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Tuesday, May 22, 2012

The Big Glass

Long-focal-length super-telephotos are obviously well suited to wildlife work. They also can give you a new perspective on your landscape photography.

Labels: LensesGear
Canon EF 28-300mm ƒ/3.5-5.6L IS USM; Sigma 300mm ƒ/2.8 EX APO DG HSM; Tamron 200-500mm F/5-6.3 Di LD
Long lenses magnify camera shake along with the image, so they're best used atop a sturdy tripod. For landscapes and other nonmoving subjects, a ball-type tripod head is ideal because it allows you to easily position the camera as desired, then lock it there with the twist of a knob. Bird-in-flight specialists generally prefer gimbal heads, which allow you to track moving subjects while providing good support. For some photographers, a rifle-stock support such as the BushHawk is comfortable and fast to use. Besides eliminating camera shake as a source of image blur, a tripod can lock your composition in so you can study it carefully. Focus using the zoomed live-view image and you won't accidentally alter the framing as you squeeze off the shot. A monopod is easier to carry around than a tripod and still provides much more support than handholding a long lens.

Note that since most long lenses are heavier than most camera bodies, you generally attach the lens rather than the camera body to the tripod head. Heavier long lenses come with tripod-mounting rings for this purpose.

What Do You Get For Your Money?
You can pay as little as $1,400 for a new top-brand 400mm lens or as much as $11,499 (same manufacturer, incidentally). What's the difference?

All-Around Performance. The higher-end lenses deliver better image quality, assuming proper shooting techniques are used because they employ better optical elements and more precise production tolerances. Higher-end lenses also provide better autofocus performance, faster maximum apertures and more rugged construction, with better weather, dust and temperature resistance.

Special Elements. Aspherical elements are generally used to minimize the distortion and spherical aberration (focusing of light rays coming through the edges of the lens closer to the lens than light rays coming through the center) that plague wide-angle lens designs—especially fast wide-angle lenses—but you'll also find them in some wide-to-telezooms to handle these problems and keep overall lens size down. One aspherical element can do the job of two or more "regular" elements. Aspherical elements aren't needed in prime long lenses.

Multicoatings. An ideal lens element would transmit all the light that strikes it. But in reality, each element reflects a portion of the light that strikes it, and so transmission is reduced. Each glass-to-air surface in a lens does this; in a zoom lens, which might have 20 or more such surfaces, the light loss can be extreme. So better lenses employ multiple coatings on each element surface to minimize reflections and increase transmittance. Multicoatings also help provide good color characteristics, and reduce ghosting and flare. Some lenses also employ fluorine coatings, which minimize problems with dust and water, and make the front element easier to clean.

Aberrations In Long Lenses
Long lenses are prone to chromatic aberrations.

Longitudinal chromatic aberration results in unsharp images because short (blue) wavelengths are focused farther from the image plane than medium (green) ones, and long (red) wavelengths are focused closer to the image plane than shorter ones (see diagram). Longitudinal chromatic aberration causes color fringing because, with off-axis light rays, short wavelengths are focused farther from the optical axis than longer ones. To combat chromatic aberrations, lens manufacturers incorporate special low- and ultra-low-dispersion and anomalous-dispersion elements in their long lenses—fluorite, AD, ED, ELD, FLD, LD, SLD, Super ED, UD, XLD and Canon's DO (diffractive optics).


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