Adjusting Stops to Foot-Candle Levels
Foot-candles are the units used to measure light in the photography and film industry. A foot candle is defined as the 1 foot radius uniform sphere of light that would be created if there was one candle burning inside it. Most people associate wattage with a lights brightness, but that is inaccurate. Wattage is a unit that measures the amount of electrical output the light is using. Foot-candles are the accurate unit of measuring the intensity of light.
Understanding How F-Stops Work
Stops refers to f-stops. F-stops are the units used to measure the size of the camera's aperture. An image is exposed when light enters the camera through the lens. The light then passes through the aperture which controls how much of that light reaches the photographic medium. If you're in a low light situation you want to have the aperture opened wide to allow as much light as possible to enter for exposure. If you're in a very bright situation then you want to keep the aperture tightly closed to limit the amount of light that passes through.
Using a Light Meter
To get an image as evenly exposed as possible you should use a light meter to help you adjust the stops to foot-candle levels. Light meters cost a few hundred dollars to own but if you're serious about photography then it is an essential tool to own. The first step in using the light meter is to adjust it to the proper film speed or digital ISO that you're working with. Then hold the light meter in front of your subject and make sure that the sensor is facing the camera's lens. Click on the button and the light meter will tell you what stop the aperture needs to be at for proper exposure.
Dealing With Contrast
The odds are pretty good that your frame won't have an even foot candle distribution. There might be parts where it's either too bright or too dark for the f-stop. If there are areas that are too dark add lights to them so that it will get brighter. If that is not a viable option then you might have to consider killing the light in the area so that you can shoot with a lower f-stop.
Let's say that you're shooting outside and can properly expose the subject, but that ends up blowing out your background, there's no way to fight the sun on this so your only option is to add light onto the subject so that the f-stop can be closed. This way you'll achieve a more balanced look.