Give a little, take a little: reciprocity

Now that we know some of the basic controls available on your camera, let's try and understand how they relate to each other.

A correctly exposed image can be made using several combinations of aperture and shutter speed.  The relationship between the aperture and shutter speed that enables us to vary the settings and still maintain the same exposure level is called reciprocity. Each combination results in the same level of exposure but the aesthetics on the image will vary a great deal  (we'll talk about these differences in much greater detail in future posts).

Here's how reciprocity works.

The Basics

At its core, a photograph is a recording of light on a medium (film, sensor etc.).  When we make an image we try to manipulate the amount of light that strikes this medium. Some of this can be controlled on the camera while others are external to it; selecting the time of day to shoot, using reflectors and strobes to control contrast etc.

In this post we'll talk about three basic controls on a camera that every photographer needs to understand to get the most out of their exposures;  aperture, shutter speed and ISO.  Together these three variables make up the exposure triangle.  Develop and understanding for how they work and interact and you're off to good start.

The information discussed below is relevant to all types of cameras, DSLRs, point-and-shoots and mobile devices but the controls on point-and-shoots and mobile devices will be more restrictive. Here's an image of the info screen as seen on a Canon EOS 40D. There's lots of information here but in this post we'll focus only on the items that are circled.