Measuring and Exposing Digital Images

In the darkroom, the exposure level necessary is searched for through test strips . The exposure level is regulated with the diaphragm's aperture and the exposure time. The equivalence law of aperture/time combinations we saw is maintained. Logically, the more closed the aperture of the diaphragm is the longer the exposure time has to be; nonetheless we will have a better focal depth, which is very useful when enlarging negatives without crystals in the negative holder. This prevents the risk of a lack of focus when the film has a slight curvature or when the focus mechanism of the enlarger is loose. Therefore, unless the exposure time is too long (because of a large enlargement factor or density in the image) it is not advisable to use the maximum aperture. The time regulation is made by interrupting the current of the enlarger by hand or with a enlarger timer which provides the time we need without error.

The correct exposure time is that with which the print will reach the wanted density after the developing time is OVER.

Without a lab photometer, the necessary exposure is determines with test strips. In them we will be able to appreciate the characteristics of the positive image, the necessary exposure and the contrast of the image for that type of paper. It is necessary to make a test in the zone of the image that can provide all this information. Not only the main zone is important, but also the lights and shadows we want to appear with detail. If we don't have the most adequate paper contrast for the image predetermined, we will make a test on normal paper (num. 2). Once the piece of paper has been placed, we will cover ¾ of it with a dark cardboard. Next we will give successive exposure with set values, uncovering ¼ of the image at a time. We will develop the strip the same as a print, completely ; once set, it is advisable to see it in white light.

Of the four exposure levels, one of them probably provides us a density in the main zone that is very similar to the one searched for (on the other hand we will repeat the test with new time values). If in every line of the strup there are also lights and shadows, we will be able to see the contrast of the image. The reproduction of the lights and shadows of the scene regarding the main zone can be satisfactory for that exposure level. If, with a correct exposure level for the main zone, the density of the lights and shadows in insufficient or excessive regarding what we want, we will use a paper of a more adequate characteristic contrast for the situation. After the necessary tests, we will have the most adequate paper and exposure level for printing the image.