Wildlife photographers need long lenses because it's usually difficult to get close enough to wild animals to get good shots (and sometimes dangerous to try to do so). If they want to photograph wildlife in motion, especially birds in flight, they also need excellent AF performance. Today, we have a range of long lenses to suit many budgets. The newer consumer supertelephoto zooms are surprisingly good, allowing one to get a quality lens that goes out to 400mm, 500mm and even 600mm.
But the ultimate wildlife lenses are the pro big guns: the 300mm ƒ/2.8, 400mm ƒ/2.8, 500mm ƒ/4, 600mm ƒ/4 and 800mm ƒ/5.6, the 200-400mm ƒ/4 zooms from Canon and Nikon, and Sigma's 300-800mm ƒ/5.6 zoom. These bulky, costly beasts feature the best optical designs and glass, the best AF systems and the most rugged construction, and are the standbys of wildlife pros throughout the world.
On the downside, these big guns are a bit unwieldy, although we do know a few who handhold them for bird-in-flight shots (far more use them on tripods with gimbal heads, though). If you're used to handholding Canon's EF 400mm ƒ/5.6 (2.8 pounds) and think buying the EF 400mm ƒ/2.8 (8.5 pounds) will automatically get you better animal action shots, it won't, at first. Shooting with the big lens is a lot different than shooting with the light one, and requires different techniques (and, preferably, a tripod). But once you master it, the 400mm ƒ/2.8 will deliver better images due to its better optics and AF performance.
When you start using really long lenses, be aware that the field of view is quite narrow, making it hard to find the subject when you bring the camera up to your eye. Even if you're used to using a shorter telephoto lens, like a 70-300mm consumer zoom, when you buy a 600mm big gun (or even the much lighter 4.3-pound Tamron SP 150-600mm supertele zoom), there will be a big learning curve: A 600mm lens has half the field of view of a 300mm, and it will be difficult to even find the subject the first several times you use the longer lens. The zoom actually offers a big advantage here, in that you can zoom to the widest focal length to acquire the subject, then zoom in on it.
Zoom Or Prime?
Zoom lenses offer the advantage of a whole range of focal lengths in a single unit. With a 70-300mm zoom, you get 70mm, 85mm, 100mm, 135mm, 200mm and 300mm, and all the focal lengths in between. This can save a lot of cost and bulk in obtaining and carrying several prime lenses, and lets you change focal length with the twist of a wrist—much quicker than physically changing lenses, and avoiding dust on the image sensor from multiple lens changes. If you're so inclined, you can also zoom a zoom lens during a long exposure to produce an interesting "explosion" effect.
Prime lenses are sharper than zoom lenses and generally faster, although the fastest 500mm lens is Sigma's $25,999 200-500mm ƒ/2.8. It's easier to correct all the possible aberrations and distortions and other lens flaws for a single focal length than for a whole range of focal lengths. That said, many fine wildlife shots have been made with pro zooms—image quality may not match a pro supertele prime, but it can still be very good.
Sigma's $25,999 200-500mm ƒ/2.8 notwithstanding, the fastest zoom lenses that go out to 400mm have a maximum aperture of ƒ/4, and Canon and Nikon both offer 400mm ƒ/2.8 primes. The fastest zooms to go to 500mm (excluding the aforementioned Sigma 200-500mm) have maximum apertures of ƒ/6.3 at 500mm, while Canon, Nikon and Sony offer 500mm ƒ/4 lenses, and Sigma has a 500mm ƒ/4.5. The zooms that go to 600mm also have a maximum aperture of ƒ/6.3 at that focal length, while Canon and Nikon offer 600mm ƒ/4 primes. If you like to shoot in dim light (dawn and dusk, for example), the faster primes will let you use a lower ISO or a faster shutter speed.
Zooms offer an advantage when you need reach and are on a budget. The lowest-priced 500mm prime lens (excluding cheapie mirrors) costs $500, while there are a number of good supertele zooms that go out to 500mm and even 600mm for under $2,000, several closer to $1,000.
Variable-Aperture Or Fixed?
In general, lower-priced zoom lenses change aperture as they're zoomed; they have a larger maximum aperture at the shortest focal length and a slower one at the longest focal length. For example, the Tamron SP 70-300mm ƒ/4-5.6 has a maximum aperture of ƒ/4 at 70mm and a maximum aperture of ƒ/5.6 at 300mm (and, yes, this full-stop difference extends through the aperture range; the minimum aperture is ƒ/32 at 70mm and ƒ/45 at 300mm).
