Photometer
A photometer is a device which measures the amount of light using a specific film speed and it translates it to a combination of diaphragm and shutter speed. This diaphragm and shutter speed would be the correct combination to expose a photograph under a specific light situation. Photometers have a light-sensitive cell which, in modern photometers, is operated by a battery.
Hand-held photometer Sekonic L-758DR. This photometer measures reflected and incident light.
The first thing we need to do is entering the ISO film we are using or, in the case of digital cameras, the ISO setting we want to use. Then, we need to point the photometer towards the object or scene we want to photograph and measure the amount of light in the surrounding environment. As a result, the photometer will give us a combination of diaphragm and shutter speed we need to use to take the picture (see the lesson on exposures).
This is the screen of a hand-held photometer. In the upper right-hand corner, you can see the ISO, in this case 25. In the middle, you can see how, after measuring the photograph, it gives us a shutter speed (where it says "T") of 125 or 1/125th of a second, and under "F", it gives us the diaphragm of f/9.5.
