Exposure measurements.
While measuring the color temperature of light is a special measurement method in color photography, especially for reversible images, this light meter is well known for its black and white technique. Any experienced amateur can start shooting without a light meter, however, it is necessary, and at least necessary for photos taken on colored reversible materials. Therefore, exposure measurements should be discussed together with the problems of color photographic technique. At the moment we are dealing only with photoelectric devices and therefore we will only deal with them. As is generally known, the most important part of the light meter is the photocell made of small metal plates, covered with a thin layer of cuprous oxide or selenium. If light falls on the photocell through the aperture of the instrument, an electromotive force is created between its two plates and a current flows in the circuit, which is measured by a sensitive galvanometer connected to the circuit. By properly calibrating the instrument in the aperture values and exposure times, we can use it when taking pictures. However, we must pay attention to this, that the scaling of the instrument is not always accurate enough, and especially for reversible materials. Therefore it is recommended, for every amateur, when dealing with a new light meter, or if this is your first time taking color photos, he checked and calibrated his instrument in conjunction with his camera.
You can easily calibrate the instrument yourself, if we take about five photos of possibly different motives, in bright and low light conditions, using the same shutter speed and changing the aperture by half a value yes, so that the measured value is in the middle. After development, the obtained images are compared and fixed, which are the best, i.e. at which the correct exposure was used and by what factor they differ from the results of light meter measurements. It is naturally beneficial, if the ratio is the same for light and dark themes, which happens in most cases. We can then expose materials shorter or longer, by a whole or half of the aperture value, than the light meter shows. However, we can tell, that for long exposures at full aperture, a different factor should be used than in good lighting conditions. It is better to switch the scale to other sensitivity values than to recalculate the measured illumination each time; with automatic cameras it is not necessary at all. When irradiation according to the indications of the instrument (1/60 sek. at the aperture 8) pictures are too dark, and stronger exposure at the next aperture (1/60 sek. at the aperture 5,6) leads to better images, therefore, for colored reversible films with a sensitivity of 16°DIN, set the scale instead of to 16 which is 13°DIN. The opposite should be done accordingly.