Color brightness and exposure metering

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Our perception of colors depends on many factors. For example, the ambient brightness initially determines which type of receptor (rod or cone cells) we see with and thus determines which color spectrum can be perceived at all. But the receptors themselves also perceive different colors with different levels of brightness. The diurnal cones are most sensitive to green-yellow, while the nocturnal rods prefer blue-green. This means that we perceive yellow and red more strongly in bright areas and prefer green and blue in partial shade. A striking yellow sunflower, which appears much more brilliant to us in daylight than the surrounding green, consequently appears dull to us at a lower light level, as we perceive a proportionally smaller amount of yellow. Our perception of color therefore changes with the brightness. In addition, we determine brightness and color values in relation to each other in order to create consistent perceptual worlds. The chapter dealing with lightness and color explain all these relationships.

Compared to this, the functioning of an exposure meter is downright simple. Thanks to its calibration to a medium gray of, as we have seen, 12 % remission, every film is exposed approximately the same in every camera. The light meter simply reduces all the brightness values in the image to this gray and thus creates its own clear world. This works well as long as only medium brightness and contrast values occur in the subject, i.e. the medium gray makes up a disproportionately large part of the image. In any more extreme case, however, when large bright or dark areas dominate the motif, any light meter is overtaxed from a technical point of view and the head behind the camera must intervene to correct the situation, must manually underexpose or overexpose.

A pure white surface, for example, reflects 90 % of the incident light. If the exposure were based on such a surface, you would have to expose 2 2/3 stops longer than the measured value in order to obtain an image with the correct color and brightness. Conversely, with a deep black subject, the exposure would have to be halved, as black only reflects around ten percent. – I know what you’re thinking! With a subject that reflects an above-average amount of light, the exposure must be lengthened because the high reflection causes the light meter to erroneously shorten the exposure. Conversely, a subject with below-average reflection causes the exposure to be too long.

If you are not sure about the reflection and exposure of a subject, a standardized gray card will help. If we measure it under subject lighting, it is the ultimate reference if you take into account the necessary extension of the exposure mentioned in the last section. But an asphalted road, preferably already somewhat worn, or the blue sky on the side facing away from the sun also come close to this ideal and can serve as a substitute.

Furthermore, the vast majority of exposure meters share the preference of our visual system for certain color values, which is reflected in deviating exposure values, especially for the colors at the ends of the visible spectrum. Figures 61 and 62 show that a green or yellow surface appears much brighter in the image than a red or blue one. The following values give an indication of the magnitude of the correction „over the thumb“ (each silicon diode used for exposure measurement has a slightly different characteristic curve):

Red and Blue -2/3 bis -1,0
Orange +2/3 bis +1,0
Yellow and Green +1,0 bis +1 2/3

Due to the same technology in exposure metering, digital cameras also face this problem.

Exposure metering in the film plane can also lead to similar problems since the film layers of different manufacturers have different colorations and therefore do not reflect the light to the same extent in each case. With monochrome subjects, this can lead to metering fluctuations in the range of an entire exposure value. Comparative shots are indispensable to understand the reaction of your own exposure meter, including the built-in TTL device in the camera. A gray card, light measurement, or one of the expensive, specially corrected light meters from Zone VI Studios can also provide precise assistance. You can still find them used. With colored light and self-luminous objects such as lanterns, fire or colored glass, however, even they fail, and the photographer must correct according to experience and feeling.

Next Possibilities of exposure measurement – Object metering

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