In the previous post in Photography Basics we saw what is the procedure to expose a picture correctly, especially of subjects that don’t reflect the expected amount of light. But a couple of important and more advanced topics were skipped. In this post I’m going to talk about one of them that, without being essential for taking pictures, is really useful when a good exposure is desired but in our scene there are present different subjects that reflect light in different ways.
A measurement by any other method… would not measure as well
As we previously saw, the photometer is the tool that measures the ambient light in order to achieve a correct exposition value. When the scene’s illumination is homogeneous and there are no problematic subjects, it knows how to do well its job, but when there are differences of illumination in different places of the scene, or different subjects (yeah… sometimes we need to photograph that black cat walking on the snow and all our plans go awry) measuring with the photometer gets complicated. To solve this, photometers usually provide 4 different modes of measure light. Each mode measures a different part of the scene and treats the lighting value in a different way, so choosing the correct one will help to obtain a measurement closest to the optimal value. Each camera may have different modes, but most of them are based on the following:
Evaluative metering: Is the standard method of measuring light, the most used and in some cheap cameras the only one available. It works in a simple way: just measures the amount of light for every pixel of the image and calculates the mean of the values. We obtain the average lighting of all the scene. This mode works well in scenes with a very homogeneous illumination, where there is an object or background that fills most of the frame, or when the different areas of the picture all reflect the same amount of light. It works well for general pictures (for example a landscape), but fails when two important points of the picture reflect a different amount of light.
Partial metering: This works in the same way as evaluative metering, but only averaging the central 6-9% of the frame, and all other information is discarded. As a result, the measurement is more precise on the subject that is situated on this zone of the picture. If we wanted to calculate the illumination of our (now famous) black cat, the evaluative metering would average its illumination with the background, probably showing that there is more light than the true value. Using partial metering will evaluate only the cat, obtaining a more precise measurement of the light that the cat is reflecting to us.
But what if the cat is not in the center of the frame? In this case, we can put the cat on the center, adjust the parameters until the photometer shows the desired value and, after that, recompose and move the frame until the cat is in the place we want. As the light the cat reflects doesn’t change if we move our camera, the values adjusted will be valid.
Yeah, but… What if I’m using a semiautomatic mode? In this case, the camera will measure automatically in the center when we take the picture. In order to avoid this, a function call Exposure Lock exists (sometimes called AE-LOCK, AE or simply *). When we activate this function the camera will measure light in the central area, will store the desired values and will use them for the next picture taken. We can put the black cat in the center of our frame, adjust the parameters, activate AE-LOCK so the camera stores the measurement and finally just recompose and take our picture. Even if the cat is no longer in the center, the exposure will be measured on it. This mode is also useful in backlight photography.
Spot metering: Works exactly the same as partial metering, but only on the 1-4% central area of the frame. As the area is smaller, the measurement is more precise, but also more susceptible of variation with small movements of the camera or the target. It works in the same way as partial, but provides better exposure value if the place we are measuring is very small or is surrounded by an environment with very different lighting.
Center-Weighted Average metering: It’s a hybrid between evaluative and partial modes. It measures the light on all pixels in the frame and averages them, but gives more importance to the pixels close to the central point. It works well when it’s necessary to expose a difficult subject well, but also the background is important and we need to keep it under a reasonable value.
So, in the initial example of our black cat walking on the snow, two different approaches can be taken:
If the dynamic range of the scene doesn’t overflow the dynamic range of our camera (which means: if we can expose the snow well and our cat is not pure black, or we can expose our cat well and the snow is not pure white) then partial/spot is the recommended measuring method. You can focus one of the subjects, adjust exposure and recompose. The exposure is a value for the overall picture, so if the snow is well exposed for a set of parameters, the cat will also be correctly exposed. The measurement the photometer will provide for each of them will be different, but the parameters that you will use to set the mark in the correct point will be the same whichever the subject you choose is.
But if the dynamic range of the picture is greater than the dynamic range of the camera, we won’t be able to expose both subjects correctly on the same picture (without the use of some advanced techniques as HDR). This means that we can use partial/spot mode to expose the snow correctly (but our cat will be too dark) or we can use it to expose the cat correctly (but the snow will be burnt and without details, pure white). Or we can use Center-Weighted mode to expose the cat not-so-well-but-not-terribly. In this last case, the illumination provided by the photometer will correspond to the light the cat reflects, but with a little addition of the light from the snow. This will lead to a picture where snow is a little bit grey but not too much, and the cat will be a little bit darker than necessary, but not excessively; we change a little bit of quality from out cat to increasing a little bit the quality of the snow.
Does your camera have any other methods? Or perhaps you know more kinds of scene where those methods are good. If you want to add something, comments are open for your collaboration!