Long lenses are terrific tools when you can’t get close to your subject, providing dramatic frame-filling images of distant wildlife and sports subjects. Long lenses are also useful to isolate a subject from busy surroundings and for telephoto compression effects. There’s a variety from which to choose today—both prime (single-focal-length) lenses and tele-zooms. Following are some considerations when buying and using long lenses.
What’s A "Long Lens"?
Anything longer than a camera's "normal" lens is long, but it generally takes a lens of at least twice the camera's normal focal length to produce a noticeable long lens effect. For a 35mm camera, 50mm is considered normal, so anything from 100mm up is long. For a 6x7cm medium-format camera, 105mm is normal; 210mm and longer is long. For a 4x5-inch view camera, 180mm is normal, so anything from 360mm up is long for this format. For a typical digital SLR with an APS-C-sized image sensor, 32mm is normal and 64mm on up is long.
There's nothing magic about "long." Principles of optics don't suddenly change when you attach a lens that exceeds the camera's normal focal length. But the longer the focal length, the bigger the subject's image will appear in the image frame and the narrower the angle of view. And because we generally shoot from farther away when using longer lenses, perspective is compressed—objects at different distances in the scene seem squashed together due to the great shooting distance, what's known as the telephoto compression effect (more on this later).
Additionally, while photographers tend to call any lens longer than a camera's normal lens a telephoto, technically the term refers to a specific optical design that employs refracting elements in such a way that the physical length of the lens is shorter than its focal length. Most of today's long lenses are of telephoto design, so it's okay to call them telephotos. Just be aware that not all long lenses are true telephotos.
Focal Length Vs. Format
Whether a given focal length is telephoto, normal or wide-angle depends on the camera format with which it's used. On a 35mm camera, a 200mm lens is quite telephoto—it provides a big image of the subject in the image frame (four times the size produced by the format's normal 50mm lens) and has a fairly narrow angle of view. The smaller image sensor of an APS-C sensor digital SLR crops in more on the image formed by any lens; a 200mm lens on an APS-C D-SLR will produce about the same image size within the frame and the same cropping as a 300mm lens on a 35mm camera.
Conversely, the larger image size of a 6x7cm. medium-format camera "sees" more of the image produced by any lens, so the 200mm lens has a wider angle of view (about equivalent to that of a 115mm lens on a 35mm camera), and the subject's image is smaller within the frame. A 4x5 view camera's big 4x5-inch image area sees even more of the image produced by the lens, so the 200mm lens is barely telephoto, equivalent to about a 58mm lens on a 35mm camera. The diagram above makes this easier to comprehend.
Prime Or Zoom?
All other things being equal, a fixed-focal-length lens probably is a little sharper than a zoom that goes out to the same focal length, simply because the zoom has to be optimized to perform at a whole range of focal lengths, not just one. The fixed-focal-length lens is also lighter because fewer elements are needed to produce one focal length efficiently. But today's major-brand tele-zoom lenses are quite sharp (some sharper than older prime lenses, due to use of modern design, manufacturing and materials) and not all that bulky; in fact, we now have zooms that go from wide-angle to supertelephoto (28- 300mm), yet are less than 3.5 inches long (when not zoomed all the way out).
The tele-zoom offers the versatility of a range of focal lengths in a single unit. This facilitates changing framing when you can't easily move toward or away from your subject, and it means fewer lens changes in the field—which in turn means less chance for dust to settle on your D-SLR's image sensor. Another advantage is that for those on tight budgets, you can get a longer focal length for less money in a zoom. There are a number of major-brand 70-300mm and 75-300mm zooms on the market for less than $300, while no major-brand fixed-focal-length 300mm lenses are available near that price. Sigma and Tamron offer tele-zooms that go out to 500mm, yet sell for less than $1,000,far less than the cost of a good fixed-focal-length 500mm lens.
