Lens Focal Lengths
Lens focal length is a key element. It affects angle of view, or magnification, perspective and the physical size of the lens itself.

Angle Of View And Magnification. For any camera, the longer the focal length of your lens, the narrower its angle of view becomes and the more it magnifies distant objects and seems to bring them closer. Short focal lengths do just the opposite, giving you a wider angle of view while making objects seem farther away and smaller within the image.

Choose a focal length, then, by what you want to accomplish—if you often find yourself trying to capture a large part of your subject, but can’t back up enough to include everything, a wide-angle lens may solve the problem for you. Wide-angles are great for photographing indoors, landscapes or any tight situations. If you’re always pursuing the smaller details of your subject and you can’t come close enough to isolate them, then a telephoto is your answer. Tele-photos are ideal for people photography, wildlife and making the sun’s image larger at sunrise and sunset.

Magnification Factors. As much as angle of view is controlled by lens focal length, it’s also governed by the size of your image sensor. If you use the same focal length on two sensors of different size, then the angle of view, or magnification, changes.

On a 35mm film camera, a 50mm is a normal lens. When a lens of that focal length is used on an 8-megapixel advanced compact, it gives an angle of view equivalent to 200mm on the 35mm camera. That’s because the advanced compact’s image sensor is four times smaller than 35mm film, so its angle of view at a given focal length is four times narrower and its apparent magnification is therefore four times greater.

Since the image sensors of most D-SLRs are smaller than 35mm film, lenses naturally have a narrower angle of view when they’re mounted on D-SLRs than they do with film. The apparent magnification for most D-SLRs is from 1.3x to about 2x that of the 35mm film format. A 20mm lens on a D-SLR with a 1.5x magnification factor, for example, sees the same angle as a 30mm lens would when it’s mounted on a film camera; your 50mm behaves like a 75mm, and a 200mm acts like a 300mm.

In many respects, a digital SLR’s magnification factor is good news. It increases the reach of your existing telephotos, with neither the light-loss penalty of a teleconverter nor the bulk of using longer focal-length lenses. Until recently, though, magnification factors were a problem on the wide side—when you mount your 28mm lens on a D-SLR, it sees the same angle of view as a 42mm lens does with film. This problem is now answered with new ultra-short lenses for D-SLRs.

Perspective. You can use distance together with focal length to expand or compress your picture’s perspective at will. Move back and use a long lens for compression; come in close and use a wide-angle for a sense of depth. This works because distance controls perspective—the closer you are to something, the more perspective foreshortening there will be, with foreground objects looming large and background objects receding into the distance. When shooting up close, then, the wide-angle’s job is to encompass the enlarged foreground objects so you can capture the resulting impression of depth. On the other hand, if you back up and use a telephoto, the same foreground object now will appear similar in size to the background, making the scene compressed in perspective.

Sometimes subtle tweaks can be important. Portrait photographers, for example, use medium-telephoto lenses—from about 75mm to 105mm—to shoot head-and-shoulders images because a shorter lens—like a 50mm—can distort a person’s face when you come close enough to fill the frame.

Lenses For D-SLRs With Smaller Image Sensors
A special class of lenses now offers true wide-angle coverage for D-SLRs whose image sensors have magnification factors of 1.5x or more. These “digital-only” lenses cover just the smaller sensors’ image area, allowing designers to create ultra-short focal lengths to compensate for the D-SLRs’ magnification factors. They offer the 35mm equivalent of 18mm or wider while maintaining high optical quality. If these lenses have a downside, it’s that they won’t function with film SLRs, or with D-SLRs using full-frame or nearly full-frame imagers.

Zoom Lenses
As with everything else in life, there are trade-offs to make when selecting lenses. Zoom lenses offer faster access to a variety of focal lengths than fixed-focal-length lenses, and they can be set at in-between lengths like 33mm or 165mm. In some cases, a single zoom can replace a gadget bag full of lenses. Then again, most zooms aren’t as fast as their single-focal-length counterparts.

Some zooms offer maximum apertures that remain constant throughout the zoom range. As mentioned, they’re generally larger and costlier than zooms with variable maximum apertures. This latter group provides maximum openings that are from about a 1⁄2-stop to 11⁄2 stops faster at a zoom’s shortest focal lengths than at the longest ones.

For example, a 28-105mm ƒ/3.5-4.5 zoom has a maximum aperture of ƒ/3.5 at the 28mm setting and ƒ/4.5 at the 105mm end. With telephoto zooms especially, you’ll notice the lens speed is reduced right where you need it most—at the long end—but the advantages of compactness and greater affordability may be more important to you.

ƒ-stops
A casual glance through any manufacturer’s lens catalog shows a wide range of available maximum apertures. Many lenses open up to ƒ/2.8 or wider, others to no more than ƒ/5.6 (or even less), and you’ll often have a choice between fast and slow versions of the lens you’re thinking of buying. Just choose one according to your needs.

Fast lenses are bigger, heavier and more expensive, but they let you shoot in dimmer light with a faster shutter speed or with a lower ISO. The main difference between an expensive ƒ/2.8 zoom and its less expensive ƒ/3.5 counterpart is that often the ƒ/3.5 lens is only that ƒ-stop at its widest focal length, while the ƒ/2.8 lens is constant throughout the range.

