Basics 2: Aperture

Now that we have a basic understanding of exposure, we can look at what we can control to influence it. I like to start with aperture, even before talking about focus, because it carries a lot of influence in how an image is presented. The aperture is the opening in the shutter that allows light to hit the film or sensor in a camera. A larger opening lets in more light allowing an image to be captured faster. A smaller opening lets in less light at one time but has a secondary effect of allowing more of the image to be in focus.

So why talk about aperture before focus?

Before we focus on our subject we need to choose what our subject is and how much attention we want to draw to it. If our subject is a bird in flight, we care about the bird but not the background. If our subject is a majestic landscape, we might want the distant mountains as clear as the stream in the foreground. Aperture controls how much of the scene is in focus, and so I often want more control of aperture than anything else. When we do talk about focus in a later post, we’ll see how our choice of aperture makes our focus points more or less relevant.

Technically, and artisitcally, aperture serves two functions. With dual purpose comes a necessary trade-off. If we want more of the subject in focus, we will need more light. If we need to brighter image for the conditions, then we have to choose how much of the subject to blur. Conversely, if we have a brightly lit subject but want a blurry background, we may have too much light for our chosen aperture and will need to use one of the other controls to compensate.

Aperture is expressed as a ratio of the focal length over a number. That number represents a scale of opening sizes. The resulting calculation gives us the effective diameter of the opening. The scale is typically one of several common values, or “stops”, such as 1, 1.4, 5.6, 8, etc. A lens with a 50mm focal length at f/2.8 will have an effective diameter of 17.8mm. That same lens at f/11 will have an effective diameter of 4.5mm. Now we have a more intuitive measure 17.8 is larger than 4.5, so we know it is a larger opening, so will let in more light. That’s a lot of math to keep track of… so we see the usefulness of the f-number scale.

The simplest way to interpret an aperture notation is to ignore the fraction and think only about the number in this way:

Smaller Number = More Light = More Blur

I may get scoffed at for stating it that way, but a simple way to remember a setting is often the most practical. You might also think of aperture as a scale, where the notation is just labels along the way. going to the left gives more blur and a birghter image. Going to the right gives less blur but darker results.

When do we want a wide aperture (small number) like f/2.8 or lower?

  • Dark settings
  • Portraits, where we want a blurred background
  • Moving subjects, where we are constrained by the need for a fast shutter
  • Any artistic setting where we want to focus attention on a specific part of the frame

When do we want a narrow aperture (big number) like f/14 or higher?

  • Landscapes (much of the time, but not always)
  • Product photography
  • Settings with excessive light, such as daytime or snowscapes

Mid-range apertures like f/8 are good for events, street, and casual photography, where you may need to quickly capture a reaction, an expression, or a spontaneous action without time to adjust settings. Photography is often about being in the right place at the right moment, and you don’t want to miss a great image becuase you needed a second to change a setting. Many photographers will use their camera’s “aperture priorty” mode for this reason.

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