Higher-end zooms generally maintain a constant aperture throughout the zoom range. Sigma's 300-800mm ƒ/5.6 has a maximum aperture of ƒ/5.6 at 300mm, 800mm and everywhere in between.
Ironically, the "constant-aperture" zoom actually has to change the diameter of its aperture as the lens is zoomed to maintain a constant ƒ-number throughout, while a "variable-aperture" zoom can maintain a constant-aperture diameter throughout the zoom range (which would account for the change in ƒ-number as it zooms, the ƒ-number being the ratio between the focal length and the diameter of the entrance pupil (the apparent size of the aperture opening as viewed through the front of the lens).
Letting the ƒ-number change as the lens is zoomed allows designers to create more compact and less costly zooms. Built-in TTL metering automatically compensates for the change so, in practice, the main consideration is that variable-aperture zooms get slower as they get longer. Since lenses are generally sharpest when closed down a stop or two from wide open, this can be a disadvantage in dim light, as you'll have to use a slower shutter speed or a higher ISO setting, both of which can have a negative effect on image quality. That said, though, lots of great wildlife images have been made with variable-aperture zooms.
Faster lenses are bigger than slower ones—larger in diameter and much heavier. Canon's 300mm ƒ/4 IS measures 3.5x8.7 inches and weighs 2.6 pounds, while their 300mm ƒ/2.8 IS measures 5.0x9.8 inches and weighs 5.2 pounds. Those 2.6 pounds are easily handholdable for most photographers, while 5.2 pounds is not. Fast lenses let you shoot at a lower ISO or faster shutter speed in a given light level, and being pro lenses, offer better optical and AF performance. But they're also much bulkier and more costly. Pros and serious amateurs who can afford them generally go for the big guns for the better performance, but many (especially those who prefer to work handheld or have limited budgets) are quite happy with slower lenses.
All-out pro lenses also tend to be bulky, mostly because of more rugged construction and better glass. Tamron's SP 150-600mm ƒ/5-6.2 supertele zoom measures 4.2x10.2 inches and weighs 4.3 pounds, while Canon's 600mm ƒ/4 supertele measures 6.6x17.6 inches and weighs 8.6 pounds, and Nikon's AF-S 600mm ƒ/4G measures 6.5x17.5 inches and weighs 11.2 pounds. Again, the consumer zoom doesn't match the optical and AF performance of the pro primes, but it's much more compact—and costs one-tenth as much.
A lens field of view depends on its focal length and the size of the sensor in the camera. A 300mm lens has an angle of view (measured diagonally) of about 8° on a full-frame DSLR, about 6° on an APS-C DSLR and about 4° on a Micro Four Thirds mirrorless camera. Since photographers generally think in terms of focal length rather than field of view, this means a 300mm lens on an APS-C camera frames like a 450mm lens on a full-frame DSLR, and on an MFT camera, frames like a 600mm lens on a full-frame DSLR. You put that Tamron or Sigma 150-600mm lens on an APS-C body and zoom out to 600mm, and you get the framing you'd get with a 900mm lens on a full-frame camera.
Field of view is something to keep in mind if you're used to using a compact zoom. As mentioned, a 600mm big gun will have a big learning curve. That 600mm lens has half the field of view of a 300mm, and it will be difficult to even find the subject (especially a small bird) the first several times you use the longer lens.
Three recent supertele zooms, while not in the "ultimate big-gun" category, are of interest to serious wildlife photographers. Tamron's SP 150-600mm ƒ/5-6.3 offers surprisingly good image quality and AF performance, relatively light weight (4.3 pounds) and built-in Vibration Compensation—for $1,069! Sigma's pro-oriented 150-600mm ƒ/5-6.3 Sports lens features rugged weatherproof construction and presumably excellent performance (we haven't tested it yet) for $1,999. Canon's long-awaited successor to the original EF 100-400mm ƒ/4.5-5.6L, the EF 100-400mm ƒ/4.5-5.6L IS II USM includes optical, AF performance and image-stabilizing improvements for $2,199. All are good options for wildlife photographers who need lots of reach, but don't have the budget for the pro big guns.