Light Metering
David Haardt, 2002
why light meters do and will make errors
Any light meter (handheld, center-weighted, spot, as well as the single element of a multi-zonal metering system) believes that the subject has on average the reflectivity of 18% grey (so-called "average grey value" introduced by Kodak Eastman).
why even the best autoexposure systems can never be perfect
The multi-zonal metering systems (called "matrix" or "evaluative" or "honeycomb" or "segment" metering by the various camera manufacturers) try to compensate for that inability by applying an algorithm or by comparing the light situation to internally stored template images. However these procedures can be very delicate because you don't know how and how much the camera did already correct exposure, so exposure compensation on basis of a multi-zonal metering is totally useless. The results you get with such autoexposure systems are not repeatable and more or less stochastic.
what we can about it
That's why most serious people (including myself) don't use multi-zonal metering systems when there is enough time, but they use either center-weighted or spot metering, either built-in in the camera, or externally in a separate exposure meter. They do so because this allows them to correct for exposure without guessing what the camera might have done.
the problem we are facing
If you meter a black subject (actually we mean a very unreflective one, but black is a good approximation - this is where the Nikon F5's colour matrix meter tries to hook up) and photograph it without exposure correction, it will appear too bright (i.e. average grey). If you meter a white (here, read very reflective) subject and photograph it without exposure correction, it will appear too dark (i.e. average grey). So without exposure correction, black and white subjects will look the same on your image! That's why it's crucial to do compensations.
the grey card
A grey card is a piece of cardboard paper or plastic which is gauged to 18% grey reflectivity. You can buy such grey cards in any photo store (look that you get a waterproof, non-fading one), then hold it in front of your subject, meter it, remove it, and then photograph the subject with the meter reading you got from metering the grey card. Of course the use of a grey card is only practicable in "slow" photography, e.g. landscape or macro work. If the subject is far away, or if we need to be fast, then we have to develop a metering technique which achieves the same effect as the grey card, but without having to use one.
zone system for beginners
A rather easy way to do so might be a simplified zone system*. When your subject is darker than average grey, underexpose by one stop compared to what the meter (center-weighted or spot) suggests. When your subject is totally black, underexpose by two stops. When your subject is lighter than average grey, overexpose by one stop. When your subject is totally white, overexpose by two stops. Of course you can and should do fine-tuning to these rough guidelines with half stops or thirds of a stop.
*) The zone system is an exposure method developed by the famous Ansel Adams to achieve perfectly predictable exposure with black-and-white film by controlling exposure, film, development, and printing; basically you are selecting which parts of the image shall have which brightness in the final image, with consideration for the exposure latitude of the film material used - you are "placing" image parts "onto zones", as Adams expressed it. Adams' original zone system included 11 zones, starting at Zone 0 (total black) over Zone II (first suggestion of texture) to Zone V (middle grey, 18% reflectance), and upwards over Zone VIII (whites with texture) to Zone X (pure white). So Adams suggested four texture-less zones (0, I, IX, X) and seven textured zones (II-VIII). Of course this preciseness is only possible with black-and-white film. With colour film, we need to resort to the simplified system which I have described above because of its far narrower exposure latitude. For a detailed but understandable, and original, description of the zone system, read "The Negative" by Ansel Adams.
taking it to the extreme: spot metering
A spot meter allows you to exactly select the part of the subject for which you believe to know the appropriate compensation. If you believe that the cap of the third man from the right is exactly average grey, and if that cap is exposed to the same amount and direction of light as the overall subject, then you can spot meter that cap, and take the meter reading for your image. The problem is that spot metering requires very much experience and knowledge about light and exposure to be able to select an appropriate image part.
I prefer it the easier way
I personally in most cases prefer a center-weighted metering (with my camera, or with my Weston Master V selenium hand exposure meter), and then correct in a range of +/- 2 stops just as I explained above. A selective metering method which many cameras offer might be a good way to combine the broad "security" of center-weighted metering and the preciseness of spot metering. My method worked very well for me so far, and I even exposed 4x5" large format chromes (Fuji Velvia) with it, without any exposure "error" so far which would have been due to my method.I didn't invent the wheel
By the way, I have derived that method from the book "Naturfotografie" by the famous German nature photographer Fritz Pölking which I can only recommend to anyone interested in outdoor photography.
a useful everready grey card
Another useful hint: An average caucasian hand is about one stop brighter than a grey card, so you can meter your hand, add one stop (just as I wrote above: subjects brighter than average grey need to be overexposed compared to the meter reading), and then take that exposure. Works very well for landscapes...
about incident light metering
The methods I described above are all referred to as reflected light metering - you meter the light which your subject reflects. A different approach is incident light metering. Normally you put a white dome over the metering cell of your handheld meter (to achieve a very broad angle of metering, the opposite of spot metering where we have a very narrow angle of metering) and then you meter from your subject towards the camera, i.e. the other way round. With that method, you meter the light prevailing in a scene. The advantage is that this method doesn't account for the brightness of the subject, so we do not need to compensate as I described in my simplified zone system, but the disadvantage is that this method also doesn't account for the reflectivity of the subject. I personally didn't ever use incident light metering so far. It is also only applicable if your subject is rather close, and if you have enough time.
different builds of light meters
Basically spoken, there are selenium and battery dependent meters. Selenium is an element which generates voltage from light without any additional power source. The advantage of selenium meters is that they always work, also in very cold climates where batteries give up. The disadvantage is that they cannot produce a meter reading in very low light. Battery dependent light meters (Silicium, CdS) are more common today. They have a very broad metering range, but are not reliable with very cold temperatures. Simple light meters have the size of a cigarette packet and are dominated in design by a large dial. You hold the meter against the subject, press a button, get a meter reading on a needle, and then you turn the dial so as to point the needle reading against an index. More modern light meters look like very small computers, they directly write down the meter reading in combinations of diaphragm values and exposure times with an LC display. I personally use a Weston Master V meter which is pretty "primitive" being a selenium meter with a very simple design. Nonetheless such meters are quite expensive, and absolutely useable.
about flash meters
Flash meters are electronic light meters which can not only meter light constant over time, but also short impulses of light, i.e. flashlight. They are especially important in studio work.