It is not essential to use any special type of lighting, but as magnification increases, so the light intensity must be increased if exposure times are not to run into minutes or even hours. There are three suitable types of lighting, tungsten, fibre optics, and electronic flash.
For photographing insects, the same basic rules apply as for any other type of photography, namely that both the quality and quantity of light must be appropriate to the subject. For larger-than-life work, daylight will almost always be too weak, so you will have to provide extra lighting, either flash or continuous.
Single light sources
Probably the most versatile single light source for photographing larger insects is a ring flash, which has a tube attached around the lens to provide almost shadowless lighting. This can be hand held, and can be used in the field as well as in the laboratory. However, it is not suitable for use at lens-subject distances below about 10cm. It is also unsuitable for insects in a box with a glass top, for insects with shiny wings or elytra, and for subjects that need shadows to show their shape (for example a group of uniformly-coloured seeds). A less common use for a ring flash is off the camera to provide dark field illumination, using a mask to shield the lens but not the subject from the flash. This type of illumination is useful for transparent subjects, such as live mosquito larvae or cleared insects on microscope slides.
A single conventional flashgun does have its uses, but can produce very dark shadows that hide part of the subject and produce very high contrast that is unsuitable for reproduction in print. Matters can be improved in a number of ways, by arranging the subject and lighting so that the shadows fall outside the picture area, by using a diffuser to increase the effective size of a light source and so soften the shadows, or by using a reflector to lighten the shadows. Reflectors need to be placed very close to the subject, at about one quarter of the light-to-subject distance for a mirror and even closer for a white reflector.
Two light sources
With two light sources, almost any subject can be lit satisfactorily, but you do sacrifice portability, as it is almost impossible to hand hold a camera and two flashguns. If you have the strength and a steady hand, there are brackets available commercially for attaching two flashguns to your camera or lens. The two lights can be arranged to provide uniform lighting, or one can be used as the main light and the other as a fill light to lighten the shadows. With a pair of portable flashguns, the light output of the one used as fill can be altered by either changing the distance from the subject, or by using a diffuser made from cloth or tissue paper. Be careful if you try to use plastic for the diffuser, as many 'white' plastics have a distinct mauve or orange tint by transmitted light. A stand built from laboratory scaffolding will enable you to set up the lighting quickly and repeatably. A more flexible approach, though considerably more expensive, is to use powerful mains-operated equipment, which will have variable power and interchangeable reflectors to tailor the light to meet your requirements for almost any subject.
For taking dorsal views of set specimens, probably the most common subject, a pair of reflectors about 30cm in diameter used at the same power and distance will provide ideal lighting for nearly all specimens. There will always be reflections in shiny specimens, but they can be minimised by altering the angle of the lights, which should be arranged asymmetrically if there is any danger of the reflections being confused with markings. Iridescent specimens need to be covered with a light tent, made from a cone of white paper with a hole in the top for the lens to point through. The light passes through the paper and bounces around inside, surrounding the insect with light and displaying its iridescent colours. This technique sometimes works for shiny specimens, but can mask their colours. If you are troubled with shadows on the background, the specimen can be pinned into the top of a column of Plasticene, and this will both make the shadows less sharp and allow more light to get under the specimen. Insects will look better if you remove their labels for taking the photograph, but if the specimen is important then it is safer to leave the labels rather than risk damaging the specimen. Light sources that are much larger then the subject produce a soft light that can sometimes make it difficult to see sculpturing on the surface of an insect, because of the lack of shadows. If this happens, you will probably have to resort to fibre optic lighting with its very small light sources, and experiment to find the best arrangement for each individual specimen.
When taking pinned specimens from other angles, for instance to show the correct way to label them, symmetrical lighting may not give the best results; for example, it may produce double shadows. You can then use one main light to provide overall illumination, and a fill light with a larger reflector to put some light into the shadows cast by the main light. This second light should be arranged to give slightly less than half as much light as the main light, by selecting an appropriate reflector, power and distance. The same style of lighting, but on a larger scale, can be used very successfully to photograph boxes of insects and pieces of equipment. For insects, convenient reflector sizes are 30cm and 45cm, and then for the larger subjects the 45cm reflector can be used on the main light with a 1m white umbrella for the fill.
You will find it convenient to establish a basic lighting arrangement that is suitable for most of your work, with lights always in the same places and fitted with the same reflectors. By making a series of trial exposures, you will then be able to draw up a table showing the correct power output and lens aperture for every magnification, so there will be no need to measure the exposure every time, or to bracket your exposures. You will have to adjust the exposure slightly for very dark or light specimens, but you will not need to make any allowance for the background, which can be black, white or any colour to suit the specimen.