A long lens can "bring the subject to you," producing a larger image in the image frame and making long lenses particularly popular with wildlife and bird photographers. But they can also be useful to landscape shooters, as they allow you to zero in on more distant portions of a scene, and in so doing, flatten the perspective. (Note: It's the great distance, not the focal length, that compresses the perspective. If you shoot from the same spot with a shorter lens, then crop the resulting image to match the area shown in the longer-lens image, the perspective will be the same.)
Telephoto zooms offer additional benefits. First, as with any zoom, you get a whole range of focal lengths in a single package. That makes reframing much simpler (just rotate or push/pull the zoom ring, rather than physically change lenses) and minimizes dust on the image sensor by minimizing the number of lens changes in outdoor conditions. Telephoto zooms can take you out to 500mm for not much over $1,000, while a good 500mm prime lens can cost five to 10 times that. But a telephoto zoom offers another advantage over a single-focal-length lens. You can zoom back to the widest focal length to "find" your subject, which is convenient for wildlife and especially useful for birds in flight, then zoom in to frame, as desired. It can be hard to acquire a small or fast-moving subject with a long prime lens and its very narrow angle of view.
For our purposes here, "telephoto zoom" means one with its entire focal-length range in the longer-than-"normal" category: beginning at 70mm for full-frame cameras, at 50mm for APS-C cameras and at 35mm for Four Thirds DSLRs (Four Thirds System DSLRs are no longer in production, but some readers no doubt have one, and Four Thirds lenses can be used, via adapter, with Micro Four Thirds cameras).
What Does Telephoto Mean?
In general, lenses close in focal length to an image format's diagonal measurement are considered "normal" for that format. For example, a 35mm film frame (or a "full-frame" DSLR sensor) measures 36x24mm and has a diagonal measurement of 43.2mm. Lenses in the 40-55mm range are considered "normal" for this format.
Lenses shorter than the format's "normal" lens take in a wider angle of view and are called "wide-angles." Lenses longer than a format's normal lens take in a narrower angle of view, but instead of calling them "narrow-angle" lenses, most photographers (including us here at OP) tend to call them "telephotos." Actually, "telephoto" refers to a specific optical design in which the focal length is longer than the lens' physical length. But most of today's long lenses are indeed telephotos, which is kind of nice: It makes them less bulky. "Long-focus" is another term for these lenses.
Bottom line: A telephoto lens has a focal length longer than the "normal" focal length for a given format, producing a narrower angle of view and greater magnification.
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The aperture is the opening in the lens that lets light in. The larger the aperture, the more light the lens can transmit to the image sensor, so you get a brighter viewfinder image for composing and focusing, and you can shoot at a faster shutter speed in a given light level. But the larger the maximum aperture, the bulkier the lens. Apertures are expressed as ƒ-numbers. An ƒ-number is simply the focal length of the lens divided by the diameter of the aperture (actually, the diameter of the effective aperture, which is what you see when you look into the front of the lens, rather than the diameter of the physical aperture itself). For example, ƒ/4 means the diameter of the effective aperture is 1⁄4 the focal length of the lens, or 25mm for a 100mm lens.
You can see from this why there aren't a lot of fast long lenses, and why the ones there are cost a bunch. A 500mm ƒ/2.8 lens (the fastest 500mm you can buy today) has an effective aperture diameter of 500/2.8 = 178.6mm (seven inches!) wide open. That means a big, heavy and costly front element (that Sigma 200-500mm ƒ/2.8 zoom has a street price of over $25,000 and weighs more than 30 pounds!). And, yes, the fastest 500mm lens available today is the long end of a zoom—not a prime lens—quite an engineering feat.
Zoom lenses generally aren't as sharp as prime lenses of equal focal length and price. That's because a prime lens must be corrected for only the one focal length, while a zoom must be corrected for a whole range of focal lengths. And corrections that help at one focal length can make things worse at another. Various aberrations and distortions tend to be more visible in zoom lenses.
That said, however, today's better zoom lenses are excellent, and for many—including working pros—the benefits outweigh the drawbacks.
Tamron SP 150-600mmF/5-6.3 Di VC USD
Tamron's new SP 150-600mm features the second-longest focal length in a current telezoom, yet it costs just $1,069—about $7,000 less than the only current zoom with a longer focal length (Sigma's 300-800mm). Think about that: A quality zoom that goes all the way to 600mm, for just over $1,000. And with built-in Vibration Compensation. And it covers full-frame sensors, as well as APS-C. We just received an evaluation lens from Tamron, and we'll report on our experience with it at www.outdoorphotographer.com. Find the specs in the chart on page 5.
