How Does Your Camera Know?
In the beginning (and sometimes the middle and the end), a photographer just wants to wander around, point the camera at a scene and snap a picture. Camera’s usually have a basic setting that allows you to do just that – point and shoot. It’s fun – you get to go cool places, look at cool things, use neat gadgets, and get a picture to remember it all … BUT, how does your camera know what settings to use to have the picture come out with the correct amount of brightness?
Magic? It can seem that way with how good cell phone cameras have gotten at being able to handle really complex lighting situations. The “m” word we are looking for, however, is “metering.”
Cameras are programmed to know how much light is needed to make a good picture. Metering simply means the camera takes a measurement of how much light is available and then provides the correct settings so the right amount of the available light is captured by the camera sensor. Camera settings that effect how much light is collected by the camera are: the aperture (size of the opening in the lens for light to pass through), the shutter speed (how long the light coming through the lens is allowed to hit the sensor), and ISO (how quickly the sensor can collect the right amount of light).
DSLR cameras often have at least three metering modes from which a photographer may choose. The Canon EOS Rebel T7 possesses (a) evaluative metering, (b) partial spot metering, and (c) center-weighted average metering.
Evaluative metering is the default setting on most cameras. Imagine splitting the viewfinder into a pattern of equal areas – like a grid.
Let’s pretend that the correct amount of brightness for any image is 10 units. If the camera’s settings cause the picture to display as 10 units of brightness, it looks correct.
In evaluative metering, the camera reads the light level in each cell of the grid and then averages the light levels across all the cells of the grid.
If the average value of those cells equaled 5, for example, the camera setting would need to allow twice as much light from the scene to have the sensor record the correct 10 units for a proper picture.
The camera evaluates the light at different parts of the viewfinder and averages those levels together. The camera’s programming can then adjust the amount of available light collected by the sensor. Any scene that has roughly the same intensity of light across the picture or equal patterns of light and dark can be meterd effetively with this method.
Because of the brightness of the snow in the picture on the left, the camera thinks the total image average across cells is 16.2.
Ignoring our pretend numbers, the camera sees that most of the picture is bright white so it thinks it needs to let in less light to get a correct exposure. Less light means a darker exposure.
Evaluative metering would cause the camera to lessen the brightness of the image so that the average is adjusted to the fictitious gold standard of 10 units. As a result, white snow appears gray and the picture ends up underexposed.
The original picture is the correct exposure. That’s because spot metering ignored the bright white around the outside of the picture and just made sure to get the correct light settings for the middle part where the children are.
Partial Spot Metering
Partial spot metering is a more appropriate metering mode when brightness levels are not uniform across a scene. Partial spot metering only uses a part of the grid readings to determine the correct exposure. In Canon cameras, partial spot metering allows the photographer to restrict readings to 6.5% of the scene.
Using partial spot metering, we are able to select the portion of the picture we wish to have as the correct exposure and adjust the camera settings accordingly. In our fictitious example, partial spot metering of the original scene yielded an average of 10.77 units – much closer to the optimum 10. Allowing the camera to use spot metering in our example clearly produces a much more accurate exposure than did the evaluative metering.
Some higher end cameras also provide near true spot metering by allowing the photographer to restrict sampling to 2.5 percent of the scene when taking a reading.
The important take away at this point is that partial spot metering provides a much better starting point for determining correct exposure when the scene is not evenly lit or the photographer is attempting to selectively expose for certain areas of the scene for creative purposes.
Indeed later lessons will reveal the importance of partial spot metering for determining exposure for
and natural light portraiture.
Center Weighted Averaging
Finally, center weighted averaging provides a compromise between the two extremes by basing most of the exposure analysis on the center portion of the scene but also taking into account the outer portions to a lesser extent.