Digital Photography Exposure Level and Contrast

The second determining factory of the contrast in a developed negative is the exposure we give when photographing the scene. Until now, measuring was the search for the correct exposure of the main zone (2.2.2). This notion is too general to assure sharpness and contrast in the other zones. Since what really is important is the entire scene we have framed, an underexposure of the shadows or an overexposure of the lights can ruin our photograph. Therefore, it is much more advisable to get used to reasoning in terms of exposure level intervals , which is what we handle when photographing contrasted scenes.

In the search for the correct exposure for a contrasted scene we have to pay attention that the extremes of our brightness interval don't pass the exposure limits. If, for example, we make a mistake in the exposure so large that all the zones in the scene are overexposed or underexposed, we have placed ourselves on the horizontal, inferior or superior, stretches, and when developing the only thing that appears is transparency or maximum density without distancing of the tones (void contrast). We know how to avoid these extreme cases: in 2.2.2, when measuring the brightness of the zone that most interests us, we assured ourselves that our interval falls, at least in it's most interesting part, inside that latitudes.

If the exposure value of the left of the figure gives us total underexposure, and that on the right overexposure, we can represent the effect of the different exposure values by displacing our brightness interval along the horizontal axis. Theoretically, the exposure value we are looking for is one that places the brightness interval inside the latitude , which produces, after developing, a image were the different original vividness is represented with different densities. If correctly place only on part of the interval in the exposure latitude, we loose detail in the lights or shadows and the contrast is less than the original. In these cases, the photograph “comes out”, but in the figure we can see that the exposure latitude has been underused. The amplitude of the range of densities is less than the brightness and we have lost detail in some of the zones.

This precision error is produced in a large quantity of photographs; in them there is at least one zone that falls in the latitude, but because of the scenes characteristics, this exposure value leaves some brightness levels out. The greater the brightness interval we want to cover with detail, the greater the risk of that happening.

The difficult cases can be resolved at first instance by trying different exposure values, which assures at least one optimum picture. Regardless, this removes doubts and implies that only part of our material will be useable. How can we know more exactly what we're doing with the film to be sure that the exposure value we choose when shooting is effectively the best? To know so, we need to establish a correlation between our theoretical outline of the operation of film (the “ladder” and the different exposure) and the real situation we are in.

We need to modify and expand what we saw in 2.2.2: the best situation to assure a correct exposure in the “main zone” can no be so for the brightness interval we want to cover. The notion of “main zone” is very quirky, because it may sometimes be the brightest zone of our interval, as can be one of it medium brightness, or the darkest.

What exactly happens when we assign the OK exposure level of our photometer for a zone of the scene? It depends on the material we are using and the ladder we are on. A black and white (or color) negative with normal contrast has an approximate latitude of five exposure values (32:1), which can be represented like a five step ladder with a normal incline. The OK exposure level give us a density that according to the manufacturer is the “minimal to obtain a quality image”, and that in our ladder can be represented by the second step . In the figure we can see something very important that derives from the previous: the zone that receives and OK exposure is closer to the underexposure limit than to the overexposure one. Therefore, when photographing a contrasted scene with a black and white negative, the OK exposure should correspond to one of the shades of the scene, be it or not the main zone.

We can see it more clearly with examples: let's imagine two scenes with a contrast relation of 32:1, the maximum we can cover with a normal contrast negative. In the first (a), the zone that interests us the most is the brightest of the interval we want to cover; in the second (scene b), our maximum interest zone is the darkest. In both cases, when assigning the “main zone” -right end of (a) and left end of (b)- to the OK exposure we loose detail and contrast because some of the zones pass the limits.

For any scene, the best exposure value is the one that assures good detail in the shadows without the lights being overexposed . We shouldn't forget that this is impossible when the scene we want to cover has a contrast that exceeds the latitude; If the contrast is the same as the latitude, we have only one optimal possibility for exposure; for scenes with less contrast were we have more choice possibilities, it's convenient to adjust to the left of the latitude, since the grain is less than in the superior stretch.

The exposure value we select for each scene in particular is the one that assigns the different brightness of the scene to the densities of the emulsion. Now we can improve our criteria to achieve correctly exposing not only one zone, but all the interval that interests us the most, making the most of the registration capacity of the emulsion.