Variable Vs. Constant Aperture
Some zoom lenses (generally, the higher-priced ones) maintain a constant aperture throughout their zoom range. For example, a 70-200mm ƒ/2.8 zoom has a maximum aperture of ƒ/2.8 at 70mm and at 200mm, and everywhere in between. The aperture doesn't change as you zoom the lens. With variable-aperture zooms, the maximum aperture does change as you zoom, becoming "slower" at the longer focal lengths. For example, a 70-300mm ƒ/4-5.6 zoom has a maximum aperture of ƒ/4 at 70mm and a maximum aperture of ƒ/5.6 at 300mm. Just how quickly the aperture "slows" as you zoom varies from lens design to lens design; with most, you can assume halfway through the zoom range that the maximum aperture is close to the slower end of the range.
If you use the camera's built-in TTL exposure meter, it doesn't really matter which type of zoom you use. The TTL meter automatically will compensate for the change in aperture as you zoom. If you determine exposure manually with a handheld meter (or using the Sunny 16 Rule), you'll have to compensate manually for the slower apertures at the longer focal lengths.
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The variable-aperture designs are also more likely to shift focus as you change the focal length. If you're using AF, this doesn't matter, as the system will compensate automatically. But if you focus manually with a variable-aperture zoom, you'll have to focus at the focal length you'll be using for the shot. It's nice to zoom to the longest focal length to get a magnified image for focusing, then zoom back to the desired composition, but with lenses that shift focus as they zoom (which includes some constant-aperture zooms), you can't do this because the focus will shift when you change the focal lengths. Test your zoom(s) to see if focus shifts when you zoom. If in doubt, it's always safer to focus at the focal length you'll be using for the shot.
Angle Of View
Shorter focal lengths provide wider angles of view than longer ones. But just how wide or narrow a given lens' angle of view also depends on the format of the image sensor. Smaller sensors "see" less of the image produced by a given lens, producing a narrower angle of view. Larger sensors see more of the image, producing a wider angle of view. See the diagram at left.
Smaller DSLR sensors generally are assigned "crop factors" based on how their angles of view compare to that of a full-frame sensor (36x24mm, the size of a 35mm film image frame). In the early days of digital, this helped film shooters quickly understand what a given lens would do when used on a smaller-sensor camera (early digital SLRs had smaller APS-C sensors, so named because they were approximately the size of an Advanced Photo System "Classic-"format image frame). As mentioned earlier, a full-frame DSLR image sensor measures 36x24mm and has a diagonal of 43.2mm. An APS-C sensor measures around 23.6x15.6mm and has a diagonal of around 28.3mm. Since the full-frame diagonal is about 1.5X longer than the APS-C diagonal, a given lens on an APS-C camera produces the same field of view as a lens 1.5X longer on a full-frame camera. For example, a 200mm lens on an APS-C camera frames like a 300mm lens on a full-frame camera—great for telephoto fans. But a 28mm lens on an APS-C camera frames like a 42mm lens on a full-frame camera—not so good for wide-angle fans. That's why the kit zooms sold with APS-C cameras start at 18mm: An 18-55mm zoom on an APS-C camera frames like a 27-83mm on a full-frame camera.
Four Thirds System sensors measure 17.3x13.0mm, with a diagonal measurement of 21.6mm—half that of a full-frame sensor. So a given focal length on a Four Thirds System camera frames like a lens twice as long on a full-frame camera: A 200mm lens on a Four Thirds DSLR (or Micro Four Thirds mirrorless camera) frames like a 400mm lens on a full-frame DSLR.
Note that this is a crop factor, not truly a magnification. When focused at a given distance, a given focal length produces an image of a given subject as a given size at the focal plane. For example, a 100mm macro lens focused on a 20mm-high object at 1:2 produces an image of the object 10mm high at the image plane (sensor or film). This size doesn't change just because you put a larger or smaller sensor at the image plane. A smaller sensor just crops more tightly, so the object's 10mm image takes up more of the frame. For most practical purposes, it can be considered magnification, but in reality, it's not.
Several manufacturers offer lenses designed for full-frame cameras (these include 35mm SLR lenses) and lenses designed specifically for the smaller APS-C sensor. The full-frame lenses cover an image circle 43.2mm in diameter—the diagonal measurement of the full-frame sensor (or 35mm film frame). APS-C lenses cover an image circle of around 28.3mm—the diagonal measurement of an APS-C image sensor. If you use an APS-C lens on a full-frame camera, the image will vignette. Canon calls its APS-C lenses EF-S, and you can't even physically mount one on a full-frame camera. Nikon calls its APS-C lenses DX, and if you mount one on a full-frame Nikon DSLR, it will crop to the smaller DX format automatically. Sigma calls its APS-C lenses DC, Sony—DT, Tamron—Di II and Tokina—DX. (Pentax doesn't make full-frame DSLRs, so their DSLR lenses are APS-C.)
The advantages of APS-C lenses are that they can be designed to be smaller and perform better with smaller sensors. But if you intend to go full-frame some day, you'll be better off buying full-frame lenses now, even if you currently use an APS-C camera—you won't be able to use APS-C lenses on your new full-frame camera and take full advantage of its sensor's megapixels due to the crop factor.
