a brief guide for field inventory:
In this situation, you are not taking fotos as art or for a magazine, you are taking them mainly so that you can either identify the plants, use as a voucher, or use for a simple field guide. There is usually not time to use a tripod. Sometimes you are lucky and your fotos are both practical and artistic. It takes time to get good at it, and a better camera helps, especially one with a stabilizing mechanism in the camera body or the lens.
Look for dark background or make dark background.
Light behind a subject not only distracts, it also makes it more difficult to adjust contrast and other settings on the computer afterward. When the subject is at hand, you can train yourself to turn the subject or yourself so that the background is as dark as possible. When looking through the viewfinder always look at the background as well as at the subject, and you might be able to move to avoid sunflecks. Otherwise, you can carry the plant to a darker place, or arrange a dark cloth behind the subject.
Avoid distracting backgrounds.
Try not to take a foto with the subject placed on the ground (unless that is its normal habit), or in the middle of other vegetation in the same plane of focus. I.e. try to put some distance between the subject and the surroundings.
A flash photo, though not “natural”, improves sharpness, color saturation, and focuses attention on the subject while reducing other distractions. Better cameras have settings that allow you to “force” the flash even when there is enough light. Otherwise, the best time to take fotos is in early morning or late afternoon because most cameras have settings that will automatically activate the flash when light is low. Fotos at night are possible for most cameras, but it is usually more difficult to focus. Flashes use up the batteries more rapidly, so come prepared with extras or a way to recharge them.
Reduce exposure for flash closeups.
Better cameras have a setting for reducing or increasing the exposure, either by reducing the aperture or reducing the power of the flash. You will usually need to refer to the camera manual to do this, or else just by searching the menus available. I have found that most cameras overexpose the subject when taking a close up with flash, especially for white and yellow flowers, and I put a setting that reduces the exposure by 1.5 or 2. You might want to increase exposure for dark tree-trunks taken a few meters away.
Hold the camera steady or brace it against something.
Most older and less expensive digital cameras do not have built-in image stabilization, and slight shaking blurs the foto, especially if the camera is held in one hand and the plant in the other.
Don’t trust the automatic focus.
Auto-focus, even if it is accurate, usually focuses on lighter objects that may be behind the subject The viewfinder may be small, but is important in confirming that your subject is really correctly focused.
Since plants grow vertically, and leaves, flowers, and fruit are usually longer than broad, it usually fills the frame better to take vertical portraits of plants, just as you would a standing human. For animals, a horizontal/landscape orientation is more appropriate.
Size of image.
Set the camera to take images that are 1-4 megabytes in size, if that is an option. A 3 megapixel camera that only takes images smaller than a megabyte can be adequate, but only if everything else is good, i.e. in-focus, stable, and with good lighting conditions. A 5 or 6 megapixel camera is better, an 8 or 10 megapixel camera is overkill.
Take at least two or three fotos of each fertile plant, and at different scales.
For identification, it helps to see a plant at different scales: a more general foto that shows branches and the arrangement of the leaves and inflorescence on the branches, a foto of a single inflorescence with a couple of leaves, and a close-up of flowers or fruit that shows both the base and apex of the organ. It helps that new cameras mostly use zoom lenses. A close-up may not be necessary if a larger scale foto is very sharp and can be zoomed in on the computer. A hand or finger can be used as a measure of scale in the larger scale fotos, but is not needed in the close-ups.
Put leaves in same plane of focus as flowers or fruit.
Leaves are very important for identification, especially of species with no or only small flowers. Give them importance in your fotos, and make sure they are in focus along with the inflorescence. Most pictures of pretty flowers forget to include the leaves.
Emphasize underside of leaves, and include the tip and base of the blade, and junction with branch.
Leaf undersides have many more characters for identification than does the upper surface which is often covered by wax and epiphylls. However, if there are important glands/nectaries on the upper surface (i.e. Inga, many Euphorbs), it is important to also take fotos of these. To take a foto of long leaves and to get upper and lower surfaces of compound leaves, one can fold the leaf to show characters of both upper and lower surfaces and tip and base of the leaf, all in the same image.