Digital Photography Measuring Light in Practice

When we start photographing with film and see the results we'll realize that measuring light isn't as simple as when using a digital camera. The digital screen lets us let's us see the brightness of the different zones and therefore act in consequence. While with film, even when following the photometers instructions, we almost always find pictures that don't look the way we expected them to: important areas in the image might appear dark, black, burnt or washed. It's obvious that another exposition was necessary but we didn't know that at the moment of taking the picture. The problem is that both the characteristics of the scene and the film are variable, and the effects of different exposures aren't visible at the moment of taking the shot.

How can we know what to do when confronted with a scene we want to photograph? Here we're going to take a look at the problem and further on we'll complete the table (4.3.1.2, 4.8.2, 4.9.2 and 6.1).

When referring to a scene, we know that the general brightness level is a main factor to have in mind because the sensibility of the film determines the exposure value we're going to use. If the general brightness is scarce and/or the sensitivity of the film is low we need an exposure value with slow speeds and/or a large aperture. If the general brightness is high, and/or the sensitivity is also high the exposure value will have high speeds and/or small apertures.

Nevertheless, general brightness isn't the cause of exposure errors. The real problem is in the scene's contrast , meaning, the greater or smaller difference of it's brightness and darkest extremes. The variations in this sense are huge: in some cases we find scenes of homogenous lighting (exteriors with the sun behind our back) and in others, the zones in our frame can have very different values (scenes with lights and shadows). Contrasted scenes are the most difficult to photograph because we can make a very big mistake.

What does a photometer do before a contrasted scene? It normally gets and average of the brightness of the scene and that's is the exposure value it recommends. In a homogenously lit scene, whether it is bright or dull, the average coincides with the necessary exposure for all zones of the scene, and therefore the photometer is reliable. On the other hand, if we make measurements of the different zones of a contrasted scene we will notice that they need different exposure levels. Since we can only choose one exposure level it's necessary to decide what part of the scene is more important, measure it's brightness and use the correct exposure for the chosen scene.

An example: a portrait against the light. We want to photograph a person who is inside and next to a window. In this case the brightness of the background is much higher than that of the face. To avoid this interfering with the measurement the only thing we have to do is frame only the face and take the light measurement and then frame the scene as we wanted it. Therefore, in a contrasted scene we have to reject the average reading the photometer gives us and look for the correct exposure for the part of the scene that interests us the most. This system guarantees a correct exposure for the main theme and that's all we need to know for now (although we will see that this concept is improvable).

When referring to film it's essential to have in mind that the emulsion can only record with detail a quite limited range of brightness , much narrower than our sight. With a correct exposure level for the main zone, the shadows will be at inferior levels, and the lights at superior levels. There is an interval of exposure levels in which we can appreciate detail in the lights and shadows, but we have to get accustomed to the idea that this interval is limited and variable. When the contrast of the scene we want to photograph exceeds the films capacity, in the final images the zones with lighting extremes will appear indistinctly black or white, with no detail or tone. Even though in the scene the zones were completely visible they have disappeared in the images because they passed the limits. Our focus has to stay on the evaluation and control of the contrast in the scene we want to photograph. Further on we will go over this argument. Experience will teach us to adjust the exposure not only for one zone, but for a brightness interval that we want and that appears in the image with detail. Following the example of the figure, it's very different to want detail on the face and the outside than have the same portrait with detail in the inside .