Most prime telephoto focal lengths are available in superfast and slower versions. For example, I do most of my shooting with a 300mm ƒ/4 lens, while the same manufacturer also offers a 300mm ƒ/2.8. The faster lens permits shooting at a faster shutter speed in any given light level, provides quicker auto-focusing performance and offers a brighter viewfinder image for easier manual focusing. But the ƒ/2.8 lens also costs more than three times as much and weighs more than twice as much as my ƒ/4 lens, which is why I own and use the ƒ/4. Most pro wildlife and sports photographers use the superfast supertelephotos—300mm ƒ/2.8, 400mm ƒ/2.8 and 600mm ƒ/4—because they need the advantages cited above. These pros deal with the bulk by mounting the lens on a sturdy tripod and with the price by making money from the resulting photos. Most tele-zooms come in only one speed version, but when lens-speed options are available, consider the benefits versus the cost in weight and dollars of the faster versions. Again, faster lenses generally provide quicker autofocusing, important if you shoot action subjects.
Some lenses extend in physical length and rotate the front element as they're focused. This means the balance of the lens changes during focusing, and orientation-sensitive lens attachments such as polarizers and graduated filters will change their orientation as the front element rotates.
Internal focusing solves these problems and more. Only internal elements shift during focusing, so the physical length of the lens doesn't change, nor does balance. And the front element doesn't rotate, so polarizers and graduated filters retain their orientation. Because only smaller internal elements move, autofocusing is quicker with internal focusing lenses. Rear focusing provides all the advantages of internal focusing except the lens' physical length changes during focusing (albeit not as much as with front focusing).
With most AF lenses, trying to focus manually while in AF mode can damage the AF motor in the lens or camera body. There are exceptions, however, including most of the Canon USM (Ultrasonic Motor), Nikon AF-S (Silent Wave Motor) and Sigma HSM (Hypersonic Motor) lenses; check the instruction manual for your particular lens. In single-shot AF mode, this allows you to focus manually if the AF system has trouble with a specific subject or to fine tune focusing manually after autofocusing without fumbling for the AF/MF switch. In both single-shot and continuous AF modes, it allows you to prefocus manually at a ballpark setting, which speeds up autofocusing when you then press the shutter button halfway to activate the AF system because the lens won't have to search so far to find the focus point.
A lens' minimum focusing distance can be important. For instance, my 300mm ƒ/4 will focus down to 4.9 feet, while the same manufacturer's 300mm ƒ/2.8 lens won't focus closer than 8.2 feet. This allows me to get close-ups of subjects that would be too close to focus on with the faster lens.
When you can approach a subject closely, sometimes you can get a bigger image of the subject from a shorter lens than from a longer one. My 300mm lens at its 4.9-foot minimum focusing distance produces the same subject size in the image frame as a 600mm lens focused at 9.8 feet—but the 600mm lens from my camera's manufacturer won't focus closer than 18 feet. This means I can move in on, say, a spider, and get an image nearly twice as big in the frame with my 300mm lens as I could using that 600mm lens. Of course, when both lenses are used at the same distance (i.e., at any distance the 600mm lens can focus), the 600mm will produce twice the magnification of the 300mm lens.
There's a lot of focusing travel from inside five feet out to infinity with a long lens. To expedite autofocusing, many such lenses have a focus-range limiter switch. For example, my 300mm lens has a switch that allows me to limit focusing travel from infinity to 10 feet instead of the full infinity to five feet, so the lens doesn't have to hunt all the way down to five feet before racking back out toward a more-distant subject. If your lens has a focus-limiter, learn to use it; doing so will increase your percentage of sharp shots and decisive moments immensely.
Long lenses are particularly susceptible to chromatic aberrations, so manufacturers use special elements to compensate. Extra-low-dispersion elements correct these aberrations, which appear as color fringing in images made with uncorrected lenses. These elements are designated differently by different lensmakers; ED, LD, UD, SLD, ELD, SUD and HLD are some of the identifiers used. Fluorite elements also correct chromatic aberrations and are often used in conjunction with ED elements.
Aspherical elements correct spherical aberration, which appears as image softness and is especially evident in fast wideangle lenses and wide-range zooms. It's not a huge factor with tele focal lengths, and you won't find aspherical elements in many long lenses, just a few zooms.
Diffractive Optics (introduced by Canon in 2000 and indicated by DO in the lens name) correct chromatic and spherical aberrations with fewer, smaller elements, resulting in much more compact lenses.
