Let's look at lenses for a while.

Dr. Garth's Prescription for Improving your Photography #342:

Put your money in the glass. The camera is basically a light tight box that holds the film flat. The hard part of translating the real world into images on film is done by the lens.

Some people think they are getting a great bargain when they get a spiffy, state of the art camera with a cheapie brand-x lens. I don't understand the logic of this arrangement. If you must compromise on one of these components, put a good lens on an inexpensive camera.

I have a sneaky, devious reason for including a long section on lenses, when I briefly gloss over most of the equipment: it's the best way to introduce a bunch of vocabulary you'll need to understand for the rest of the book.

Focal length and aperture

Lenses are identified by their focal length and widest aperture. In the 35mm film format, a lens with a 50mm focal length is "standard"; it has approximately the same field of view and apparent perspective as your eyes. A wide angle lens has a shorter focal length, (smaller number) and includes more of the world in your photo. A telephoto lens magnifies your subject more, and has a longer focal length. The widest aperture of a lens defines how much light the lens gathers. The smaller the number, the more light. So, a 28mm, f2 lens is "faster", it gathers more light, than a 28mm, f3.5. A 28-80mm, f4.5 zoom, which includes the same focal length, is "slower" still.

A faster lens has a few important advantages over a slower one. First, it enables you to use a faster shutter speed in low light situations. Second, it gives you a brighter image in the viewfinder, making it easier to focus and compose. Third, faster lenses are generally superior, optically and mechanically, to their slower cousins.

Most people don't realize that different focal lengths have effects beyond changing the field of view. We'll look at two other effects of focal length: its influence on depth of field, and its influence on the apparent perspective of our images.

The depth of field thing is pretty simple. Wider lenses have greater depth of field than telephotos. The shorter the focal length, the greater the depth of field, assuming the aperture remains at the same setting. In combination with what we learned in our earlier exploration of apertures, we now have two ways to control depth of field. For the absolute maximum depth of field, use your widest lens with its smallest aperture. In the other extreme, a telephoto lens with its widest aperture gives you so little depth of field that an animal's nose may be sharp while its eyes are blurry.

Perspective and focal length

The apparent perspective thing is a little more complex. Wide angle lenses stretch out the perspective, enhancing the perceived distance between near and far objects. Telephotos tend to compress the perspective, making things look squished together.

To illustrate this, let's look at a baseball diamond. If you are standing just behind the pitcher's mound, home plate looks a long way away, especially if you have to throw a baseball accurately at high speeds to the plate. If you take a picture using a wide angle lens from this spot, the plate looks even farther away. But, on your television screen, the pitcher and the hitter look close enough to kiss. That's because the TV folks have a camera with a very long telephoto lens out in the center field seats, squishing your perspective.

Now, you technical nitpickers out there are probably screaming "but perspective stretching and compression aren't real, they're just illusions!" It's true that the effect is illusory; it's just a matter of the relative distances between the camera and the various elements in the photo. It's also true that if you were to take two photos from the exact same spot, one with a wide lens and one with a telephoto, and then enlarge and crop the wide one so that it covers the same area as the telephoto, you'd find that the perspective is the same. But so what? The effect seems real when you're in the field, and it seems real when the photo is on the wall. So I treat it as real.

Telephoto lenses

Most people, when they begin to take photography seriously, get a telephoto lens. There's something hypnotic about the idea of more magnification. Maybe I'm weird, but I don't find telephotos all that useful for nature photography. Medium telephotos (roughly 70-150mm) are wonderful lenses for portraiture, but that's not nature photography. Really long telephotos are necessary for wildlife, but that's a pretty specialized area which I'll touch upon only briefly.

Why, you may ask, do I find telephotos to be of such limited usefulness for nature photography? First, consider the compressed perspective they produce. I don't think the squished together perspective looks right for most subjects. Second, telephotos allow such limited depth of field I find it difficult to get the whole frame in focus.

These two factors combine to make telephoto landscapes look rather flat in general. If you include an interesting near foreground, it will probably be out of focus, and the perspective compression reduces the feel of depth among distant objects. Still, there are times when you really need a telephoto. Maybe you can't get close enough to your subject with a standard or wide lens. Sometimes that compressed perspective looks really neat, such as a view of rows of distant ridges receeding into the distance. Occasionally, you might want to use the limited depth of field to isolate an in focus subject against an out of focus background.

