Digital Photography - what kind of art is this ?

The game of light

The correct exposure has been a great problem of the photographers since the beginning of photography. The evolution of techniques eliminated the need of analyzing the weather forecast, because different mechanisms of measuring light have come to help the photographer.

The measuring of light which falls onto the subject has always been a mystery problem. At that time the photographers had a level of experience which allowed them to establish the exposure from analyzing the weather forecast. The times of exposure varied in the 19th century at a level of minutes and then of seconds. Only after the discovery of sensible emulsions times of exposure reached 10ths and 100ths of seconds. On the other hand, the optical quality of the objectives imposed the using of a narrower diaphragm for the avoiding of aberrations. Then, Ansel Adams fixed the “zone system”, dividing the levels of illumination met normally in the nature into ten zones, starting from the maximum light in a sunny day to the natural light in a well lightened room. Notice that everything is based on a certain approximation. To risen confusion I’ll say that the human eye has strength of adaptation superior to any system of reproduction of the image so everything becomes absolutely relative. The eye, commanded by the brain, quickly adapts itself to any type of changing of the light both for quantity and quality so we will not be able to evaluate the quantity of light and to find the correct exposure. Till the apparition of the level of light measuring instruments, the tables with situations of illumination and exposure were the only helping ways which came to help the photographer. The discovery of photoelectric elements with cadmium sulfur led to the miniaturization of the equipments and to the possibility of implying those into the cameras so that the measuring of exposure became automatic. A long way of more than a century and a half.

In the digital era, the measuring of light is done in real time, but the instruments integrated into the photo camera can measure only the light reflected by the subject. If the subject is darker, this will reflect less light. Simplifying: it will be blacker. The same way, in the situation in which they reflect more light: it will be whiter. The equipments of measuring on the cameras are calibrated for measuring an intermediary value among the brightest and the darkest things in the picture, doing an average and that is a correct exposure. But these equipments are easy to be “tricked” by the situations in which we have a black cat on coal or a white cat on snow. The camera does not “know” to make a difference between the subjects and to action properly, doing exposure corrections, so in both cases the images will be exposed equally, meaning that they will appear to be grey. If we still want for the white cat to be white like snow, we must apply a correction of two steps plus. If we used an apparatus which measures the incidental light (the flux of light) which falls onto the subject and this is transformed into values of exposure, we would always get a correct exposure. This happens because into the equation enters only the light which falls onto the subject. In the case of the cameras with automatic measuring, these measure the light reflected by the subject. This is where the names of the two types of measuring come from: the incidental light or the reflected light. The exposing systems separated from the cameras (used by professionals) can measure the incidental light and are easy to be recognized because there is white screen of diffusion applied which is spherically shaped like half of a ping-pong ball.

The knowing of the measuring mechanism is not enough for a correct exposure. The sensors are not capable to expose on the whole scale of 10 levels of brightness. A digital camera has the capacity of exposure of approximately four-five levels of brightness. This is called exposure latitude. If the subject has very big contrasts, then we will have to scarify a part of the details in the favor of others. In the landscape photo it is preferred for the shades to be sacrificed in the favor of correct exposure of light details. On a photo, the deep black has a greater value than the “broken” white, with no details. The generation of 20 years ago would have given anything for being able to analyze the images immediately after taking the photos. This advantage must be exploited because there are a lot of mistakes which can be fixed at the same time. When we get home, it is too late to do something. Five years ago, before doing the final step towards the digital, we used three times more photos to be sure that at least one of them was perfect. The bracketing was compulsory for the professional’s work. Now, the analyzing of the histogram immediately after taking the photo (for the bridge cameras it is done live) reduces the number of the photos to be “thrown away” because of technical reasons. A correct histogram (which is tangent at none of the margins) is the certitude of a correctly exposed photo.

September, 12, 2007