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.
Close-Up Lenses
Your eyes don't focus so great on really small things either. Do you try to pull your cornea a foot away from your retina? No. You stick a magnifying glass in front of your cornea. You can do the same thing for your normal lens. Unlike your cornea, it even has convenient threads for attaching a magnifying glass. The magnifying glass screws into the same place where a filter would go.
A camera store could never sell you a "magnifying glass" for $50 so they call these things "supplementary lenses" or "close-up lenses". Good things about close-up lenses:
- they don't require any exposure corrections
- you can throw a couple in your pocket in case you need them
Bad things about close-up lenses:
- they aren't very high quality though they might be good enough if you stop down to f/16 and if you can find two-element close-up lenses (e.g., Nikon-brand) instead of the cheapo one-element ones.
- you have to take them on and off constantly if you are taking pictures of things at different distances.
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