Wide angle lenses

While tele lenses certainly have there place in a nature photographer's camera bag, I don't use mine that often. On most day hikes, I carry one telephoto and three wide angle lenses. I use the telephoto for 10% of my shots, tops.

Wide angle lenses, with their perspective stretching, long depth of field, and broad angle of view, are perfectly suited to many types of nature photography. In sweeping panoramas, both distant mountain peaks and nearby wildflowers can be in sharp focus in the same frame. From a rock in the middle of a small stream, the brook becomes a raging river, with towering trees converging overhead.

There are a few potential problems to be aware of when you are using a wide lens, however. That stretched perspective can make distant objects look very distant, indeed. A snow capped mountain might look too tiny and insignificant in a shot with a wide lens. Is the shot really about the mountain, or is it about the flower filled meadow? If the mountain is the principle subject, you may need to use a longer lens. If the meadow is the main subject, you'll probably be better off using a wide lens.

Another potential problem with wide lenses involves the distortion that is inevitable with a very wide field of view. If you were standing in a forest with a wide lens on your camera, you might notice that the parallel lines formed by the tree trunks look parallel if, and only if, you hold the camera exactly level, so that the film plane is perpendicular to the ground. If the camera is pointing up, the trunks appear to converge towards the top of the frame; if the camera is pointing down (maybe the ground is sloping away from you) the trunks appear to diverge at the top. The wider the lens, the more dramatic the effect.

This distortion effect is inevitable due to the geometry of the situation; you are trying to translate a round world onto a flat surface. This is like a problem that mapmakers face. You've probably seen world maps where Greenland looks bigger than South America. Maps of small regions are much more accurate. The smaller field of view is closer to a flat surface.

Keep this effect in mind when you are using a wide angle lens. The distortion is not necessarily a bad thing; sometimes it looks really neat. If you use this effect, use it on purpose. For a more natural looking wide angle landscape, be sure the camera is level.

Zoom lenses versus fixed focal lengths

There are, broadly speaking, two types of lenses: fixed focal length lenses, and zoom lenses. A zoom lens allows you to change the magnification of your subject without moving. To change magnification with a fixed focal length lens, you need to move closer to, or farther away from, your subject.

These days, zoom lenses are all the rage. Some manufacturers don't really offer much in the way of fixed focal length lenses anymore, except for extremely wide, super telephoto, and macro lenses. This is progress, I suppose.

Zooms have a lot going for them. They reduce lens juggling considerably. Instead of carrying a 28mm, a 50mm, and a 100mm, and changing lenses all of the time, you can get a 28-105mm zoom and replace all three. Plus, you get all of those in between focal lengths, like 40mm, 75mm, and 63-1/2mm. But what does the world look like through a 63-1/2mm lens, anyway? What apparent perspective and depth of field does it produce? Shouldn't we move around a little to compose our photos, rather than just taking quickie grabshots?

OK, OK, I'm a curmudgeon in training, blindly railing against all that newfangled technology. But zooms do have a real, tangible disadvantage, along with their more obvious advantages.

Zooms are much slower than comparable fixed focal length lenses. A typical fixed 28mm wide angle lens has a wide aperture of f2.8, with f2 lenses readily available. A typical 50mm "standard" lens has a wide aperture of f2, with f1.4 lenses available. The equivalent zooms have apertures around f4.5. So, with the zoom, you have less than half of the light gathering power at 28mm, and less than one quarter of the light gathering power at 50mm.

Sure, you can buy a 28-80 f2.8 zoom lens, but it'll cost you an arm, a leg, both kidneys, and a cornea. Furthermore, it'll be heavy and delicate, with an enormous, easily scratched front element. That's not really something that I want to take into the rough, rugged, wet field. Still, the large, expensive zoom is one full stop slower than a humble 50mm f2.

You may well ask, “why do I care what the widest aperture of my lens is? you keep telling me to use a tripod and stop down for maximum depth of field...so what if my lens is slow?” I'll tell you why: when you look through the viewfinder, you are looking through the widest aperture of the lens. On most cameras, the lens stops down to the value you've set when you press the shutter. So, a faster lens means that you get a brighter image in the viewfinder. It also means that you're looking at the image with its minimum possible depth of field, so you can carefully place the prime focal plane where you want it.