Essentially, a camera is a box with a lens on one side that forms an image (of a subject outside the box) on a light-sensitive film on the opposite side. Refinements include a method of focusing the lens on subjects at various distances, and a viewfinder to show the amount of the subject that is being recorded. For an entomologist, only two types of camera are worth considering, the single lens reflex and the monorail.
Single lens reflex cameras (SLRs) fall into two types, those taking 35 mm film and those taking 120 and 220 roll film. They work by having a mirror in front of the film that reflects the image formed by the lens onto a focusing screen at a right angle to the film. The image can be composed and focused on this screen, and convenient and accurate focusing and viewfinding are the supreme advantages of SLRs. For the exposure, the mirror flips out of the way, so you lose the image in the viewfinder while the exposure is made.
SLRs taking 35 mm are popular, and are often used for snapshots and general photography. Unlike roll-film SLRs, in most 35 mm ones only the lens can be changed; some have interchangeable focusing screens, a few can be fitted with other viewfinders, and one takes magazine backs. The normal viewfinder is a pentaprism, and this provides an image that is right way up and round, and approximately life-size when a 50 mm lens is used. The normal focusing screen has some form of focusing aid in the centre, but this is only suitable for normal work, and will be a nuisance for larger-than-life work and photographs through a microscope. Most 35 mm SLRs have a built-in TTL (through the lens) exposure meter, which can be very useful as it takes magnification into account, but you must beware of light getting in through the viewfinder and affecting the reading. This type of meter suffers the normal problems of integrating meters.
If you want to use your SLR for larger-than-life photography or with a microscope, then you must choose one with interchangeable focusing screens, and one that includes a clear glass screen with cross-hairs in its range. It is not essential to have an interchangeable viewfinder (because you can get right-angle finders to use with a fixed pentaprism), but interchangeable finders do provide a better quality viewing image.
Roll-film reflexes are more expensive and less versatile than those taking 35mm film, but are fully useable up to life-size reproduction and produce noticeably better quality than the smaller format. Most roll-film reflexes are modular, with interchangeable lenses, film backs, focusing screens and viewfinders, and can be assembled in the best form for any particular job. Their disadvantages compared with 35 mm reflexes are their cost, size and weight, and in most cases their inability to work at high magnifications or through a microscope.
TO BE WRITTEN
Monorail cameras have developed from early bellows-focusing cameras, and modern versions are very versatile and capable of producing outstanding results. However, they are slow to use, expensive to run (they use 5×4 inch film), and require considerable experience if their facilities are to be fully utilised. There are exposure meters available that measure from parts of the image on the focusing screen. They are more suited to a photographer attached to an entomology department than to an entomologist who wants to take his own photographs.