Incorrect exposure indications.
Despite the performed calibration, we unfortunately have to make deviations from the found exposure values. This is always the case with low-contrast subjects (reproductions of color drawings and watercolors, medical skin photos, etc.), with low contrast lighting (sunless pictures of landscapes or streets), and finally with extremely bright objects, like for example. snowy landscapes. In these cases (instead of extending the exposure time or using a larger aperture, in automatic cameras, the film sensitivity setting should be lowered. When explaining this inconvenience, attention should be paid, that, on the one hand, colored reversible films should reproduce white objects as white regardless, is the sun shining, if the weather is cloudy, on the other hand, you can say, that the light meter shows correctly only in sunlight. It adds together all light stimuli from the brightest lights and the weakest radiation reflected from the shadows, putting them on your scale. It is less bright in cloudy weather, but shadows nevertheless receive and reflect more light, and the instrument possibly shows the same value, which is not enough to ensure the restoration of natural whiteness. The difference is not too great, however, reversible membranes are already responding to it. The indications of the instrument must then be doubled. Exactly the same relations hold, if the object itself does not contain contrasts, as we stated above. While reversible films should be judged "on light", negative films do not require it, because we can balance them over time
making copies. Even, contrary, they should be illuminated ,,for the shadows, i.e. so long, as it will be sufficient to obtain the correct drawing of them. In the case of non-contrast motifs with light shadows, we can expose much shorter times than reversible films, than the light meter shows, because it is unnecessary.
The situation is completely different in the third case mentioned above when taking pictures of snowy landscapes or similar, with only very clear motives. we said, that the light meter adds all the light that falls on it. So we can get the same pointer deflection, if we put a gray board in front of the medium brightness theme and approach it with the exposure meter as close as possible, that only the radiation reflected from it will fall on it. We accept, that the gray board would be reproduced in its normal gray shade corresponding to its image density. If we hold such a board in front of a pure white snow theme, this leads to correct exposure measurement, and the snow will come out really white in the picture. After taking the board, only the radiation reflected from the snow falls on the device, acting naturally stronger, and the reading of the exposure leads to the statement, that the snow turns out so dark, as in the previous image of the gray board. So for pure snowy photos you need to expose twice as long, than indicated by the electric light meter, half the aperture, or in automatic cameras, set the pointer to half the film speed. We need to think about it a bit more. If the snow should come out a little darker, to sparkle light on the ice crystals, then we irradiate according to the instrument's indications. If the motifs consist not only of snow, but a lot of space is taken up by people or objects, then the light meter will indicate correctly.