Pixiq

A good camera demands a good lens. Actually a good camera demands two, three, or even four good lenses. The combination of camera and lens forms a unit critical to the success of your photography. Together they create the image foundation upon which you build the final photograph.

In some regards, the decision about purchasing a lens is even more challenging than deciding upon a camera because there are so many lenses to choose from. Manufacturers of D-SLRs are few in number, and each makes only a handful of cameras. But lenses are produced by camera manufacturers and several independent lens makers, and they all make a great variety of lenses to cover the many applications of photography. The result? There are dozens of lens choices. And from these dozens, you must find a few that not only meet your budget and photographic needs, but also offer the quality required to deliver an ultimate image.

Choosing a Lens

Sharpness! Far and away, sharpness is the number one criterion for picking a lens. Is it sharp across its entire field and through all apertures? Is it extremely sharp at some apertures and soft at others? If it’s a zoom, does it perform well at all focal lengths? Is chromatic aberration (CA) excessive? Is pincushion (straight lines curving inward) distortion or barrel distortion (straight lines curving outward) excessive?

Optical Qualities of Lenses

• Sharpness: Consider the lens’ ability to reveal small details crisply from center to edges. Sharpness is similar to the more technical terminology of optical resolution, which often refers to a specific test pattern of line pairs with increasingly small distances between them.

• Contrast: Local contrast helps with sharpness. Overall contrast results in an image that pops. It results from optical design and materials that prevent light entering the lens from straying and causing a subtle flare or haze that degrades the image.

• Geometric distortion: This causes straight lines to bend inward (pincushion distortion) or outward (barrel distortion). Pincushion distortion is most noticeable with telephoto lenses, barrel distortion with wide-angle zoom lenses, and both are most objectionable with architectural subjects. This can often be fixed with image-processing software.

• Chromatic aberration: Often called color fringing, it appears as a color border along a high contrast edge. It results from the lens’ inability (usually a telephoto zoom) to precisely focus all wavelengths onto the same plane. This can often be fixed with image-processing software.

• Vignetting: This results from the uneven distribution of light across the entire image. The expanded corner areas typically appear darker. This also can often be fixed with image-processing software.

• Bokeh: The way out-of-focus areas appear in a picture. Judging this is subjective, though pleasant bokeh is preferable.

Image-Stabilized Lenses

Image stabilization (IS) systems are sometimes placed within a camera’s body, or may be found within certain IS lenses when using D-SLRs. Although the most important factor in choosing a lens is its sharpness, image stabilization can also be a key factor. You should use a tripod as much as possible, but given human weakness to take the path of least resistance (and not take a tripod with you at all times), image stabilization can be an important feature when deciding on a lens because blur from camera movement is a primary cause of poor image quality.

It’s possible to fall into bad shooting habits if you always use image-stabilized lenses, leading to flabby photos (meaning they’re soft and blurry). I do, however, recommend image-stabilization lenses (and cameras)–but only if you use them wrong. By wrong, I mean using them as if they were ordinary lenses without the steadying hand of technology. To use it as an ordinary lens means you shoot it at a shutter speed that is approximately the reciprocal (or faster) of the lens’ focal length. For example, if using a 50mm lens, set a shutter speed of 1/60 second or faster; if using a 250mm lens, set a shutter speed of at least 1/250 second. Using this method you’ll get near-tripod sharpness without the tripod (but a tripod is always best). Don’t make a habit of shooting IS lenses at slow shutter speeds–you’ll end up with mediocre sharpness. And don’t hurriedly take a picture when using the stabilization. Focus and hold on the subject for a few moments. You should be able to see (and possibly feel) the system kick in. Once engaged, the camera will seem to hold steady on the subject so it doesn’t appear to move while you look at it in the viewfinder.

More about Bokeh

Bokeh is a Japanese term that refers to out-of-focus areas. Depending on lens and aperture diaphragm construction, out-of-focus background highlights take on slightly different shapes, from a soft, pleasing disc to a harder edged hexagon, while other bokeh characteristics also vary with lenses.

Bokeh is both old and new. It’s always existed but until recently hadn’t been formalized as a lens quality to be concerned about. Japanese photographers elevated the pleasing appearance of the out-of-focus areas to a priority that has caught on around the world. It most often applies to the appearance of the background in telephoto pictures taken at a fairly large aperture–what we call selective focus shots. It’s of greatest concern to nature and portrait photographers, and sometimes sports photographers.

