image quality: It takes more than more megapixels What does this all mean in the real world? Obviously, your DSLR’s megapixel rating alone does not determine picture quality—the image-processing firmware and software built into your camera, and the optical quality of the lens used are also extremely important. However, in practice, a good quality 5MP or 6MP camera can deliver excellent quality 8X10 enlargements, a 6MP camera is capable of turning out superb 11x14 prints, and images shot with a 10MP or greater camera can be enlarged to 13x19. If you’re a bit less fanatical and demanding, you can go up one print size at each MP level without much sweat, and we have seen stunning 16x20 prints made from 10MP image files. However, if all you’re going to do is make 4x6 prints and e-mail images to your friends, none of this really matters too much. In fact, if you don’t want to blow out your buddy’s e-mail, you’d better reduce the file size before sending him those great shots you just took with your shiny new 8-, 10-, 12-, or 14-megapixel marvel! Digital lenses: Do they really do a better job Now that you have a pretty good handle on the megapixel department, how do you go about choosing lenses for your DSLR that will deliver all the quality built into the image sensor of your state-of-the-art DSLR? The answer is to choose high quality lenses that are designed for digital imaging. Lens manufacturers from major camera companies and leading independent makers like Tamron have (at least since the dawn of the new millennium) touted their lenses as “optimized for digital photography” and many carry identifying codes such as Di, and Di II, denoting this. Some makers of 4/3 System and APS-C-format DSLRs have even claimed that their “digital lenses” designed to cover these smaller-than-35mm formats provide better off-axis performance (especially at ultra-wide-angle focal lengths) than “35mm lenses designed for film photography.” Do such claims really have a scientific basis, or is it just advertising guys blowing smoke? Let’s take a closer look. 35mm lenses on DSLRs: What, if anything, do you lose? As anyone who’s used a 35mm SLR lens on a DSLR knows, a good quality lens for a 35mm SLR will generally perform quite well in digital imaging applications. This is especially so with lenses like Tamron’s Di series that are optimized for digital sensors but designed to cover both 35mm and digital formats. And because wide- and ultra-wide-angle lenses for all SLRs are of inverted telephoto (retro focus) design to clear the mirror box, the rear of the lens is farther away from the image plane than the actual marked focal length. Therefore, the light rays at the edges of the image field strike the image plane at a less oblique angle than they would in, say, a 35mm rangefinder camera like the Leica MP, minimizing any potential cutoff caused by the recessed light-sensitive elements of a CCD’s image sensors. And if your DSLR uses an APS-C-size sensor (about 22.5x15mm) rather than a full-frame (24x36mm) sensor, any cutoff is further reduced due to the cropping factor—you’re only using the center of the image field. The bottom line: Unqualified statements that 35mm lenses are unsuitable for digital photography are vastly overstated and a good quality ultra-wide 16mm 17mm or 21mm lens for a 35mm SLR will probably work reasonably well on your DSLR. However due to the cropping factor of 1.5-1.6 on most consumer DSLR models, you won’t get the full benefit of their wide-angle coverage—your expensive 21mm will cover the same angle as a 32mm or so on your DSLR. That’s why, in the long run, the best choice for DSLR users is opting for a high quality ultra-wide-to-wide zoom such as an 11-18mm (17-35mm equivalent) or, even better, a 10-24mm (16-37mm) equivalent like the unique new Tamron SP AF10-24mm Di-II LD Aspherical (IF). One area where digital lenses can provide a noticeable performance advantage over lenses (especially older ones) designed for 35mm photography is in reducing secondary reflections, that is, ghosting. Because CCD and CMOS sensors are much more highly reflective than film, moderate to slight levels of ghosting that are tolerable when shooting film may yield unacceptable levels of flare when shooting digital. Lens manufacturers have attacked this problem in two ways: shaping lens elements to eliminate or minimize secondary reflections, and applying special multi-layer coatings to lens elements. According to optical experts, these “digital coatings” are specially formulated to reduce secondary reflections, unlike traditional multi-coating, which is primarily designed to increase light transmission. Side-by-side comparison tests published in scientific journals and camera magazines corroborate the advantages of these digital coatings. A good example of reconfiguring lens elements to minimize ghosting is found in some super-telephoto lenses in which flat front elements have been replaced with meniscus-shaped protective front glasses so that any light reflected off the image sensor is dispersed and does not hit the rear surface of the protective glass, thus preventing bounce-back (ghosting) and increasing contrast. Tamron and other well-known lens makers do both, using efficient Di and Di-II coatings, and also shaping internal elements to reduce the amount of repeated reflections inside the lens, assuring that any light reflected off the sensor escapes out of the front of the lens in the direction of the subject. Digital-format lenses: Do you need special lenses for your DSLR? A “digital lens” can also be defined by its coverage—an image circle specifically designed to cover a smaller-than-35mm digital format. Virtually all major camera companies and independent lens makers now offer digital lenses for APS-C (22.5x15mm) and/or 4/3 System (17.3x13mm) digital formats. Indeed, digital-only lenses such as Tamron’s Di-II series will not even operate on full-frame (24x36mm) digital or film SLRs. Having an image circle that precisely matches the format has a number of important advantages. First, all things being equal, lenses that cover a smaller format can provide higher resolution across the image field—which is why lenses designed for 35mm generally deliver higher resolution across a smaller area than lenses designed for 2-1/4 or 4x5 formats. Second, a smaller format makes it easier for optical designers to produce smaller, lighter, less expensive lenses that deliver very high image quality. These include wide zooms in the 11-18mm and 10-24mm range that provide ultra-wide coverage on the smaller formats, and compact, long-range macro zooms like Tamron’s 18-200mm, 18-250mm and 18-270mm lenses that are now among the hottest sellers in the DSLR market. Virtually all these digital-format lenses also incorporate special coatings and optical designs that minimize ghosting, which is why they’re an excellent choice for high-end consumer DSLR shooters. Do your present lenses pass the test? What does all this mean in practical terms for today’s DSLR users? First of all, if you already have some high quality lenses you bought for your 35mm SLR, by all means try them on your DSLR—you’ll probably be pleasantly surprised at just how well they perform. And if you often take pictures in high-glare situations, try it with your existing lenses to see if you get any objectionable ghosting. Then shoot some ultra-wide-angle images and look for light falloff or image degradation, especially light falloff and loss of detail toward the edges of the frame. If your lenses flunk either of these tests, by all means check out those new digital lenses, especially those designed to cover your camera’s digital format. Finally, if you’re starting from scratch and you don’t have an existing collection of lenses to deal with, digitally optimized lenses are definitely the best way to go. Just make sure to go for an established brand you can trust so you’ll get all the performance built into your latest-model, hi-res DSLR. |