Light falling on the photodiodes of the digital camera matrix is converted into an electrical signal. For this to happen, the number of photons that hit each individual photodiode must exceed the sensor sensitivity threshold. If there are not enough photons, the corresponding section of the frame will be completely black. If the exposure is excessive, the photodiodes are saturated with photons and the overexposed area turns out to be white. The ratio between the exposure values required to produce a completely black and completely white color is called the sensor’s dynamic range or photographic latitude.
The matrix of a digital camera has a dynamic range of about seven to eight stages (or, in other words, zones) of exposure. In principle, you can pull up to ten or more steps from a RAW file, but you should not abuse this. Eight zones is not too small, but not too many. Significantly less than negative films (both black and white and color), but slightly more than color slides.
If the difference in brightness between the lightest and darkest parts of the scene exceeds the dynamic range of the sensor, this inevitably leads to a loss of detail, either in the shadows or in the lights, or both. All objects whose details and texture are important for the image must fit into the dynamic range. Black shadows devoid of detail, are relevant, but knocked out of the world, as a rule, inadmissible.
Waterfall Yankoski. High contrast.
Example of a scene whose contrast does not fit into the dynamic range of the photo sensor. The sunlit area of the forest above the waterfall is much brighter than the foreground objects. The best solution here is to wait for more favorable lighting.
Why is the exposure meter wrong?
Usually, the built-in camera exposure meter does its job well, but in some cases it is worth interfering with its work. The fact is that no matter how perfect the exposure meter is, it still will not be endowed with even the rudiments of reason. It’s just an electronic device that measures the amount of light that falls on it.
At the same level of illumination, different objects reflect light to different degrees – this is why some objects look dark, others light, and others have a neutral tone. A light object looks light to us, and a dark object looks dark in any light, because our brain takes into account the overall level of illumination and the difference in brightness of equally illuminated objects. In this case, the absolute brightness of a dark object in the light may be higher than the brightness of a light object in the shadow.
Turn on the spot metering and take a picture of a neutral-colored object – a concrete slab, a blue sky, a green lawn, or the face of a moderately tanned person. The exposure will be more or less correct, since the exposure meter is calibrated at the factory for a neutral gray color.
Now set the exposure to something radically black – it can be a black cat, a pianist’s tailcoat, a hearse-no matter. No matter how black they are in real life, they will appear neutral gray in the photo and you may have to reduce the exposure by a couple of steps to return them to their natural appearance.
Take a picture of something white – a sheet of paper, snow, a white Swan – they will also turn gray, and this time you will have to turn the exposure correction up.
The exposure meter can’t tell if the cat is really black, or if it’s actually white, but it’s hidden in a dark closet. It is based on the assumption that there are approximately equal amounts of dark and light objects in the world, and that if you calculate an average-neutral exposure, it is likely to be correct.
When on matrix metering, the light meter is not so stupid. It tries to take into account the brightness of individual items in the frame and, if possible, maintain tonal relationships. But scenes whose overall tone is significantly lighter or darker than neutral put the exposure meter at a dead end. As a result, the coal mine will be overexposed, and the snow-covered field will be underexposed. High contrast that exceeds the dynamic range of the camera sensor also leads to exposure meter errors. If you are not satisfied with this, you will have to learn to recognize situations in which the exposure meter can go wrong, and after recognizing it, take control of the exposure in your own hands.
It’s not really that bad. The exposure meter, of course, is wrong, but it does it quite predictably and monotonously. Over time, you will learn the algorithm of its operation and will know exactly when you can fully rely on automation, when you should use exposure correction, and when it is better to switch to manual mode.
If you are interested in always determining the exposure as accurately as possible, you should get acquainted with the applied aspects of exposure in digital photography.