Not All Lenses Are Created Equal
Have you ever wondered why one DSLR lens costs more than another? Well, the short answer is, you get what you pay for. As much as we might wish it so, lenses are not all created equal, and they almost always involve a level of compromise in order to achieve a given price point. However, there are definitely some things that you should look for in a quality lens. Sharpness, build quality, distortion control, and a fixed aperture are traditionally the hallmarks of a great lens. Here's why.
Tack sharp photos need tack sharp lenses. While all lenses are capable of producing sharp, in focus pictures, there are some that will do that more often and with greater accuracy. These are the lenses that photographers crave. Whether you are a wildlife photographer with a super telephoto lens, or a wedding photographer with a portrait zoom, edge to edge sharpness is a must. Some lenses just have a better reputation for sharpness than others, but the sharpest ones are almost always the most desirable ones. All you need do is find the ones with the best reputations for your camera manufacturer. Canon shooters need not look much further than the excellent 17-55mm f/2.8.
A digital SLR lens has one of two types of apertures: constant or variable. A large, constant aperture means that you can capture low light photos at any focal length. For instance, the much envied Nikon 70-200mm f/2.8 will let you shoot at f/2.8 whether you are at 70mm or zoomed out to 200mm. This gives the photographer much more flexibility with when they shoot, and provides more control over depth of field. Constant apertures can be found on prime and zoom lenses, but they will mean a higher price tag.
Variable apertures on lenses are generally less desirable, and cover a range of stops, e.g. f/3.5-5.6. This means that at its widest focal length, the maximum aperture setting you can set is f/3.5. At the longest focal length, the maximum aperture you can use is f/5.6. A lens with a variable aperture is not a bad lens, in fact there are many excellent f/2.8-4 lenses, but it is not as flexible as a constant aperture, and will therefore generally be cheaper.
Barrel distortion and pincushioning are terms you will hear to describe the ability of a given lens to control image distortion. The less you have of these, the better your digital SLR lens usually is. If a lens is prone to distortion, buildings will lean to the center of your photo, horizons may bulge, and faces will appear out of proportion. The wider the focal length of the lens, the more prone it will be to distortion.
In order to overcome distortion, manufacturers add extra lens elements to try to counteract the effects. However, the success of such a technique does vary from lens to lens. So, read lots of reviews on your chosen lens, and try it out if possible, in order to minimize distortion in your photos. Zero distortion is rare, especially in wide-angle zooms, but you need not worry about that. Many digital photo editors are more than capable of correcting this in post processing.
Of course, the exception to this effect is the fisheye lens. When you use a fisheye lens, you are deliberately distorting a given scene in order to achieve a specific photographic composition.
The best lenses are built to last. The best digital camera lens for your DSLR will likely have a metal lens mount and body. This makes them durable and able to withstand knocks and drops better than a cheaper plastic based lens. Another common feature on the best camera lenses is some degree of weatherproofing. Pentax make a number of weather-resistant lenses, as do Nikon, Canon and other lens manufacturers. When a lens is resistant to moisture or dust, it is more able to be used in a greater number of shooting conditions. Lastly, in order to ensure maximum performance, manufacturers save their best optical glass for their best lenses, and add more aspherical elements to help guarantee the best picture quality possible.
The Best of the Best
So, what makes a truly great camera lens? The best lens is the one that can combine as many of the above elements as possible. Inevitably, this makes it more expensive than your average camera lens, but, as stated above, you generally get what you pay for with lenses.
Sony DSLR and lens image courtesy of Sony USA.
Nikon zoom lens image courtesy of Nikon USA.
Sigma fisheye lens image courtesy of Sigma USA.