According to a generally accepted definition, the domain of macrophotography lies at the reproduction ratio of 1:1. A true macro lens must be capable of attaining a 1:1 reproduction ratio. Many macro lenses with built-in focusing helicoids actually focus between infinity and 1:1. Lower reproduction ratios than 1:1 are the domain of close-up photography, and higher ones of photomacrography.
The main characteristic - you could call it the main problem - in macrophotography is a very reduced depth of field. The depth of field can be increased by stopping down the diaphragm, but diffraction may reduce the picture sharpness and contrast. Therefore, a compromise between depth of field and sharpness should be sought when deciding the aperture. A shallow depth of field is often used in macrophotography to isolate a subject from its background.
Most macro lenses can stop down to f/22 or f/32. As a lens focuses closer, the image circle that it projects onto the focal plane grows larger. Therefore, the light that forms the image spreads onto a larger surface. This results in a darker projected image. In photography books, this is often described as an increase in the effective aperture of the lens. When a lens is focused at 1:1, its effective aperture is decreased by two stops (i.e., a diaphragm setting of f/16 requires the exposure time that would be associated with an aperture of f/32 when focused at infinity). Cameras with TTL (Through-The-Lens) metering, i.e., in practice all modern cameras, take this into account automatically, and normally you should be concerned about this only if setting the exposure time manually, or using a flash without TTL metering.
Some macro lenses, like the Micro Nikkors, transmit the effective (i.e., adjusted) aperture setting to the camera body. Thus, setting a diaphragm of f/16 on the lens barrel results in the setting being transmitted to the camera, and recorded in the picture data, to increase gradually from f/16 to f/32 when the focus is changed from infinity to 1:1. Other macro lenses do not do this, and with these you must take into account the increase in stops when using manual exposure or non-TTL flashes.
In macrophotography, focusing is preferably done by moving the camera and lens as a whole toward the subject or back. The reason for this is that changing the distance between lens and body (e.g. by rotating the lens helicoid) also changes the reproduction ratio. Focusing can be done by holding the camera by hand, or by mounting it on a focusing rack. In general, holding the camera by hand is difficult at magnifications exceeding 1:1 (i.e., in the magnification range of photomacrography), and becomes impractical above 2:1 or 3:1. Therefore, tripods and focusing racks are normally used in macrophotography, and setting up a macro shot in the field or studio is time-consuming. Plants and grasses swaying in the wind are a major problem in field macrophotography, because a movement of 1 mm may be enough to spoil the focus and/or composition. Clamps, ties and even temporary wind-breaking shelters may help to solve this problem, but a windy day is definitely not ideal for field macrophotography of subjects on vegetation. Wind can also affect the steadiness of a tripod.
Electronic flashes are commonly used as light sources in macrophotography. Unless the subject is completely static, flash is a necessity to stop subject as well as camera motion, which is amplified at high reproduction ratios. Since flashes can be placed close to the subject, even a battery-operated flash can provide more than adequate light, and allow shooting at the highest stop settings.