How meters KNOW and the f16 rule

How meters determine exposures may be a bit of a mystery, but it is all easily understood if just a bit of time is taken. Understanding some basics can help you determine if your meter is reading correctly, or if an adjustment needs to be made. This article will help you to understand how meters know what to read, and also how to check the calbration of your meter against a standard.

Because all light meters are calibrated to a certain standard, with very little equipment, you can test your meter to make sure it is calibrated properly.

The first thing to know is that meters are calibrated to something called 18% grey. The theory behind this approach is the concept that if you took all the colours in a normal scene and mixed them all together, you would get a colour that is exactly 18% grey. While this may appear to be silly, the reality is that in most situations it works. Now I am sure that all of us could think of many situations where it does not work, yet, as a standard, it still provides a good starting point.

One of the best and cheapest ways to check your camera meter is with the use of a grey card. They are available at photo supply stores, and are only a few dollars. In my opinion, every photographer should own one.

In order to test your meter there are a few things you need to know. The first is the f16 rule. It goes like this, in a sun lit scene, you will always get a proper exposure if, you set your f stop to 16 and your shutter speed equal to your ISO setting. This works every time, and if you memorize any rule this should be it. The f16 Rule will always tell you if your reading is out when you are shooting in sunshine, especially if you are shooting over sand, snow or water. (Over time and with experience, the rule will also help you to determine if the meter is reading false in other situations too, but that takes time and experience).

So to test your meter, put your grey card in direct sunlight, manually set the aperture to f16 and take a reading. The shutter speed must equal the ISO setting (or be very close...1/3 of a stop is not an issue). If it does not read the same, your meter is not calibrated properly. (Many modern cameras allow meter adjustments in 1/3 stop increments to be set as the standard, so this is not a problem, just know that it is out and adjust your camera accordingly, on my equipment for instance, my camera is set to under expose by 1/3 of a stop for every shot).

The second (less precise method) is to find a normal scene. It must be outside in bright sunlight. Set the lens to f16 and take a reading. Once again it should be equal to your ISO. If it is not, you will need to compensate. In this situation, you should do a number of test shots, because there is the possibility that your chosen scene is not exactly normal in that the mix of colour is producing something other that 18% grey. (If possible, set your camera meter to average the readings, don't use spot or centre weighted, because the area of reading might be too small to check the accuracy of your meter.)

The final way to check your camera meter, and perhaps even the best, is with an incident meter, if you have access to one. Incident meters are highly accurate because they read the light that is actually falling on the scene. First you must check the meter itself by making sure it is following the f16 Rule. If the meter is correct, you can then compare the readings to any reading from your camera and verify that they are the same. This method can be used in any lighting situation with any shutter and aperture setting. Because of that flexibility, an incident meter is an important piece of equipment for any serious photographer.