Digital Camera Image Sensitivity & Exposure

The sensitivity indicates the exposure level with which an image density considered optimal by the manufacturer will be obtained. This optimal density is approximately the one obtained in the image zones were the exposure level is strictly correct according to the photometer (OK). If the emulsion requires a low exposure level for a correct density, it is said that it is a high sensitivity film; if the exposure level required is high, it is a low sensitivity film. Naturally, the are values relative to the range of sensitivities available on the market. A film that requires an exposure level between the previous two is usually called an intermediate sensitivity. The emulsion's sensitivity follows a scale used by film and camera manufacturers, which makes the exposure more easier and more precise. The internationally used scale (ISO) is the result of a fusion of the two previous systems: the American (ASA) and German (DIN).

In the ASA system, the sensibility follows an arithmetic scale. If the film is a medium sensitivity, for example ASA 100, an emulsion with double the sensitivity will be ASA 200, and half of the sensitivity will be ASA 50. The DIN scale is logarithmic, and an increase of three units means a sensitivity increase of double. A medium emulsion as with the last example is 21 DIN, double the sensitivity is 24 DIN and half is 18 DIN. The ISO scale integrates the previous two in a single number composed by both terms: the first corresponds to the ASA and the second, in degrees, the DIN value. A medium film as in the examples, is indicated as: ISO 100/21°. Sometimes it is only one of the two terms, but it shouldn't give way to mistakes. The sensitivity is a characteristic of the film that depends on the developing . The nominal sensitivity, which appears on the container, is refereeing to a standard or normal developing, also recommended by the manufacturer. Regardless, the sensitivity can be increased or decreased through developing (11.3).

Practical Meaning

What is sensitivity for? Some amateurs don't know what to choose when the salesperson asks what sensitivity is needed. The choice of the sensitivity usually depends on the intensity of the light anticipated in the moment of taking the pictures, and the characteristics of the situation we a are going to be in. Earlier we saw that the exposure value with which we shoot depends on the brightness of the scene and the sensitivity of the film . For example, a scene whose brightness requires an EV 9 (60 f2.8) for an ISO 50 film, will be correctly exposed with an EV10 if the film is ISO 100, with an EV 11 if the film is ISO 200 and with an EV 12 (60 f8) for ISO 400. In the first of these cases (ISO 50), if the light diminishes or we have to resolve a depth of field or movement problem, we'll be in trouble. In the last case (ISO400) we could face the situation and even worse light conditions. Before the same light conditions, when using high sensitivity film, we find ourselves choosing higher EV combinations, which implies using more closed apertures and/or faster speeds than those offered by a low sensitivity film.

High sensitivity film (fast) extend the possibility of photographing to very low levels of light (including unexpected ones); let's us face closed apertures or dull lenses and movement problems in the image; and for these reasons are associated with great versatility and easy handling. Nevertheless, they are also associated with larger grain and lower contrast and sharpness. Low sensitivity emulsion (slow) offer less versatility, but the sharpness and contrast are greater, the lower the sensitivity is.

Mistake in the Reciprocity
The sensitivity of the film expresses a relationship between the exposure level and the resulting density, but this is fulfilled only in “normal” exposure conditions. If the intensity of light is abnormally high or low, even with the corresponding high or low exposure time, the exposure level gives us an inferior density to the expected (reciprocity mistake between this and the resulting density). In practical terms, this means a decrease of sensitivity. Exposure times of many seconds, or below a millisecond, can cause a greater mistake with the further we get from normal conditions. The quantity of the increase in the exposure level that compensates this mistake depends on the kind of film (it will be greater, the greater the sensitivity is) and is usually found through experimentation. In color materials, a variation of tone is also expected.

Exposure Latitude
The existence of underexposure and overexposure limits of the film means that between them there is an interval of exposure levels, which we will call exposure latitude , for which we can register an image. If the main zone receives a level considered optimal (OK, correct exposure in the photometer), the scenes zones that are different in a certain measure, will also be registered as scenes with more or less brightness and detail. The greater the difference of brightness gets with the main zone, the corresponding zone of the emulsion receives an exposure level every time closer to the limit. If the scene is very contrasted, the lights and shadows are lost from certain levels on because the pass the levels of over and underexposure. Therefore, we count on an interval of brightness were we can fill with detail. This interval is much narrower than that of vision and varies from one type of film to another . We can consider that a scene is an interval of different brightness that can “fit” into the registration capacity of the film. If a part of the measurement consists on searching for the exposure value that assure a correct exposure in the main zone, another not so important is the contrast control in the scene in regard to the exposure latitude.