It seems as if lens manufacturers are beginning to consider bokeh when designing lenses, which is a tricky task given the aesthetic nature of bokeh and the need to achieve high optical lens quality. If pleasing bokeh is important, you may be able to find assessments on the bokeh of particular lenses at test websites, and you can always go to photographers’ forums to read different opinions.

Be sure to read the instructions for your image-stabilized lenses. More intelligence is being built into these lenses, so the day is coming when they’ll work automatically in any situation. But in the meantime their effectiveness is often determined by an assortment of switches and functions. Some have to be turned off when used with a tripod; others recognize they’re on a tripod and adjust accordingly. Some have to be switched for vertical or horizontal orientation, some for panning or moving subjects, and some can’t be used with panning or moving subjects.

And finally, test your image-stabilization system at different shutter speeds to determine its limits. In the end, it may improve sharpness, but it’s no match for a tripod.

Older Film vs. Digital-Only Lenses

Sensors demand higher and sometimes different qualities from lenses than does film. So, should you continue to use older lenses originally made for 35mm SLRs? The answer is yes, especially if they are high-quality lenses that work with your digital camera and prove to be sharp when retested.

First review your D-SLR camera manual to make sure your old lens can be attached without causing harm. Some of these lenses have rear elements that extend too far and could damage the mirror when it flips up. Film-based lenses deliver an image that’s bigger than the sensor area. That means most D-SLRs use only the central portion of the image delivered by these lenses. In theory, the central portion of the image exceeds the quality of its outer regions (vignetting and image softness usually increase at image edges). And since the light rays from the center of a lens will be at a steep angle, most of them should fill the pixel buckets.

See if the D-SLR manual indicates what functionality you might lose. Some D-SLRs offer adaptive features for older lenses by selecting an onboard menu and indicating certain specs of your older lens, while newer lenses are electronically integrated with the camera body. A number of lenses are now being designed just for digital cameras and, in theory, should outperform their counterparts designed for film cameras, enabling not only autofocusing, but automated metering, exposure setting, and communication with the flash (you may have to meter and expose manually when using an old lens).

In the end you can theory this and theory that, but theory stops where practice starts. Put your best film-based lenses to the test. If they meet your needs, use them. I still use my rather ancient, manual focus micro-Nikkor in Manual exposure mode on my digital camera. Why? Because it’s sharp.

Four Thirds Lenses

Yikes. Another thing to know. Four Thirds lenses are designed from the ground up to meet the requirements of cameras based on four thirds design principles. At this time, D-SLRs based on this design come primarily from Olympus. The name derives from the aspect ratio of the specific type of sensor used in these cameras, because the long side of the sensor is 4/3 (1.33) times the short side.

The declared advantage of Four Thirds lenses is their design, which is specifically for the requirements of sensors rather than for film, making them smaller and theoretically better performing. Four Thirds lenses deliver light perpendicularly to the pixels, pouring all the light into the pixel bucket like a faucet into a cup directly beneath it. You do risk that these lenses will become obsolete whenever competing sensors in the future commonly become larger than the most prevalent current size (APS-C dinemsions). That may take several more years, but is the present direction of sensor technology.

Trade-Offs in Choosing Lenses

If money is no concern, you can simply buy the best rated lens for your type of photography. But if the lens you dream about requires a home equity loan to purchase, then you need to make trade-offs when choosing lenses. The good news is that trade-offs won’t hurt most of your photography. Ideally you could still purchase one superb quality lens to meet your highest priority photography. So analyze your photography and decide where you can make trade-offs.

For instance, you might judge that fifty percent of your photography will not put excessive demands on a lens (you mostly shoot within a moderate range of focal lengths at apertures well within the extreme settings), but about twenty percent of your work is for sports or bird photography that requires a super telephoto used with a wide aperture (a “fast” lens).

For the first scenario you could certainly find a moderately priced 3x or 4x zoom lens that performs nearly as well as more expensive lenses with features you don’t need. But finding a super-telephoto lens that gives superb performance when shot wide open (f/2.8) for a low price won’t be easy; in fact it may not be possible.

