There are three main controls for creating an exposure: ISO, aperture, and shutter speed. Many put these settings in the context of a triangle as a way to show how they interact with each other, but as I stated previously, I like to think of them as forming a pyramid. I feel that the pyramid model is a better metaphor for how you build an exposure one piece at a time.
Last time we discussed ISO, the base of the pyramid. This post will go over the next level, aperture.
I would say aperture is the most confusing of the three settings because it uses unintuitive terms and mathematical ratios. In an attempt to alleviate any misunderstandings we'll take a look at what aperture actually is, and use real world examples to explain how it works.
What Aperture Is
At it's most basic, aperture is the opening in your lens that allows light to enter your camera. The size of this opening can be changed in order to control the amount of light that gets in. A lens's aperture is denoted as a ratio known as the lens's F-number, or f-stop. These numbers are made up of the sequence
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/34, f/45, f/64, and so on.
This ratio, which is the focal length of the lens vs. the diameter of its opening, can lead to uncertainty for learning photographers. Unintuitively, the smaller the f-number the bigger the opening. similarly, the larger the f-number, the smaller the opening.
You can get around this common sticking point by thinking of the f-number as what it is: a fraction. The closer a number is to 1/1, the larger the opening. With this thinking it's easy to see that 1/2.8 is much larger than 1/22, and therefore will allow more light into your camera.
Aperture and Exposure
So how does all this actually effect your exposure? Just like ISO, aperture is measured in stops. Moving along the above sequence changes the size of the opening, and therefore the amount of light that enters the camera. Every step up the scale doubles the amount of light, and every step down halves the light.
This means that f/1.4 is twice as bright as f/2, and it's four times as bright as f/2.8. Similarly f/16 is half as bright as f/11, and sixteen times less bright than f/4.
Apertures control over light is why I place it at the second level of the exposure pyramid. If ISO controls how sensitive to light your sensor is, aperture controls how much light is actually available to hit the sensor.
Effects of Aperture
Not only does your aperture effect your exposure, but it has an effect on the actual appearance of your final image as well. By changing the f-stop you're using, you are able to control how much of the image is in focus. The larger your aperture (the lower the f-number) is, the less of the image is in focus. Conversely, the smaller your aperture (the larger the f-number) the more of the image is in focus. This effect is known as depth of field.
This doesn't mean it's best to shoot at the highest f-stop available though. At a certain point the opening will get so small it begins to diffract the light, which causes the image to be less sharp. This point is different for every lens so you will have to experiment with your gear to find out its limitations.
Two Out Of Three
There is only one level left on the exposure pyramid, which will be covered on the next blog post later this week. You are well on your way to taking full control of your tools, and knowing your equipment inside and out is the first step towards this. When you aren't working against your gear, you're able to unleash your inner creativity.