On the SLR cameras it is possible to select three modes of light metering: spot, center weighted and matrix. When setting the spot metering, the camera’s light meter reads the light value only in a limited area (usually, depending on the model, variable between 1% and 4% of the framed area) in the center of the viewfinder (some cameras allow you to tie the exposure measurement point to the focus point, so the reading does not necessarily happen in the middle).
When you choose the exposure meter, which center weighted, the unit will take into account the entire area of the frame. It assigns a greater weight to the measured brightness values in the central area of the frame.
When operating in matrix mode, the area of the viewfinder is divided into a large number of small sectors and the exposure meter detects the intensity of light in each of them. The reading is then compared with a series of situations stored in the camera’s algorithms, which based on the resulting analysis, selects or suggests the correct exposure.
Some camera models also have a reading mode called semi-spot that measures the light only in the middle. This is done on an area variable between 8% and 13% of the viewfinder that is wider than that properly defined spot.
In difficult light situations, the choice of the correct metering mode can make the difference between a good and a bad photo. Poor light is uneven that is characterized by different areas of the frame, which are illuminated differently. The subject reflects a much higher or lower amount of light than the background.
Matrix mode
The matrix mode tries to indicate an exposure that allows you to capture the scene in the best way possible. It is good when the light is fairly uniform and there are no strong differences between the subject and the background. This means that it is fine for most photographs but if you are in a tricky situation, it will be difficult to get a correct exposure.
In other words, a dark background could lead to overexposure of the main subject, especially if this occupies a minority part of the frame and, conversely, a light background (as in the backlight), could lead to underexposure of the main subject.
When to use the spot mode
The spot metering mode is the most reliable. If you want to learn how to photograph, it would be good to learn to use this mode. It is possible to calculate the exposure exclusively on the subject or on a portion of the subject that interests you while excluding the rest of the frame. In this way, you will be certain that the subject of your photo will be correctly exposed.
This does not mean that by using the spot mode you cannot also take into account marginal areas of the frame. With this mode, if you operate in manual mode, you can read the exposure on all the areas of the frame. As such, you will determine the differences in terms of stop. You have full control of the exposure process. This is a really useful aspect for those who are learning to photograph.
The spot mode allows you to obtain the correct exposure in some tricky light situations, such as the backlight or a white subject on a background in the shade. The same applies to a dark subject on a very bright background, a ray of light that filters through the branches or the sunset.
Before the arrival of digital cameras, professional photographers used exposure meters to measure the intensity of light at different points in the frame and then averaged the values. Digital cameras do the same thing automatically.
Shooting in manual mode is the best way to be able to freely express your creativity. However, sometimes you have to do it quickly and you do not have time to experiment with different settings, such as aperture and exposure times. On these occasions, you can help out with the camera’s metering modes for proper exposure. There are different metering modes in all modern SLRs.
This mode provides the most precise control over exposure and uses only 3% of the values for the calculation, usually using those at the center. Frames that have very light parts and other very dark parts can trick the camera. To get a good reading, you will need to measure in a point that is a middle ground between extreme values. A characteristic use is the photo backlight, you measure a point in the brightest area and everything else becomes a silhouette.
Center weighted metering mode
This mode reads the values of light but decides to give different values to the values read. It favors the area at the center of the scene. For SLRs, the final value is calculated for 75% from the readings at the center and 25% from the readings outside. This mode is ideal when the subject takes up most of the photo.
At best, this mode will provide results comparable to those achieved with the matrix. In any case, the reading mode with prevalence at the center can give good results when the subject is in the center of the frame and occupies most of the frame.
The matrix and center weighted metering modes can achieve correct exposure of the scene in most cases but not for a creative interpretation of the scene. With the latter, the exposure meter measures the intensity of the light cones coming from the whole scene. It gives greater weight to the cones coming from the central area, meaning a large area of the frame.
It is a preferred mode and chosen as default because, most of the time, when composing a photo, the most important parts are located towards at the center. With this mode, a strong beam of light on one edge would not compromise the entire picture, it would have a minimum weight because the exposure is based above all on the central parts.
The exposure meter continuously evaluates until the end of the shot.