Canon, Nikon and Sigma offer a number of telephoto and tele-zoom lenses with built-in image stabilizers that counteract camera shake. Canon lenses are designated "IS" (Image Stabilizer), Nikon lenses are "VR" (Vibration Reduction) and Sigma lenses are "OS" (Optical Stabilizer). I love stabilization because it gives me sharp shots at shutter speeds I couldn't handhold otherwise. For maximum sharpness, long lenses should be used on tripods, but if you have to (or prefer to) work handheld, a stabilized lens is worth its weight in gold (yes, even at today's gold prices!).
The new Pentax K100D and Sony DSLR-A100 (and the pioneering but out of-production Konica Minolta Maxxum 7D and 5D) provide in-camera stabilization: instead of shifting a group of lens elements to compensate for camera shake, they shift the image sensor itself. This has the advantage of providing stabilized shooting with all glass, not just special stabilizer lenses. On the downside, sensor-shift stabilization steadies only the recorded image, not what you see in the finder, so you can't see how well it's working as you shoot.
Tele-extenders (also known as teleconverters) are a cost-effective way to get long focal lengths. These little tubes fit between the lens and the camera body and increase a lens' focal length by the indicated amount—1.4x or 2x for the most popular extenders. As a bonus, they don't change the lens' minimum focusing distance in the process: if you attach a 2x extender to a 300mm lens that focuses down to 4.9 feet, you have a 600mm lens that focuses down to 4.9 feet.
The major drawback to extenders is that they reduce the amount of light transmitted to the film or image sensor—by one stop for a 1.4x extender and by two stops for a 2x extender. That means a 300mm ƒ/4 lens becomes a 420mm ƒ/5.6 lens with a 1.4x extender attached and a 600mm ƒ/8 with a 2x extender.
With many AF cameras, the AF system won't function with lenses slower than ƒ/5.6, meaning that with a 2x extender attached, that autofocus 300mm ƒ/4 lens becomes a manual-focus 600mm ƒ/8 lens—and the viewfinder image is pretty dark, making manual focusing in dim light difficult. But with practice, you can get the knack, and a 300mm ƒ/4 lens plus 2x extender costs less than half the price of the 600mm ƒ/4 lens. By the way, AF performance will slow noticeably even when a 1.4x extender is used because of the decrease in effective lens speed.
Tele-extenders do reduce image sharpness—minimally when a quality converter matched to the lens (or focal length) in use is employed, somewhat more when cheap or mismatched converters are used. I've seen some excellent results with my 300mm/2x converter combo; when I do get soft shots, it's because I misfocused manually, not because the lens/extender combo lacks sharpness.
Besides a relatively inexpensive way to acquire very long focal lengths, extenders provide versatility options. For example, you can buy one camera manufacturer's 400mm ƒ/5.6 supertelephoto lens for around $1,100. Or you could buy its 70-200mm ƒ/2.8 zoom lens for around the same price, add a $300 2x teleconverter, and turn the 70-200mm into a 140-400mm ƒ/5.6 lens. The 400mm ƒ/5.6 prime lens will probably be a tad sharper, but with the 70-200mm and 2x combo, you also get focal lengths from 400mm down to 70mm and a fast ƒ/2.8 maximum aperture from 70-200mm—very handy if you photograph a lot in dim light.
Perspective is really a function of camera position, not focal length. If you put the camera on a tripod and take a shot with a long lens, then remove the long lens and attach a short lens and take another shot of the scene, then blow up the shorter-lens shot so that it shows the same area of the scene as the long lens shot, the perspective will be identical. Why do some think increasing the focal length flattens perspective? Because we generally shoot from farther away when using a long lens, hence the "telephoto flattening" effect. TV news photographers covering stories on rush-hour traffic often use this technique to stack the cars up dramatically. You can also use the effect to enhance "heat mirage" distortion on hot days.
Depth Of Field
All other things being equal, the longer the focal length, the less the depth of field, and the wider the aperture, the less the depth of field. So depth of field is extremely limited with those superfast, very long focal-length lenses the pros use. When using very long focal lengths, it's best to focus carefully manually and close the aperture down a stop or two.
You can use a long lens wide open to really isolate a subject from the background— the narrow angle of view crops out peripheral distractions, while the minimal depth of field throws background elements completely out of focus. It's best to focus manually when doing this; an AF system might not focus precisely where you want in selective-focus situations.