With many zoom lenses (especially those using internal focusing), the maximum focal length decreases as you focus closer—in some cases, a 70-200mm zoom winds up with a maximum focal length of maybe 140mm when set at 200mm and its closest focusing distance. For most purposes, this isn't a big deal. If you're at the lens' minimum focusing distance, a really long focal length isn't as important as with distant objects. If you do insect and flower photography, consider the minimum focusing distance and magnification: If your 70-200mm lens focuses down to 0.25X, it doesn't really matter if it's doing that at 200mm or 140mm; you're still getting 0.25X. (Of course, this focus breathing means you have less working distance—less space between you and the insect—but at these distances, that's not as critical as at true macro shooting distances.
Internal focusing offers its advantages. First, the lens doesn't change physical length during focusing, good for balance. Second, the front element doesn't rotate during focusing, so polarizers and graduated and other orientation-sensitive filters maintain their orientation. Note that while internal-focusing lenses don't rotate or extend as they focus, many do rotate and extend physically as they're zoomed.
Most zooms today use a zoom ring, which you rotate to change focal lengths. But with some, you push or pull the zoom control rather than rotate it. The push-pull type is probably more prone to sucking dust into the lens, but we haven't found that to be a big problem. So, mostly, it's a matter of personal preference. Some photographers feel more comfortable with the rotating ring, others, with the push-pull control.
Most telephoto zoom lenses incorporate low-dispersion elements to minimize the effects of chromatic aberrations, improving image quality. They have designations such as LD, SLD, ED, ELD, HID, ULD and the like, depending on manufacturer and degree of correction. Fluorite elements are even more effective at compensating for chromatic aberrations, but also very costly, and are found only in some high-priced lenses. Lenses with low-dispersion elements are "better" than those without, but you should always test a telezoom before buying (you can rent many for a couple of days), rather than just depend on the presence and number of such elements.
Aspherical elements correct spherical aberration, which is more of a problem with wide-angle lenses, so they aren't often found in telezooms.
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Sigma 150-500mm F/5-6.3 APO DG OS HSM
This has been the go-to lens for wildlife photographers on a budget, the only telezoom with built-in stabilizer to go out to 500mm until the just-introduced Tamron 150-600mm. We know several bird photographers who use the Sigma 150-500mm lens with very good results, and we liked our test example when we tried it. A good stabilized zoom that goes out to 500mm for a bit over $1,000—very nice. If you're a wildlife photographer on a budget, we suggest that you check them both out.
If you use a Pentax or Sigma DSLR and need a long lens, this is your best option. Pentax's only lens longer than 300mm is a 560mm prime that costs thousands more, and the Tamron 150-600mm isn't available in Pentax or Sigma mounts.
Most newer telezooms incorporate AF motors. The best ones are quick, smooth and quiet: Canon's USM, Nikon's AF-S, Olympus' SWD, Pentax's SDM, Sigma's HSM, Sony's SSM II and Tamron's USD. The main thing to check regarding a telezoom's AF motor is whether you can change focus while in AF mode. If you're photographing a bird in flight, for example, and the AF system loses focus and focuses down to its minimum focusing distance, it's a lot quicker if you can just turn the focusing ring back to infinity and press the button to start AF again, rather than have to switch to manual focusing mode, reset focus, then re-enter AF mode. It's also helpful to be able to "ballpark"—focus on the flying bird manually before activating the AF system—something you can't do if the lens won't let you focus manually in AF mode. (With some lenses, rotating the focusing ring manually while in AF mode can damage the AF motor; you don't want this type of lens if you're photographing birds in flight.)
Many Canon, Nikon, Sigma and Tamron telephoto zooms come with built-in optical image stabilization (Canon's designation for this is IS, Nikon's is VR, Sigma's is OS and Tamron's is VC). This feature moves a group of lens elements as you shoot to minimize the effects of handheld camera shake. Olympus, Pentax and Sony DSLRs have built-in sensor-shift stabilization, which moves the image sensor rather than lens elements to compensate for camera shake. This has the advantage of being available with any lens you put on the camera, not just special stabilized ones. The drawback is that you don't see the stabilizing effect in the eye-level optical viewfinder. If you work handheld with a long lens, stabilization is a wonderful boon; if you work from a tripod, check the instructions for the lens or camera to see whether you should switch the stabilization off.
Each glass/air interface in a lens causes a loss of light due to reflections, and telezooms generally have lots of elements. So, manufacturers coat the surfaces of the elements to reduce reflections, minimizing this light loss. A telezoom with good coatings on all element surfaces can transmit a much greater percentage of the light than a lens with uncoated elements. The coatings also help to produce good color rendition. The newest coatings (found, naturally, on the newer lens designs) are even more effective than older coatings.