Before we start, to get the most from this post you should have read the previous post on exposure.
The metering system built into your camera is used to adjust the shutter speed & aperture (lens opening) so that the correct amount of light falls on the sensor. Although your camera may be limited to one metering mode, most allow a couple of settings. Typical metering settings include matrix, area, center weighted and spot. Each can be useful when attempting to properly expose an image.
Click “Read the rest of the entry” to continue.
The simplest metering system is area. The camera meters the entire area of the viewfinder, averages it and uses that to set the shutter speed and aperture. Although a simple system to build into the camera, it doesn’t work very well. Unless the lighting of the entire composition you have made is average, the metering will probably be wrong. The area metering system would work well for taking photographs of a gray sky, but add highlights or shadows & it will cause problems. Because of this it is rarely used in modern cameras.
The simplest solution to this problem is to build a center weighted metering system. This type of metering assigns the most importance to the center 40% – 60% of the image. Of course, this assumes that the photographer is going to put the most important part of the image in this area; good composition generally suggests placing your subject almost anywhere other than center. Still, it will usually work better than area metering.
Spot metering is useful for the photographer that understands how to use it. A typical spot meter measures a small 2° – 3° area, again usually in the center of the viewfinder, although some DSLRs can often choose the section of the viewfinder that contains the spot.
The problem with a spot meter is because it measures a small area the photographer has to interpret the reading. For example – aim the spot at a black horse, take a picture, then do the same for a white horse. When you review the images, each horse will be gray! The backgrounds will be different – the white horse will have a dark background while the black horse will have an overexposed background. The reason – The information from the meter reading is used to provide the middle of the exposure level, making both gray. If fact, the only way you can obtain a correct exposure in this situation is to aim your spot meter at an 18% gray card and use that exposure reading. More information about gray cards later.
Another way to use a spot meter is to measure the brightest part of the image, remember the reading. Measure the darkest part of the image you wish to contain detail. How many stops are between them? If it is within the capability of your camera, set it for the middle between the two readings & make your exposure. Check the histogram to insure you didn’t overexpose & correct if you did. If the range is beyond the capability of your camera, you must either change the lighting (tough to do in most situations, although don’t forget the possibility of adding fill flash) or go with less shadow detail. In almost all cases it will be better to lose shadow detail rather than to overexpose the image.
Higher end cameras, both point & shoot & DSLRs offer matrix metering. Depending on the manufacturer, the viewfinder is divided into sections, anywhere from as little as 4 to hundreds or even thousands. The camera compares the light level in each section, notes which section is being used for camera focus, etc and uses a database in the camera to choose the exposure based on the result. There are big differences between each manufacturer’s matrix systems, however for general use it works well. This may be the only choice for some cameras.
The matrix system is probably the best metering system to use since if will often correct for most lighting conditions. It can still be fooled by unusual lighting conditions, and, of course, no metering system can get around the fact that no camera is capable of recording a 14 stop high contrast scene – if the image is important, be sure to check the histogram or blinking highlights to insure you didn’t overexpose.
No matter which metering system you are using, if your camera offers exposure compensation you have a useful method of adjusting for metering error. By the way, you might want to take a look at some of the other sections of “Creating Better Pictures” in the left hand menu at the site in the previous link – there is lots of good information at this Nikon site. To adjust exposure compensation, look for a dial or button with an EV +/- symbol. If your camera also has flash compensation, be sure you are choosing the correct adjustment – you want exposure compensation, not flash compensation.
If the camera’s metering system is over exposing your image, turn the dial towards minus. If it is underexposing, towards plus. How far is something you will learn with practice. For example, if you are taking pictures of a snow scene on a bright day you may need to increase the exposure because the metering system is trying to make the white snow middle gray. By dialing in +1 to +2 stops, the snow will become white rather than gray. Dial the opposite direction to increase exposure for dark scenes. Don’t forget to dial it back to zero when done!
If you become serious about photography, a useful accessory is an external light meter. Although the meter in your camera may work well in most situations, there are times an external meter is better.
There are two general types of external meters – reflective & incident. Reflective meters are the type built into your camera. They are designed to give the correct reading when aimed at a 18% gray card that is illuminated by the same light as the subject. Since most of us don’t carry around an 18% gray card, the fact that reflected light from a typical scene is close to 18% lets us aim at the scenery rather than a card. Another often used substitute is the back of the photographer’s hand & open up one additional stop. Of course one’s complexion will change the adjustment, but once you have determined the amount of correction necessary, you can use it in place of a card.
Still, for the most accurate reading, a gray card should be used. One of the most useful external reflective meters is the spot meter. Since it is not built into the camera you don’t need to recompose your image trying to use it. They are also available in very narrow measuring angles – a 1°metering area is not unusual. Most in-camera spot meters don’t provide under 2° – 3° metering. If you wish to meter the moon, or the highlights of a distant or small object, a 1° spot meter is a useful tool.
Incident meters measure the light falling on the subject. They are held in front of the subject & aimed at the camera. They have a diffusing dome covering the meter sensor so they average the light falling on the meter. Because they measure the light falling on the subject rather than reflected light, there is no need for a gray card – the reflectivity of the subject has no effect on the reading. If you can walk an incident meter across the subject aiming it at the camera, it will let you compare the lighting levels across your scene. This will be more accurate than a spot meter used at the camera location & aimed at the subject since the spot meter is measuring reflected light from different objects, not an 18% gray card – remember the black & white horse problem. Of course, if you are taking a picture of the Grand Canyon, you might not be able to walk the meter throughout the entire scene. If the lighting conditions are different at the subject & you cannot take the meter to the location, a spot meter or the camera’s matrix meter may be better.
A good tutorial on the use of incident meters is at the bottom of this Luminous Landscape Page, as well as some information on the Sekonic L508 II, a hand held meter that can be used to measure flash output, incident light or as a reflective spot meter.
Even if your camera has an excellent metering system built into it, it will be fooled some of the time. Be experimenting with difficult lighting situations (strong back light, high contrast situations, snow, nighttime, etc) you can learn when your metering system is wrong & how to correct for it.