Light meters give an objective assessment of light, but unfortunately not always accurate. Even the most perfect ones will inform us incorrectly, if we use them uncritically.
Using on purpose light meter, we have to correct its reading visually, subjective assessment of light conditions.
The issue of optical light meters will be summed up in a few sentences, because they are no longer produced (a few remained from the interwar period). Measuring them, based on the eye's sensitivity to small amounts of light, it is basically inaccurate, because this sensitivity in people changes from hour to hour.
The principle of measurement with an optical light meter is as follows: The lens of the light meter is directed at the object of measurement. Inside the light meter is a device that allows you to regulate the amount of light entering it (Iris). The light falls on the glass, on which the measurement mark is located (geometric figure, digits). We start the measurement with the aperture closed, while opening it, until we see the measuring mark. Then on the slider rings we read the aperture and shutter marks appropriate for the correct exposure. You can do the opposite - start with the aperture open, closing it. When the measurement mark becomes invisible, we read the measurement.
Widely used today, photoelectric exposure meters are the most accurate and objective to date (Of course, until their photocell loses sensitivity. This happens quite often and causes light meter errors in less advanced photographers, of screening).
The angle of view of the light meters is relatively large, almost twice the angle of view of a standard photographic lens. Therefore, the measurement of the amount of light coming from the whole of the photographed object to the lens should be made at a distance of about twice that distance: from the subject to the camera. Modern light meters have a smaller angle of view than the old ones. Measurement from a distance twice smaller than the object-to-camera distance is considered correct.
The light meter used in this way gives us only an idea of the average-proportional value of the light entering the lens. In the next chapter we will talk about the photometric paradox. Let's consider lighting conditions here, what we find, photographing an object. As we know, the span of contrast of objects may vary. Tonal contrast is the difference in brightness between individual seen or photographed objects. Np. there is a lot of contrast between white and black paper, and between white and grey, small. Items photographed may not be enough, relatively high contrast. In the first case, their contrast will fall within the contrast of our final positive (at best this contrast is 1 : 40). In the second case, it is technically impossible to reproduce the actual contrast; so we have to compromise, we must dispense with the darkest details, relatively in the brightest parts of the image.
The decision to omit tonal details depends on many factors. It depends on the size of light or dark details and on that, what size will these details be on our final positive. The general rule is freedom to lose details, which will be small in the final image. Hence the same topic, designed for small sizes, may have less tonal detail, than at high magnifications. If bright details are smaller than dark ones, we will display the image in this way, for the latter to be sufficiently exposed, and clear, if there is no other way out - overexposed. We'll just expose our negative "to the shadows".
In addition to the size of the details, their importance to the image is important here. It may happen, that the details are clear, albeit minor, are more important to us than dark and relatively large ones. Then we'll shine "into the light".
It doesn't end there. In nature, we even encounter large surfaces, which may constitute tonally undifferentiated planes in the image, like for example. sky, shadows in some cases, inside doors and windows in bright buildings, etc. Of course, there are no fixed rules here and each photographer must decide for himself, whether the details of a given image plane must be tonally differentiated, or they can be smooth.
In the exposure tables we find the parameter: the brightness of the object. This matter is not simple. In fact, in our photo, dark objects should come out dark, and clear - clear. If the emulsions of negative and positive materials we use had unlimited contrast capacity, it would be simple: all photos under constant lighting conditions would be exposed equally, for example, a black and white butterfly. However, emulsions do not have this ability. If we were to illuminate these two topics equally, then we would get either an image of a dark butterfly underexposed, black, no tonal differentiation either, with more light, image of a bright butterfly too bright, with gray in these places, where it should be gentle, bright tonal drawing. This is why, on the whole, we must take into account, in addition to the brightness of the illuminating light, the brightness of the object.
Modern light meters have a device, allowing to measure light "on the light source". It is a fast opal or mesh, which we put on the photocell of the light meter. It passes approx 1/32 incident light. This measurement gives us information about the lighting with the elimination of the element of the object's own tonality and the degree of its brightness.
We use such a light meter to carry out both measurements: "on the object" and "on the source of light". If they are identical or slightly different from each other, we apply the exposure according to the indications of the light meter. Of course, when the contrast of the subject being photographed is very high, not within the tonal tolerance of our photographic image, we have to decide for ourselves, which details must come out in the photo, and which we can opt out of.
We use it in some cases, that negative emulsions have a higher contrast capacity than photographic paper emulsions. Then we decide on indirect exposure, and even exposure "on shadows", and the details in the bright places of the image we will try to bring out during the positive process, or by lighting, or any tone resolution technique.
Additional measurement of details is about this, that the photographer, approaching the object, measures its individual brightest and darkest parts up close, and then the most important. This measurement is especially helpful when photographing high-contrast subjects.
So it turns out, that one light meter measurement tells us little, so we have to take some measurements and finally we have to critically evaluate these measurements, then select the appropriate exposure.
At the memory table, we have given, that it concerns membranes developed with energetic developers. This does not apply to light meters. They are scaled for emulsions developed with an ultra-fine developer (Atomal F). If our negatives will be developed in a fine-grained developer (borax, D 23, Final) - the measurement reading will be twice as large, if we use an energetic developer (for example, Polish "Uniwersalny") — it will take four times less exposure.
A separate issue is exposure when using development methods that force the emulsion to be more sensitive than specified. Then the whole process is fundamentally changed and the way of reading the light meter readings also changes. Because emulsions work then on the border of sufficient exposure and underexposure, the lighting must be especially accurate.
Modern negative emulsions give much better results in economy exposures than in overexposures (smaller grain, better focus). That's why we try to expose as little as possible, however, that's it, to get a negative with worked out details in the shadows (of course with the details needed for our image). We must remember this, that most light meters are calibrated with a margin and their reading can be reduced by at least a factor of two.
Because it is a whole complex of issues, the remarks quoted above should not be put into practice without repeated testing. However, if you want to reduce the exposure in relation to the information of the light meter, we should also change other parameters of image processing, and especially the calling system. Otherwise, we will get underexposed photos.