Reflected Light and Algorithms, oh my!
Your in-camera meter measures how much light is reflecting off the scene it is metering. If everything reflected the same amount of light we would be home free. Obviously, a nice polished piece of chrome on a motorcycle will reflect more light than the skin of a pretty girl sitting on the bike.
Since the amount of light being reflected from different parts of the scene varies, in-camera meters measure the light intensity in different parts of image and uses an algorithm to determine the most suitable exposure for the final picture. We get to tell the meter which algorithm to use. Well, the camera manufacturer gives us the chance to select from several metering modes that each represent a different algorithm.
Middle Gray. The Key to Your Camera's Meter.Since different materials reflect differing amounts of light the camera's meter has to be calibrated to some standard. Camera manufactures use middle gray as that standard. Specifically, they use a reflectance off of any color in the range of 10% to 18% reflectance.
If the scene you are making an image of has a wide range of reflectance it will generally average out to the same reflectance of middle gray.
If you are metering off a subject that has significantly more or less reflectance than middle gray your camera meter will indicate an exposure that is in reality either over or under exposed. Metering a polar bear on a background of fresh snow will cause an underexposure and metering a black dog against a background of coal will cause an over exposed image. In both cases the cameras meter is indicating a proper exposure for middle gray which is not appropriate for either situation.
Different manufacturers call their metering modes slightly different names but they all work the same. So the modes most often available are spot, center-weighted, partial, and matrix/evaluative. There will be a section in your camera manual that will explain the particulars of each mode for your camera model.
Spot & Partial Metering
Spot metering typically only measures the amount of light in the center 5% to 10% of the viewfinder area. If you want to meter different parts of the scene you would have to move the camera so the center of the viewfinder is on each part of the scene you wish to meter.
Spot metering is the most accurate metering method with Partial close on it's heels. Both can be very useful when you want a specific part of the image to be properly exposed. It can also be used to measure the total dynamic range of an image (lightest to darkest), perhaps for deciding the number of exposures and EV steps to make for an HDR image.
Partial metering is also very accurate since it is not averaging a plethora of different reflective sources. It is very useful for metering high contrast scenes. A back lit subject is a high contrast scene. The side of the subject facing the camera is in shadow but the background is fully lit. Metering on the shadowed subject gives you in-camera meter information that allows you to set an exposure so the subject will be properly exposed when you release the shutter. In all probability the background will be badly overexposed but that is why shooting back lit subjects is such a technical challenge. We will cover those technical challenges in a later blog issue.
Spot and partial metering modes give the photographer the greatest control for making creative exposures.
The results produced by center-weighted metering are usually quite predictable. As the illustration shows, emphasis is placed on the center of the image but most of the scene is also considered. Center-weighted puts a real crimp on making a creative exposure. If you're unsure what you might want to do creatively using center-weighted will usually give you an acceptable exposure. For that reason many amateur photographers use Center-weighted as their default metering mode.
The algorithms for Matrix/Evaluative metering are the most complex. They are usually tied to a file in the camera that contains the metering information recorded from several thousand images the camera manufacturer selected.
In these metering modes the camera essentially compares your metering information with one or more of those several thousand images and biases your in-camera meter to produce a close exposure match. Their results are not very predictable because of the complexity of the algorithms. These are the modes most likely to be used by your camera when it is in AUTO mode.
Exposure Compensation (EC)
All the metering modes give you the opportunity to insert your judgement if you find a particular mode over or under exposing consistently. Most cameras will allow compensation of up to + or -, 2 stops.
Exposure Compensation can also be used in those situations with little variation in scene reflectivity where a camera meter always makes exposure errors, like with the polar bear on a background of fresh snow. Just add +1 stop of EC or what ever works with your camera for very light scenes. For dark scenes subtract exposure on the minus side. Don't forget to reset the Exposure Compensation back to zero or the default setting you prefer. My daily carry compact DSLR is a Nikon D60 and I prefer leaving the EC set to + 0.7 and spot mode most of the time.
In the next installment of Digital SLR Basics we'll talk about how to get tack sharp focus.
See you then,