Aperture is one of the least effective ways to control the exposure in a scene. While the aperture is considered part of the “exposure triangle” it is actually the one setting that should be left alone. It’s much better to control the exposure using shutter speed, ISO, or some other external method (such as a light).
Typically you’ll want to pick a nice bright aperture and leave it there. This is true for many reasons. I recently read an interview of the cinematographer who filmed “The French Connection”, an old crime movie. The cinematographer, Owen Roizman, stated that he felt like using lenses stopped down was wrong. The truth is, when many lenses are shot wide open they look better, its a basic fact, especially for old manual focus prime lenses.
What is also true is that when old manual focus lenses were shot on film, many of the aberrations such as CA, purple fringing, even blooming around bright areas, would be substantially less evident than how those things appear on digital cameras. If you have used old manual lenses on a digital camera and thought, geez, these are loaded with issues, many of those issues didn’t show up in the film.
The good news is that most aberrations can be cleaned up very easily when editing using a RAW converter such as Lightroom, so the film vs digital debate is still mostly a moot point.
There are many aspects to a lens that make them almost universally better wide open, but it essentially boils down to bringing focus (attention) to the subject.
Once you as a photographer start to understand how aperture affects perception you’ll begin to think exactly what I said at the beginning of this article: using aperture to control exposure disregards some important creative aspects of the lens.
In case you’re wondering, the image above clearly shows the aperture in a 50mm lens. The aperture in this lens is made of 7 curved aperture blades. The curvature of the blades helps reduce distracting shapes in the bokeh balls.
When to change the aperture
If it’s not a good idea to use aperture to control exposure then why change the aperture at all? Of course there are still creative reasons to stop the lens down, it may be something basic such as creating a sun star in an image, or it may be something more subtle such as controlling the look of your work when different focal lengths are used.
When the distance to subject changes
One of the interesting things about how lenses work is that depth of field is reduced the closer you are to the subject. If you want to get the whole face in focus for a portrait a smaller aperture of f/5.6 or so is usually required.
When doing selective focus
A common technique these days is to use depth of field to separate a person or thing out of a group of people or things, or to place focus on someone speaking to another. In these cases it will usually require a wider aperture although often not always totally wide open, usually in the f/2.8 or wider range.
To control the “look” across multiple focal lengths
For some projects it makes sense to always have a similar look in how things are done. This can be true for both photo and video. The aperture can play a role in making sure the look stays consistent from shot to shot despite lens changes.
To get more detail out of the lens
For landscape photographers it is usually a good idea to use a smaller aperture as this will usually result in sharper pictures. Most lenses get sharper as they are stopped down but that is only a rule of thumb, not an absolute.
To control some types of aberrations
Certain lenses aberrations can be affected by the aperture. These include different types of flares, bloom, CA, fringing, vignette, overall softness, contrast, sun stars and even bokeh balls.
In the above image, selective focus draws the eye downward.
If you’re only thinking about the aperture as a method of controlling exposure you’re missing out. It’s not just a part of the exposure triangle, it is also part of the creative process for good photography.
If you’re interested in learning more about the aperture, check out my post on how the aperture in a camera works: What is the aperture in a camera?