The word photograph comes from the Greek meaning "to write with light". So photography is the act of writing with light. And in truth, that is really a very good description of what it is we do with photography. Photography is all about light.
The light we see with our eyes and with our camera falls into one of two categories. It's either emitted light (such as from a light source of one sort or another like a light bulb or the sun) or it's reflected light. Most of what we see on a day to day basis is reflected light. So, to really blow your mind, when you really think about it, except for emitted light, you're not really seeing anything around you. What you are seeing is the light reflect off of things around you. In much the same way that you might not think of looking at a reflection in a mirror as actually looking AT something, looking AT something is really nothing more than seeing reflected light. Kinda mind boggling isn't it? And perhaps to some, it seems like semantics. But to a camera, it's really a very important concept. Your camera sensor records the light that reflects off your subject. Period. Therefor, in reality, that light is the most important aspect of your photograph.
The camera's job is to capture the light in such a way as to render an image that is representative of what we see when we took the picture. All cameras, from the first camera made in the 1820's to the most sophisticated imaging device made today, capture images in pretty much the same way, by allowing a certain amount of light for a certain period of time to strike a medium of some sort that records the light. The job of the photographer is to see, understand, and, to the degree possible, control the light the camera sees in such a way as to get the image the photographer wants to capture. In order to do that, you have to understand the way the camera sees the light.
In the following blogs, we'll first talk about the three major controls a camera has for controlling light: the shutter, the aperture, and the medium. While there are certainly other aspects of the camera that control the appearance of the image, these are the three controls that affect the exposure itself.