Many people new to photography don’t realize that not all lenses can “zoom”.  In fact, the lenses I use most often for bird photography don’t “zoom” at all: they have a fixed focal length which provides only a fixed amount of magnification.  They’re what photographers call prime lenses.  A zoom lens, on the other hand, provides a range of magnifications.  The act of “zooming in” is a matter of increasing the magnification (which means increasing the focal length).  The availability of both zoom lenses and prime lenses means that you have yet another decision to make when shopping for a birding lens: whether to buy a zoom lens or a prime  lens.
    As it turns out, there are advantages to both types of lens.  Traditionally, zoom lenses had the advantage of flexibility (since you can vary the focal length in the field, as needed), while prime lenses had the advantage of optical quality.  For a number of years, manufacturers had difficulty designing and manufacturing zoom lenses with the same optical excellence as their simpler prime cousins.  The problem is that for zooming to work, the lens has to have additional glass elements that move in a precisely choreographed pattern (during zooming) in order to satisfy the nonlinear requirements of a high-quality optical system.  Any deviations from these requirements can result in various types of aberrations or optical distortions, such as unnatural color shifts, ghosting (parts of the image faintly duplicated and shifted from center), halos, or vignetting (a dark doughnut around the outside of the image).
    The word on the streets is that modern zooms no longer suffer from these problems—at least, not those from the top manufacturers such as Nikon and Canon.  Apparently, the technology has advanced to the point where top-of-the-line zoom lenses are now comparable (even if not 100% equal) to prime lenses in their optical quality—barring, of course, any random manufacturing defects, which can affect any individual lens, whether zoom or prime.  Given the larger number of optical elements and moving parts in zoom lenses, the incidence of such manufacturing defects may, conceivably, be greater in zooms than in prime lenses, though this is purely speculation.



    The degree to which the gap has been closed between primes and third-party zooms (such as those offered by Sigma, Tamron, and Tokina, among others), however, is unclear.  Certainly, Sigma in particular has made enormous strides in both its design and manufacturing quality and in the range of lens models that it offers.  One Sigma lens in particular, the 300-800mm f/5.6 “Sigmonster” lens, has become very popular among bird photographers.  At a recent trip to a warbler hotspot in Ohio, I noticed that the number of serious photographers using Sigma lenses rivaled the number of those using Nikon gear (though the number of Canon lenses still far outstripped either group). 
    In addition to having more moving parts, zoom lenses typically have a significantly larger number of glass elements than their non-zoom counterparts.  For example, the zoom version of Sigma’s 800mm “Sigmonster” lens has 18 elements, while the non-zoom version has only 13.  Another example is Canon’s popular 100-400mm f/4.5-5.6 zoom lens, which has 17 elements, as compared to Canon’s 400mm f/5.6 prime lens, which has only 7 elements.  Having fewer elements gives two advantages: fewer parts that can be mis-aligned at the factory, and fewer glass-to-air and glass-to-glass transitions for the light path to traverse.  As we’ll see in the next section, any time light rays enter or leave a glass element in a lens, some of the light is reflected instead of passing through as intended, resulting in a potential loss of brightness and contrast in the resulting image.  With proper lens coatings, however, the amount of light lost to reflection can be substantially reduced.
    At the upper range of focal lengths (where serious bird photographers typically work), the issue of zoom-versus-prime doesn’t arise as often as at smaller focal lengths.  The most serious bird photographers have 500mm or 600mm f/4 lenses from either Canon or Nikon, and these are only available as prime lenses.  The exception is the Sigma 300-800mm zoom lens mentioned above, which seems to be growing in popularity, though direct comparisons of image quality between this lens and the (prime) alternatives from Canon and Nikon are, as yet, difficult to find.  Certainly, some very impressive bird photos have been taken with the Sigma lens.  Users of this lens cite the convenience of being able to find the bird at the wide end of the focal-length range (300mm) and then rapidly zoom in to 800mm before taking the shot.  I can certainly attest to the difficulty (at least, for novices) of finding a small bird in dense foliage using an 840mm lens setup (600mm + 1.4× teleconverter).  The problem is that at ~800mm your window on the world (field of view) is very small, so that it can take a lot of searching just to get the bird into frame.  However, I’ve found that with a little practice you can get so good at pointing the lens right at the bird (before even looking through the viewfinder) that searching for the bird via the viewfinder is rarely needed.  Thus, while the finding the bird is easier with a zoom argument may sound very convincing to a novice, it’s really not necessary once you’ve developed a modicum of skill with a long lens, and in fact the time it takes you to zoom in and out can potentially result in lost shots.
