Types of photometers
1) camera photometer: the photometer is found inside the camera and it measure light from the lens. It only measures reflected light. We need to point the camera (with the lens) at the object or scene we want to shoot, measure the amount of light and take the photograph.
There are several systems photometers use to indicate the proper combination we need to use and they will depend on the model of camera we are using. It can give us the combination digitally, showing us the diaphragm and shutter speed numbers as in the previous image. In mechanical cameras, there can be a scale at the window with an arrow which can show us the proper exposure. It can also show "+" or "-" sings and a "0", which will be the correct combination.
This is a simple diagram of how the photometer can indicate the proper exposure. When the green light reaches 0, it means we have to use the indicated diaphragm and shutter speed combination for a properly exposed photograph.
2) Hand-held photometer: it is separated from the camera. There are some digital ones and some old ones operated by arrows. They can measure reflected or incident light or both. The digital photometers used by most photographers today, read both. The photograph at the beginning of the chapter is a hand-held photometer.
Reflected light: it is the light reflected from an object. To read it, we need to point the photometer towards the subject.
Incident light: it is the light that reaches the subject. Photometers have a tiny white ball. To read the light, we need to place the photometer where the subject is, and point it at the camera. The photometer will consider the amount of light reaching the subject or object from the place where we placed it.