Taking close-up pictures of small things is called "macro photography." I have no idea why. Perhaps because the small things in macro photography are generally larger than the things you are taking pictures of when doing "micro photography". If you really want to be pedantic then you should say you are doing "photomacrography".
What Kind of Camera
Point and shoot digital cameras can have remarkable macro capabilities, but for best results you want a single-lens reflex camera. These allow you to attach special-purpose macro lenses and show you in a bright optical viewfinder what you will get on the sensor.
A typical setup might be a Canon Digital Rebel XTi (Black) (review) with a Canon EF-S 60mm f/2.8 Macro USM (review). This lens is designed for the small-sensor Canon cameras and gives a working distance equivalent to 100mm on a full-frame camera. The lens is specified to focus down to "1:1" or "life size". This means that the smallest object you can photograph that will extend to the corners of the final digital photo will be the same size as the sensor inside the Canon Rebel camera, 15x22mm. A professional photographer might use Canon EOS 5D (review) and a lens designed for full Canon EF 100mm f/2.8 Macro USM (review). Confusingly, this lens is also specified to focus down to "1:1", but this time the sensor is 24x36mm in size, the old 35mm film standard. So you can't take a photo of something quite as small as with the cheaper equipment.
In the film world, the 35mm camera systems had comprehensive range of macro lenses and accessories and some medium format systems, such as the Rollei 6008 would have at least a few lenses and extension tubes. Only the extremely patient ever did macro photography with a 4x5 inch view camera.
Doing it all with a Normal Lens
In the good old days a 35mm single-lens reflex camera came with a 50mm "normal" lens. These lenses were extremely light, rugged, and high quality, so naturally the consuming public abandoned them for heavy, fragile, low quality zooms. But that's another story... Anyway, suppose that you are out in the woods with your Canon EOS 5D, a full-frame camera and a 50mm normal lens, and you want to take a picture of the tip of a pine needle. [Everything in this section applies equally to using a 30mm prime lens, e.g., Sigma 30/1.4, on a small-sensor camera such as a Canon Rebel or Nikon D-series.]
First, though, you want to take a picture of the moon. That's pretty far away, so you feel comfortable setting the lens focusing helical to "infinity". The "nodal point" of the optics will now be 50 millimeters from the plane of the sensor. [Note: exposure for the moon should be roughly f/11 and 1/ISO-setting.]
The effort of setting up your tripod is so great that you become tired and fall asleep. When you wake up in the morning, there is a bear standing 10 feet away. You refocus your 50mm lens to get a picture of the grizzly. As you turn the helical from "infinity" to "10 feet", notice that the optics are racked out away from the sensor. The nodal point is a bit farther than 50 millimeters from the sensor plane. The lens is casting an image circle somewhat larger than the 24x36mm sensor. Some of the light gathered by the lens is therefore being lost but it isn't significant.
After snapping that photo of the bear, you notice that his fangs are glistening. These aren't going to appear very large in your last shot, so you move up until you are about 1.5 feet from the bear. That's about as close as the lens helical will let you focus. The nodal point is now pretty far from the lens. Extra light is spilling off to the edges of the frame , but still not far enough to require an exposure correction. The bear's face is 1.5 feet high. You've oriented the camera vertically so that the face fills the 36mm dimension. 36mm is about 1.5 inches. So that means you are working at "1:12". The subject is 12 times the size of the subject's image on the sensor.
You're losing some light, but also you notice that you don't have too much depth of field. A 50mm lens focussed down to a foot from the subject only has a depth of field of 1/16th of an inch at f/4. No problem. You haul out a big electronic flash and stop down to f/11. Now your depth of field is a whopping ... 1/2 inch.
Looking down, you become fascinated by some pattern's in the bear's claws. Each one is about 1.5 inches long. You'd like to fill the sensor's long dimension (36mm) with a claw, which means that the subject and its image will be the same size. You want to work at "1:1". But the folks at the lens factory skimped on the helical. You can't rack your optics out far enough to focus at 1:1. It looks like that pine needle tip photo is completely out of the question.Why did Canon limit your ability to focus close? For starters, at 1:1 the lens would be so far away from the sensor that it would cast a huge image circle. The standard 24x36mm frame would only be a tiny fraction. So only about 1/4 of the light gathered by the lens would reach the film, i.e., you'd have a two f-stop underexposure if you used the same exposure setting that you'd used for the picture of the bear when he was 10' away. A scene that required a lens setting of f/16 at infinity would require a lens setting of about f/8 at 1:1. All this other light would be bouncing around inside your camera and lens, reducing contrast. Finally, a fixed stack of optical elements can't be designed to form sharp images at so many different focussed distances.
