In the photo below, the person in focus is wonderfully isolated from the blurry background. This is an effect we refer to as bokeh, made possible by a small aperture. Because of the bokeh, our eyes are less distracted and drawn to the subject. We consider this effect beautiful because it comes very close to our own cognitive experiences. In fact, we can recreate it without a camera. Just focus with your eye on a nearby object and you will notice how everything in the background starts to blur.
What is Aperture?
Aperture is a hole inside your lens through which light enters your image sensor. The size of the hole—which determines the amount of light passing through—is regulated by a series of aperture blades. Opening and closing the aperture blades can give you a wide or narrow aperture. A wide aperture lets more light into the image sensor. A narrow aperture lets less light into the image sensor.
f-stop is a measure of how wide or narrow the aperture is. It can be confusing to beginner photographers because the f-stop is inversely proportional to the aperture. For example, a smaller f-number such as f/1.8 indicates a large aperture (wider opening) which means more light will pass through the lens. And on the opposite, a large f-stop such as f/11 indicates a smaller aperture (narrow opening) and less light will arrive at the image sensor.
There are times when you want to do the opposite and get more depth of field. For example, in the landscape photo below, we want to get both foreground wildflowers and the background mountains in focus. To do so, you need to use a smaller aperture (such as f/11) to get a larger depth of field.
Aperture and Exposure
As we mentioned earlier, aperture refers to the size of the hole through which light passes through your lens. The following series of F-stop numbers are known as the F-stop scale. It is important because jumping up one full stop (for example from f/1.4 to f/2) means that half the light will pass through your lens. Jumping down one full stop (such as going from f8 to f5.6) means that double the light will pass through your lens!
You want to shoot a portrait with ISO 400, an aperture of f/4, and a shutter speed of 1/500. After you take the picture you decide you’d like a little more bokeh so you open up the aperture to f/2.8. According to the above f-stop scale, doing so would double the amount of light available to the image sensor. As a result, you need to increase your shutter speed one full stop (for example, from 1/500 sec. to 1/1000 sec.) in order to keep the same exposure.
Controlling Aperture Creatively
So far we explained how aperture controls the amount of light that passes through to the image sensor. We also covered how aperture affects the depth of field. But what does all this means to your photography? More importantly, how can you use this knowledge in your pursuit of taking better photos? Here are a few ways you can use your knowledge of aperture creatively.