I'm going to try out posting twice a week for the next month to see how it goes. I will be alternating between the art of photography, learning your camera, adventure posts, and assorted other topics. For this post we will continue covering basic camera settings.
Whether you know it or not, metering is the foundation of your exposure. It is how your camera decides what an appropriate exposure is, and the method it uses can make a big difference in the resulting image. Unless you know how to set your ISO, shutter, and aperture from memory, then it is worth while to understand what your meter does, and how each mode works.
In the earliest days of photography, a photographer had to know how to expose an image for any condition they might encounter. They had no way to know what would work other than years of trial and error, and the knowledge of their mentors. For this reason a good camera technician was worth their weight in gold.
As photography emulsions improved, and small differences in timing made large differences, photographers needed a more precise way of measuring their exposures. They began using a variety of instruments, from various charts to a small device called an actinometer. It was the size of a pocket watch, with a dial that when moved exposed a piece of light sensitive paper. The photographer would time how long it took The paper would begin to darken to the same shade of gray as their reference card. Using this time, they would then use a chart to determine the needed exposure for the conditions.
The issue with this type of metering, and others around this time, is that they are still subjective. They rely on human eyes to work properly. Eventually photographers needed a more objective means of finding an exposure: one that gave repeatable results.
In 1884 Charles Fritts invented the photovoltaic cell: a small device that would produce an electrical current when light hit it (think solar panel). The current was very small though, and hard to measure.
Things continued to progress, and various methods where developed to make the measurements from a photovoltaic cell more usable. Eventually this development lead to a handheld sized device photographers could carry with them to get accurate readings of a scene (you can learn more here). From that point it was only a matter of time until the first camera with a built-in light meter was produced.
In 1935 Zeiss released the first camera with a built-in exposure meter: the Zeiss Ikon Contaflex. Today, the fact that every camera, other than viewfinder cameras, comes with its own light meter is something we easily take for granted. By removing the need for a separate tool to make an accurate light reading, and automating a number of processes, photography has become increasingly accessible and popular within the masses.
The meters in modern cameras breaks up the frame into multiple sections, up to thousands, and then measure the reflected light coming through your camera's lens. It then averages out the total values to a reference middle gray (similar to the early actinometer!), and spits out suggested exposure values to give a standard image. Their are a variety of ways that cameras can do this.
Segmented Metering (Evaluative, Matrix, Etc)
This Mode uses all of the information collected by the frame when deciding on an exposure. It It breaks up the frame into 5 segments giving the center of the frame higher importance in the calculation, with the rest of the image at a slightly lower priority.
This is a good mode to use day to day. You need to be aware of its shortcomings though, as it will average any scene to middle gray no matter how bright or dark the scene actually is.
With this mode, the camera takes readings from the center of the frame, and ignores the rest. Many, but not all, cameras have a circle at the center of their viewfinder that relates to the size of the center segment that is being measured.
I would suggest using this mode when you want to make sure your subject is properly exposed, and the background is less important.
Lastly, the spot mode measures a point at the very center of your frame. If used properly, this mode can be used to create an exact exposure for your desired results.
You can use the knowledge that the point will measure to middle gray in two ways. The first is to use a reference gray card placed in the scene to get an accurate reading. The second is to pick a spot in the composition that you want to be middle gray and use that to make your exposure. Using either of these methods will be sure to give you precise results.
I have found little reason to change this setting on my camera. I used to use spot metering when I had a DSLR, but now that I use a mirrorless camera, and can see how the image will look before I take the picture, I use segmented mode almost exclusively.
Each mode has its purpose, but in the end all they give you is a suggestion; it is up to you as the artist to decide how the final image is exposed. To that end, I suggest you pick a mode that works for you and learn it well. This way, you will be able to consistently get the results you desire.
You can find another good resource here. It's the first part of a series, but if you have the time to watch the whole thing, he does a great job demonstrating how each mode can give different results.