Let's combine what we've learned until now: the aquarium
Combining everything we've learned up to this point, let's look at a case study: the aquarium. The items inside are pretty close, so you need a macro lens. If you put a rubber lens hood on the front of the lens, then you can mush it up against the glass and avoid reflections. Now you need light. Well, you can just get a flash on an extension cord and point it into the aquarium from just about anywhere.
Here are some examples from the public aquarium in Monterey, taken with a Nikon 8008, 60mm AF macro lens (set for manual focus), SB-24 flash, SC-17 extension cord. I wiped the glass with a handkerchief, asked my friend to hold the flash, and pushed the lens hood up against the glass:
People often write in wondering "How did you manage to get a lawyer in that last frame..."
With a depth of field of around one millimeter for precise macro work, camera positioning and focus become critical. If you have a good tripod and head, you'll find that you have at least 10 controls to adjust. Each of them will move the camera. None of them will move the camera along the axis that you care about.
That's why people buy macro focusing rails, e.g., Adorama Macro Focusing Rail, (compare prices). These are little rack and pinions capable of moving the entire camera/lens assembly forward and back. You use the tripod to roughly position the camera/lens and then the macro rail to do fine positioning.
The photos below are snapshots from the garden of the Getty Center. They were taken with a fancyCanon EF 180mm f3.5L Macro USM (review), but without a tripod. It was thus impossible to focus precisely or stop down enough to get sufficient depth of field. The results are rather disappointing...
Beyond 1:1 the Canon Way
(Flower interior at above left was captured with a traditional EOS film body; the jelly bean image at above right was taken with a D30 digital body (party like it's 2001).)
Beyond 1:1 with Nikon, et al
If you don't have a Canon EOS system and the special 1-5X lens, going beyond 1:1 requires more than buying a lens and turning the focus ring.
First, you can get a bellows (flexible accordion) and/or some extension tubes. These will let you push the lens farther away from the camera body. Extension tubes are rigid and tough; they only let you separate your body and lens in fixed increments. Bellows are delicate but they let you continuously control the lens distance from the body. How much magnification this extra extension will get you depends on the focal length of the lens. If you have a 1000mm lens that already needs its nodal point 1000mm from the sensor plane to focus at infinity, then a 50mm extension tube isn't going to be worth much. However, if you have a 50mm lens, then that same 50mm extension will take you all the way to 1:1.
Second, you probably want a "reversing ring" for your lenses so that you can turn the back element of the lens toward your subject. Why? Think about the normal way you use a lens. You are taking a picture of the Statue of Liberty. The Statue of Liberty is larger than 24x36mm. So you point the front element of the lens at the statue and the back element at the (smaller) sensor. Your lens is designed to work like this, taking the large and compressing it into the small. However, if you are working at 10:1, where the tip of a pine needle is going to take up a big portion of the frame, you want the lens to take the small and expand it into the large. So you want to just flip the lens around.
Third, once you've reversed the lens, you probably want some way to retain the automatic diaphragm. You want the aperture to remain fully open until just before your exposure and then close down to the selected shooting aperture. Rollei medium-format cameras have an all-electric interface between camera and lens, so this is done with clean and reliable electric contacts. Canon EOS would work the same way except that, after more than 15 years, Canon hasn't bothered to manufacture a bellows for the EOS system. An independent company, Novoflex, does make a bellows for Canon EOS, but for most people the Canon MP-E 65mm f/2.8 1-5X Macro (review) is a better choice. Nikon has mechanically stopped-down diaphragms for backward compatibility so they give you a strange dual cable release contraption.
More: John Shaw's Close-ups in Nature .
Beyond 1:1 the Lazy Way
At left are a couple of Ant Robots built at the MIT AI Lab by James McLurkin. Photographed with Canon EOS-5 and Canon 50/2.8 macro lens (lit by off-camera 430 EZ flash). This lens only goes to 1:2.
At right is a detail of the ant claws, which was taken with the Raynox Micro-Explorer, (compare prices). The Raynox is a set of close-up lenses, 6X, 12X, and 24X. These images are the result of mounting the 6X lens on a Canon 35-350L zoom lens.
A couple more example MicroExplorer shots (at left is an Ant robot detail; at right is a quarter on a $20 bill, full frame at f/8 (I think)). Note that vignetting is not as severe as it was at f/16 (above left).