That smaller, less expensive lens may be ƒ/4.5 or even ƒ/5.6 at the longer focal lengths, which is a considerably smaller ƒ-stop. That can mean a difference of two whole stops of shutter speed, which in low light also may mean the difference between sharp and not sharp. If you’re not working at the limits of available light, though, the slower lens could do as well for you, and it will be more compact, too.

If you check compact digital camera lenses, you’ll find this applies to them as well. The extra speed of a faster lens may not be important to you, but it’s worth checking.

When you’re deciding, keep in mind that with D-SLRs, you’ll find it easier to compose your pictures with the brighter viewfinder image produced by faster maximum apertures, and your D-SLR’s autofocus system will perform better. Although AF systems commonly operate with lenses as slow as ƒ/5.6, they’re neither as discriminating nor as quick-working with lenses of that speed as they are with lenses that open to ƒ/2.8 or faster. If you like to focus manually, you’ll find that easier with faster optics, too.

Accessory Lenses
D-SLRs accept bayonet-mount interchangeable lenses, each of which has its own complete optical system. Along with all the glass, these optics have built-in focusing mechanisms, as well as an internal lens diaphragm to control your ƒ-stop settings.

Owners of advanced compact digital cameras still can extend their focal-length range significantly with accessory, or add-on, lenses. These optics attach to the front of your existing lens, reducing or lengthening the built-in’s focal length. Wide-angle adapters shorten your focal length by about 20% to 30%, while telephoto adapters can provide anywhere from a 50% to a 300% focal-length increase, depending on the adapter’s make and model. A 50% to 70% increase is common.

Because these accessory lenses use the built-in lens’ focusing system, diaphragm and electronic components, size and expense are kept to a minimum while still providing strong image quality. Modern accessory lenses make use of the latest lens-making technologies, including low-dispersion glass and even phase-fresnel designs (see next page).

Lens Technologies
LOW-DISPERSION GLASS. As light passes through glass elements, it tends to disperse, or separate out, by color (prisms are an example of dispersion taken to extremes). The separated red, green and blue wavelengths focus at slightly different points and with unequal magnifications, causing fuzzy images and color fringing. This is known as chromatic aberration, and it’s mainly a problem for telephotos. Lenses specifically designed to correct all wavelengths are known as apochromatic, or APO, lenses.

Optical elements made of special glass, such as low- (LD), extra-low- (ED) and ultra-low- (UD) dispersion glasses, help minimize chromatic aberrations. Since telephoto lenses always have been particularly prone to these aberrations, they have made increasing use of the premium glasses. Very short focal-length lenses for SLRs and D-SLRs rely on optical designs that are basically inverted telephotos, so they also use the low-dispersion glasses to fight chromatic aberration. That’s why so many “designed for digital” lenses feature LD, ED or UD elements.

ASPHERIC LENS ELEMENTS. Aspheric lens elements help deliver sharper images and prevent straight lines from bending in your photos, as they sometimes do with wide-angle lenses and some zooms. Because aspherical elements can take the place of multiple conventional lens elements, they allow lighter, more compact designs. Advanced lens-making technology has overcome the challenges in manufacturing aspherical lens elements, which until recently had been prohibitively expensive or impossible to make. They’re now common even in lower-priced compact digital cameras.

INTERNAL FOCUSING. Internal focusing (IF) does exactly what its name implies, achieving focus by moving the rear or middle groups of lens elements contained within, rather than by moving the entire lens barrel and all the elements as a single unit. Compared to the latter system, IF saves weight, especially with big telephoto lenses. Internal-focusing lenses also can be made more compact, and because of the lighter weight of the few moving components, autofocus is faster. Unlike some simple zoom lenses, the fronts of IF lenses don’t rotate, which is important for users of polarizers, graduated ND filters or any other accessory that depends on a constant position.

DIFFRACTIVE OPTICS (DO) AND PHASE-FRESNEL LENSES. These advanced designs would need a short course in physics to explain fully. The quick version is that lens designers have removed about a third of the size and weight of a typical lens by incorporating a high-tech optical element that uses concentric diffraction rings to focus light. The weight savings are due to the special element’s distinct ability to combat spherical and chromatic aberrations, which eliminates the need for several conventional lens elements.

STABILIZATION SYSTEMS. A photographic rule of thumb says that the slowest shutter speed to use when you shoot handheld is the one nearest the reciprocal of the focal length you’re using; with a 50mm lens, you’d shoot at 1⁄60 sec., with a 135mm lens, you’d shoot at 1⁄125 sec., and so on. Image-stabilizing systems give you sharp images at shutter speeds slower than you could otherwise use, typically two to three stops slower.

The stabilizers work by shifting some of the lens elements to compensate for camera movement, thereby keeping the image motionless on the image sensor. Canon’s Image Stabilizer (IS), Nikon’s Vibration Reduction (VR) and Sigma’s Optical Stabilizer (OS) are all examples of this technology. Konica Minolta’s Anti-Shake system provides similar results, but because the system shifts the image sensor rather than the lens elements, Konica Minolta lenses don’t need stabilization technology.