More important than the camera, even, lenses are at the center of any photographic system. The cheapest camera you can buy will make as sharp an image as the top of the line model if both are equipped with the same lens. If you have to, save money and buy a less expensive camera, but always buy the best lens you can afford.
Normally, when discussing lenses, we divide them according to focal length. Shorter focal lengths (from 12 to 35mm) are termed wide-angle; lengths from 35 to 60mm are normal; and lengths over 60mm are considered telephoto (some people will make further distinctions between short, medium, and ultra telephoto lenses). In addition, there are fixed-length or prime lenses and zoom lenses; the latter can vary their focal length. There are also specialized lenses such as macro lenses and tilt/shift lenses.
Wide-angle lenses are very useful for the biological imager. In the field, they are sued primarily to document habitats; with their wide angle of view you see the forest, not the tree. In the lab, wide-angles are great where you need to photograph a piece of equipment in cramped quarters. And, of course, a short extension tube turns the wide-angle lens into a macro lens with a unique perspective. Wide-angles range from "fisheye" lenses which view 180o at a time (with considerable distortion) to lenses in the 28mm range. Some wide-angle lenses are rectilinear, which means that they are designed to minimize the circular distortion one often associates with wide-angle lenses. Getting away from the extreme wide-angles are lenses from 19 to 35mm which are often more affordable. These will have angles of view ranging from around 90o to 75o. Wide-angle lenses are often compact and light. One thing you do need to watch, however, is lens flare, the tendency of the lens to pick up internal reflections of the sun or other light sources. This is prevented by care in composing the image and by use of a lens hood (which also increases image sharpness by eliminating stray beams of light). Another potential problem with wide-angle lenses is the inclusion of unwanted objects (such as your feet) in the frame. If you are just starting out, you may want to purchase a wide-angle zoom instead of a wide-angle fixed lens.
It used to be that every camera sold came with a "normal" 50 mm lens. Not so anymore, as "normal zooms" in the 35-80mm range have become the most popular lens to include in a camera "kit". This makes sense, since the zooms are more flexible, fast film is of better quality, and the new zooms have better optical quality than zooms 30 years ago did. Still, most manufacturers still make and sell a lot of 50mm lenses. These lenses have an angle of view of about 46o, and produce a view that roughly corresponds to what a human sees in clear focus. For this reason, the pictures produced seem "normal". In addition, many of these lenses are fast (large maximum aperture), of high quality (and reasonable cost), and are compact and light. While these lenses have their uses, they are not as useful for the biological imager as they are for the average photographer. Sure, you need to have this focal range covered, but you don't need a 50mm prime lens.
These are the big guns of photography. Telephoto lenses have narrow angles of view, and thus make distant objects appear larger. Angles of view range from about 30o for a 85mm lens to 4o for a 600mm lens.
Eventually, most biological imagers will need a good telephoto lens of at least 400mm. Most bird photographers routinely use 500mm and 600mm lens, often with extenders or multipliers to decrease minimum focusing distance or increase magnification. The table below breaks down some of the the common focal lengths and the subjects:
Still, despite the disadvantages, zoom lenses have a place in the biological imager's photo bag. For instance, by carrying the 3 lenses above (+ a multiplier), a photographer could cover from 19 to 600 mm with 3 lenses (and there are 35-350mm zooms available that would cover from 35mm to 700mm with just 1 lens and a multiplier)! I find, however, that with longer zoom lenses I am constantly using the longest focal setting and rarely the intermediate ones. For that reason, my personal recommendation is to start out buying a "normal" zoom (35-80, 35-105, 35-135mm) and two prime lenses, a macro and a 300 or 400mm telephoto. as time goes but, add a wide-angle zoom as well.
You just can't substitute for a good macro lens. Some zoom lenses, as mentioned above, have a "macro" setting that gives 1:3 or 1:2 resolution, but this is insufficient for many insects, fungi and flowers. You can add extenders to any lens to get closer, but you lose light and camera shake looms as a problem. You can buy diopter or close-up lenses which screw into the filter mount on the front of a lens. Like reading glasses or a magnifying glass, these will also magnify close-up objects, but they never seem to produce as clear an image as a macro lens will. For information on what close-up lenses and extension tubes can do with a variety of lenses of different focal lengths, click here.
A true macro lens will produce a 1:1 (life size) image on the film without any extenders or special lenses, and can be used as a normal or telephoto lens out to infinity. Some manufacturers, Canon included, will sell lenses that do require such accessories as macro lenses. Be aware when you buy such lenses that they will be more difficult to use. When trying out an accessory in a store, remember that in the field you will have to do the same thing standing in a swamp swatting mosquitoes.
Macro lenses seem to cluster at a few focal lengths. 50mm macros are common, as are 100 and 200mm models. The longer focal lengths will give you a greater working distance - the distance between the front of the lens and the subject. This makes lighting easier, and gives you a better chance with wary subjects. Depth of field is normally less at longer focal lengths - but depth of field is a function of magnification, not focal length per se. Therefore, a 50mm, a 100mm and a 200mm macro lens working at a magnification ratio of 1:1 will all have the same depth of field. Of course, the disadvantage of the longer lenses is that they are bigger and heavier, and for the same money a shorter lens will have a larger maximum aperture.
If you need to go to greater magnifications than 1:1, there are several options available. There are "macrophoto" lenses that will go to 5x (but aren't good at distances over a few feet). Alternately, you can add multipliers and/or extenders to your macro lens. Even better is to use a bellows unit, which is a sort of variable extension tube that can give you any amount of extension you need. Bellows units are tricky to use in the field, however.
I've already mentioned the special macrophoto lenses; other lenses that biological imagers might consider include tilt and shift (TS) lenses and special mounts. TS lenses try to recapture some of the flexibility of the zoom cameras. Knobs on the side of the lens allow the photographer to bend light inside the lens; done with care this can correct for distortion or provide incredible depth of field. They do tend to be costly, however.
Another option is to use an existing optical system, such as a microscope or a telescope. Special adapters, usually T-mounts, can allow you to mount your camera on the instrument. The advantages are obvious; you could easily wind up using a $25,000 microscope as a lens and you can get magnifications up to 1,000x on a microscope. The main disadvantage is light. Most telescopes and microscopes just don't deliver enough light to the camera to allow for reasonable shutter speeds. With microscopes (particularly dissecting 'scopes) you can sometimes add flash to help with this problem, and some microscopes even come with flash built in. Modern cameras retain a high degree of automation when perched on a microscope or a telescope (astrophotography, however, requires exposure times in minutes, something your camera just isn't programmed for). Mirror telescopes will suffer from the same problems that mirror telephoto lenses face (see above). And, anytime you work at high magnification, camera shake is a real problem.
Most Useful Lenses for Biological Imaging
So you are just starting out - what lenses should you get? As I said before, personally I like to make sure I have a good macro lens, a good telephoto, and wide-angle and "normal" zoom lenses. Then, build from there depending on your interests. I have put together a buying program for the typical, money-limited college student just getting serious about photography. You can go to that link from here: