Tip of the Day - Aperture, part 1

I taught a photography mini class not too long ago and I realized in talking to those students that not everyone knows the three basic physical elements that determine the exposure of a photograph: aperture, shutter speed, and ISO. A basic understanding of these three things gives a photographer much more control over what kinds of picture he/she will achieve.

Basically, aperture (also called f-stop) is the size of the hole that lets light through the lens (similar to the pupil of your eye). Shutter speed is how long the shutter is open to let light through that hole. ISO is another term for film speed, which is a measure of a film's (or digital camera's sensor's) sensitivity to light. These three elements combine in many different ways to achieve different results.

Before we discuss how aperture creates story, let's look at how the three elements work together. To achieve a properly exposed photograph (saying nothing, for the moment, about composition or color or anything else) the camera must receive the "right" amount of light on the film or the sensor. Let's call that amount X. Amount X is roughly the same for each ISO setting or film speed. X is equal to 100 for 100 speed film or an ISO setting of 100 on a digital camera (there is some complicated math behind this, if you are interested). Depending upon how much ambient light is available in the scene you are photographing, the photographer (or the camera) adjusts the aperture and shutter speed to get the needed amount of light to the sensor or film.

A large aperture means that lots of light gets through the lens to the sensor at a time. A small aperture means less light gets through at a time. If lots of light gets through quickly, then the shutter does not have to stay open as long. If little light gets through, then a longer shutter speed is needed. Additionally, a sensitive sensor or film means that less light is necessary to achieve the correct exposure. Kind of basic stuff, but, as I said, not understood by some photographers and therefore worth reviewing.

I'll discuss shutter speed and ISO in more detail another day. Today I'm posting about the ways that aperture creates effects on the image you get (or the image you are trying to get). Take a look at part 2 of this tip, which is posted separately.