Today’s film and digital cameras (including the point-and-shoot cameras) have complex, sensitive, and very accurate light meters built in to help you get the correct exposure. Light meters work by assessing the amount of light available and calculating the shutter speed based on the f/stop and ISO settings. Changing any of the three of shutter, f/stop, or ISO will change how the exposure is calculated by the light meter.
A Little Background
Before light meters were built into the camera, photographers used hand held light meters. These hand held devices had two modes of metering light – direct or reflective. The direct mode would have the photographer point the light meter towards the light source and take a reading, whereas the reflective mode would have the photographer point the light meter at the subject and measure the light that is reflecting back towards the meter. Once the measurement was recorded, the light meter would display the correct shutter speed for the f/stop and ISO selected, and these settings could be changed to see the different combinations that would give a correct exposure for the measurement.
The in camera light meter works in the same way as the reflective mode of the hand held meter. You are pointing your camera at the subject and the camera’s light meter reads the light reflected back to the camera. The in camera light meters have evolved over the years and have become very sophisticated offerring three modes of metering light, each with specific benefits for specific types of shooting. These modes are spot, matrix, and center-weighted. Some point-and-shoot cameras will only offer one mode which is matrix.
Spot Metering
Spot metering measures the light at yup – you guess it – just a spot. It ignors any light in the rest of the frame and bases it’s exposure calculation on just the light it measures at this spot. The size of this spot varies, but is usually about 4mm and is centered on the focus point of the camera. If you can and move the focus point, the spot metering follows.
Spot metering is useful when you have a very high contrasty scene such as seen here:
Fetchin House
For this scene I spot metered on the bed rail by the headboard (assuming the footboard is nearest the window). With the understanding that the camera’s light meter on spot metering is going to calcuate the exposure based on 18% gray or zone 5, this would have caused the scene to be overexposured by about two stops, so I adjusted what the meter was telling me to underexposure by two stops. I made this adjustment based on the zone system and estimating that the bed rail was about zone 3. Since the light meter calculates adjusting everything to zone 5 I have a two stop difference.
I find that a good understanding of the zone system is very helpful when using spot metering. Otherwise you need to find something in the scene that is close to neutral gray (18% gray) and meter off of it in order to get the correct exposure. If you understand the zone system, you can meter off of anything in the scene and make the necessary adjustment to the exposure as I did above.
Matrix Metering
Matrix metering is the most complex and versatile metering modes available and because of this it is the mode that most point-and-shoot cameras use. Matrix metering uses a complex algorithm which accounts for all areas of the frame, using contrast between lights and darks, colors, and even elements within the scene to calculate the best exposure for the scene. The more expensive SLR’s even have a database of scene information that the algorithm uses to dial in the exposure setting based on known scenes. Nikon calls this 3D-matrix metering.
For most scenes, particularly landscapes, matrix metering works really well. In fact it would have worked pretty well for the image above but I would have had to make some minor adjustment to the exposure compensation to get it right, because it would have compromised on either the darks or the lights depending on how the camera’s light meter saw the scene.
The following image was shot using matrix metering:
Peaceful
Center-Weighted Metering
Center-weighted metering works in much the same way as the matrix metering with the exception that it places more emphasis on a 12mm circle in the center of the frame. If you are shooting portraits, this is the metering method to use.
I had the opportunity to listen to Adam Jones speak last month. For those of you who don’t know who Adam Jones is, he is one of 26 Canon Explorers of Light. Basically means the guy knows what he’s doing because Canon has put their faith in him. I was surprised to learn that Adam shoots over 90% of his images in matrix metering mode. I thought being the high end shooter that he is he would have been using spot metering for absolute control, but no, he said “…don’t make it any more difficult than it needs to be.”
Pretty good advice I think since I like to work on the KISS principle (keep it simple). As always, if you have any suggestions for articles you would like to see here, please contact me with your request and I’ll do my best to address it. Thanks for reading!
