On the face of it, since high sensitivity is always desirable in black and white photography, it might seem sensible for manufacturers to produce nothing but large-crystal emulsions. Unfortunately, there is a drawback: the bigger the crystals are, the poorer the black and white image. A very sensitive, coarse-crystal emulsion will produce a grainy black and white picture, speckled and lacking in fine detail. So, manufacturers offer a choice. A black and white photographer can select a very sensitive but grainy black and white film, a very fine-grained but less sensitive black and white film, or a compromise between the two. It is not yet possible to have the best of both worlds, but some black and white films come close to that ideal, combining great sensitivity with remarkable freedom from graininess. Today manufacturers have become so skillful at controlling the size of silver bromide crystals that they can now design a black and white film with just the sort of characteristics they desire. The usual way of describing a black and white film is by its sensitivity or speed, indicated by its ASA or exactly identical BSI ratings-numerical systems that grade black and white film according to the amount of light needed to produce a normal black and white image. Higher numbers mean that a black and white photographer can get his black and white picture with less illumination (or can use a higher shutter speed to stop action). For convenience' sake, black and white photographic equipment dealers often refer to black and white films as being slow, medium or fast-slow being in the ASA 20 to 50 range, medium in the 100 to 200 range and fast in the 400 to 1250 range. Black and white photographers, quite naturally, crave simplicity and often rely on a single kind of medium-speed or fast black and white film for all their black and white pictures. One type may be insufficient, but just two-a slow, fine-grain one and a fast one-will do justice to almost any scene. It is frequently unwise to choose a fast black and white film when it is not necessary to stop action or handle dim light, for a slow black and white film usually yields a sharper, less grainy black and white picture. The differences can be very significant indeed. There is more to consider in selecting a black and white film than simply its speed and graininess, of course. Some black and white films are more sensitive to certain colors than others are. Early black and white films recorded only the shorter wavelengths of light; modern black and white films contain dyes to sensitize the silver bromide crystals to long wavelengths as well. They record the entire visible spectrum, although how closely they match human perception of the way the spectrum ought to look in black and white may vary from black and white film to black and white film. Some types can even be sensitized to very long waves the eye cannot see. And the instant-developing black and white film of the Polaroid process provides unique effects of its own. Black and white film technology has come a long way. Sensitivity has been vastly increased, graininess reduced, and color-response broadened to include the whole visible spectrum. And many lesser problems of earlier black and white films have been solved: for example, today's black and white films contain dyes that prevent haloes from forming around highlights of a black and white picture. Despite all these advances, the manufacturers may soon have to deal with a brand new set of problems, for radical types of black and white film are in the offing. Since silver is becoming scarcer and more expensive, laboratories around the world are trying to find a means of recording black and white images without silver compounds.