These are decisions only you can make because the debates and arguments can go on and on. Whether you’re dealing with cost vs. quality, convenience vs. complexity, or other issues, don’t expect clear-cut answers. But don’t be afraid to make trade-offs.

For my landscape and still life photography, I shoot most pictures at f/8 to f/16, using a short-range zoom lens. I picked my lens by reviewing it on the several lens review websites. It scored well with some limitations. But when stopped down to f/8 and not shot at its extreme focal length ranges, it could rival the quality of far more expensive lenses. And in my personal testing against some high quality prime (non-zoom) lenses, it held its own.

Specialty Lenses

The discussion about fast lenses for sports and bird photography brings up a good point. There are a variety of specialty lenses that give superior results in their area. If you are dedicated to a special type of photography, then achieving superior results requires specialized lenses. Know the lenses you need and buy them. This simple summary only hints at the types of lenses specialized photographers might need.

• Sports photography requires fast-focusing, large aperture telephoto lenses that can capture football players leaping over the goal in brilliant sunlight as well as boxers throwing punches in a dimly lit ring.

• Nature photographers need macro lenses designed to allow close focusing and still give superior sharpness–well beyond the domain of the all-purpose zoom lens. Image-stabilized lenses prove invaluable when backpacking or getting into positions difficult for a tripod.

• Architectural photographers need shift lenses that provide true perspective of the buildings they photograph, not the wildly distorted perspective of an everyday wide-angle lens that converges and bends the lines of a building.

Accurate Focus

Though we tend to take accurate focus for granted, I have a word of advice: Don’t. Accurate focus is especially important if you are shooting with shallow depth of field. Shallow depth of field typically results from shooting at large lens openings (f/2.8–f/4), using long telephoto lenses for near and medium distances, and taking close-ups. Inaccurate focusing can result in unsharp photos.

A prime example is the use of selective focus to separate your subject from a cluttered background, perhaps a single plant in the forest or a bride at her reception. Especially in the second situation, you would almost certainly rely on autofocus to keep pace with the fast-changing environment. To make sure of getting the best results in such situations, test the focus accuracy of the lenses and focus modes you commonly use with your lens set to it’s largest apeture (smallest f/number).

Avoid Broad-Range Zooms

Many zoom lenses with a broad range don’t stand up to the highest quality needs, particularly those that cover from wide-angle to telephoto focal lengths. This is generally true of 6x zoom lenses (the focal length at the long end is six times that of the short end) or greater, such as 18–125mm and 28–200mm lenses. That’s not to say these lenses won’t have a sweet spot that can give good results, or that they don’t make great travel companions when you want to travel light. It’s just that with all the glass they carry, it’s hard for them to match the quality of simpler lenses.

If you frequently use one or two zoom lenses, test them thoroughly so you can understand their strengths and weaknesses and to make sure they’re capable of giving you exceptional image quality.

Zoom Creep

Beware of zooms that slowly creep and drain away quality. My sharpest zoom does just that. Perched on a cliff above Taughannock Falls, with my camera mounted on a tripod and pointed down at the falls below, I watched in wonderment as my zoom collar slowly slipped through the focal length range: 17mm, 24mm, 50mm, 70mm–gravity in action.

Not good. During long exposures, zoom creep can blur the picture. You can visually check your zooms for creep. With the zoom attached, hold your camera waist high in front of you and gently “saw” the camera up and down from your waist to your knees and watch to see if the lens slips and changes focal length. Some zooms have a zoom lock, a little slider on the barrel, that lets you lock the focal length.

Lens Review Websites

Several websites provide in-depth testing and analyses of lenses. Review these before buying a lens. Realize that they can’t account for manufacturing variability. And since they can only test one or two lens samples, and if the manufacturing process isn’t reliable, then the lens you buy may not live up to its testing. In that case, you could always perform your own tests. If it’s an underperformer, return it.

The sites I commonly visit for new lens reviews are:

• www.slrgear.com

• www.photodo.com

• www.popphoto.com

• www.photozone.de

Practical Lens Tips

• Buy at least one high quality single-focal length (primary) lens.

• Test and analyze your most frequently used lenses.

• Buy 3x or 4x zoom lenses, not 6X and greater.

• Use an f/stop two or three stops from the smallest possible on your lens.

• Avoid the extreme minimum and maximum range of your zoom lens.

• Use a lens hood.