The exposure triangle (How-To properly expose an image)

For my first how-to post I'm going to start with the basics of exposure.

Every camera, film or digital, new or old works on the same principle. How much light hits the sensor or film plane determines if your image is exposed properly. Too much light and you have an image with blown out highlights that lacks detail. Even more light and you have a flat white image that wouldn't require any pigment at all to print.

The same is true when it comes to underexposure. Not enough light? Thats an image that is dark and lacks detail in the shadows. Even less? A flat black image.

To calculate how we set our camera to get that perfect exposure in-between the two extremes, we use something called the exposure triangle.

In the triangle we have our three main camera settings. Shutter speed or how much time we let the image expose, aperture, or how much light we allow to pass through the lens, and ISO, the defined sensitivity of our image sensor or film.

When imaging a static object this is an easy calculation. Use a tripod, pick your aperture, and set the ISO to it's native sensitivity (ISO100 in most digital cameras). Shutter speed can be left to auto.

What if we're taking a picture of a fast moving subject or we don't have access to a tripod you ask? Thats where we need to start playing with our settings. A good rule of thumb is 1/1000th of a second shutter speed to freeze fast motion. This means we either need lots of light or a very high ISO sensitivity (or both!). In this instance we might open our aperture up as wide as it will go, and then use the lowest ISO sensitivity that still maintains a well exposed final image. This is represented in the camera auto modes as Tv, or shutter priority.

Now you know the basics of shooting action. What if you want creamy background blur for a portrait? Or a very wide depth of field for that gorgeous landscape? Here we need to manipulate our aperture setting to determine depth of field. For our portrait we might choose to shoot the widest aperture our lens allows, with the base ISO, and then determine the correct shutter speed for the exposure. For our landscape, we should chose an aperture value that expands depth of field, and gets more of our pretty view in focus. When you choose to expand your depth of field you will need more light! These situations are represented in the camera auto modes as Av, or aperture priority.