    At the lower range of focal lengths, the issue of zoom-versus-prime is a bit more relevant, since there are more options in both categories.  Considering just the Canon product line, there’s the highly popular 100-400mm f/4.5-5.6 zoom, the near-legendary 400mm f/5.6 prime, and the somewhat controversial 400mm f/4 DO lens (I’ll ignore the 400mm f/2.8 lens, since it’s too heavy for handheld work, and too under-powered for most other uses).  As noted above, the 100-400mm zoom has more than twice the number of glass elements as the 400mm prime (17 versus 7), suggesting that the prime should be sharper.  However, the zoom has two advantages over the prime in this case: this zoom lens has image stabilization (IS), and it offers the flexibility of being able to reduce the focal length when photographing large birds at extremely close distances.  (Although Canon could add IS to its 400mm f/5.6 prime lens, it hasn’t done so as of yet, despite popular demand; Nikon, on the other hand, doesn’t even offer a 400mm f/5.6 prime lens, and its 80-400mm zoom focuses too slowly for birds in flight).  For a general “walkabout” lens, the 100-400mm lens is very popular, since the 100mm end is useful for scenery shots and zoo animals, while the 400mm end provides just enough magnification for medium-sized birds at intermediate distances.  Also, for photographing birds in flight (BIF), 400mm is a very popular focal length, both among amateurs and pros.





    Among the third-party zoom lenses, one stands out as being particularly popular: the Sigma 50-500mm f/4-6.3 lens, also known as the “Bigma” lens.  This lens has recieved high marks from many reviewers, and reportedly boasts both high optical quality and excellent durability, as well as a very reasonable price tag (around US $1000).  Until recently, the main criticism of this lens was the lack of any image stabilization.  However, a new model is now available from Sigma, which zooms from 150mm to 500mm and offers Sigma’s version of image stabilization (termed optical stabilization by Sigma, or “OS”).  This lens also carries a very, very affordable price tag of about $1000, making it a very attractive option for birders.  The lens weighs 4 lbs, which is at the upper limit for a hand-held package, though it should be quite manageable for a healthy adult male, with some effort, and can easily be mounted on a monopod for use by women.  The new Bigma has roughly the same number of elements as the old: 21 versus 20.  Constrast this, however, with Sigma’s 500mm prime lens, which has only 12 elements.  Another thing to note is that these 500mm zooms tend to be much smaller and lighter-weight than the 500mm f/4 prime lenses that pro bird photographers so clearly prefer.  While the latter are invariably used on a tripod, the 500mm zooms offer the advantages of both hand-holdability and variable magnification (i.e., zoom).
    Sigma offers several other zoom lenses of note.  The first is their 120-400mm zoom lens, which also features image stabilization and a price tag of about US $900 (Canon’s 100-400mm zoom is several hundred dollars more).  Rather more expensive is Sigma’s 200-500mm f/2.8 lens, which reportedly weighs about 35 lbs and costs about $38,000 (!).  This latter lens has been unofficially dubbed “The Sigmarine”, and I’ve yet to actually see one in use in the field.
    One additional piece of advice: zoom lenses have, at least traditionally, been known to be less accepting of teleconverters than prime lenses.  That is to say, those who’ve tried using a teleconverter with their zoom lenses have seemed, overall, less satisfied with the resulting image quality than those who’ve tried using TC’s with their prime lenses.  This might be a result of the increased number of optical elements in zoom lenses.  Because teleconverters also contain multiple glass elements (5 elements for Canon’s 1.4× TC and 7 for their 2× TC), the combination of increased numbers of glass elements in the zoom lens and added elements of the teleconverter might simply cause some critical threshold to be crossed, in terms of acceptable image quality.  Note, however, that Canon’s 400mm f/4, 500mm f/4, and 600mm f/4 lenses all have 17 elements—the same number as in Canon’s 100-400mm zoom lens.  As always, it’s better to test out a lens yourself, if possible, than to rely on theoretical considerations such as numbers of elements or whether a lens is a zoom or a prime.  Barring that possibility, I recommend finding a set of bird photos that you like that were taken with the lens combination you’re considering purchasing (perhaps on a photo hosting site such as Flickr, which allows searching for photos by lens model).  Just keep in mind that those photos may have been extensively post-processed after coming out of the camera.  Some lenses produce images that are quite sharp right out of the camera, while other lenses produce images that are highly amenable to sharpening in post-postprocess (i.e., in Photoshop); still others produce images so poor that all the sharpening in the world can’t make them look good.
    For me, personally, I prefer prime lenses all the way—except for extremely close-up shots of large birds (such as herons and egrets), for which I use my 70-200mm f/2.8 lens—a $1700 zoom lens that is worth every penny (see the two examples of birds photographed with this lens, above).  But the only time I use this lens is in settings where I’m photographing semi-large, semi-tame birds at close range.  In places frequented by large numbers of people, ducks and even herons/egrets can become fairly trusting of people, allowing them to approach to with 10 or 15 feet.  This is especially true in places where the birds often receive food from people: at duck ponds where children feed the ducks, at fishing hotspots where birds feed on discarded fish scraps from fishermen, etc.  In these places, a small-focal-length zoom (such as a 70-200mm lens) can be ideal for framing the bird at close range, or for getting flight shots as the birds fly by very close to the camera.  But in every other birding situation, I find that a large-focal-length prime lens serves my needs better.  With a 500mm or 600mm f/4 lens, you can put on or take off a 1.4× or 2× teleconverter as needed, giving you almost as much flexibility as a zoom lens, but with the optical quality of a prime.