Half-week homework: Aperture science

If you’ve ever experimented with manual adjustment of your exposure settings at all, you’ve certainly played around with the width of the aperture in your lens. But how well do you really know your glass?

The basic concepts are easy. f-number* goes down, light coming in goes up, and the slice of focus (or “depth of field”) becomes narrower. f-number goes up, light coming in goes down, and depth of field grows wider, making more of the scene in-focus.

*The “f-number” is calculated by dividing the focal length by the diameter of the aperture opening, which is why fast lenses at long focal lengths are much larger and tend to be more expensive.

Got it!

In a real-world context, however, you’re not going to look at a scene and visualize your final image in terms of angle of entry, are you? You may have a specific amount of the image you’d like to keep in focus, or a particular intensity of bokeh, but how do you determine what aperture setting will give you the results you need? If you’re like most people, you’re going to throw the camera in aperture-priority mode and then roll the dial around until it looks good on the screen.

Surprisingly, even with an awesome WYSIWYG-capable electronic viewfinder, this is not the optimal way of doing things. This week, your challenge is to pick a lens—or lenses!—and get to know it intimately, so you can select the right f-number on purpose next time.

Pick a subject for your experiment, and position it in front of a medium-distant, detail-rich background. Then, using a tripod and manual focus to make sure the scene is the same every time, take the same photo at every aperture setting so you can compare real-world results. My examples are below; take a look and then check out my analysis further down.

What to look for

  • Background blur: If you’re shooting with bokeh in mind, you might be tempted to automatically crank the lens open as wide as possible. That’s not necessarily always the best option, however; the entire toy isn’t in-focus at f/1.8, and the background is pretty smooth. The shot at f/2.5 provides delightful blur with a few more details for a less abstract shot (which may or may not be desirable, but worth considering) and keeps the back wheel of the toy in-focus.
  • Depth of field: The most useful thing you can take away from this experiment is probably a general understanding of the way depth of field scales with the width of the aperture on your lens, specifically. Shooting at f/5.6 might be fine for a group standing at an angle on a smaller-sensor camera, but on a larger-sensor camera or depending on the distance from the group, you might end up with a blurry face in the back! The more you are able to visualize the actual slice of the scene that will be in focus using a certain f-stop at a certain distance, the faster and more effective your photography will be, and you’ll have instances of biffed focus.
  • Sharpness: Another reason you might not want to shoot at either extreme of your aperture is that, focus aside, most lenses can achieve peak sharpness somewhere in the middle. The edges of a photo suffer particularly at wide apertures. Many lenses are sharpest at around f/11 (plus or minus a stop), though I actually think my example here (an Olympus M.Zuiko Digital ED 75mm f/1.8) looks sharpest at around f/5. Remember that every lens (even the same model lens!) is going to be a little different, which is why it’s so important to get to know your lens.

As I mentioned a couple times, note that the distance between you and your subject makes an impact, too. After a certain distance, there’s little variation, but for subjects up close, even a foot makes a huge difference. For the photos above, I was about 10 feet from the rail on which the toy is sitting. For the pictures below, I got as close as I was physically able to and still focus (about three feet).


When you get this close, the background at f/1.8 is completely obliterated, and even at the narrowest-possible aperture the background is comparable to that at f/5 from 10 feet.

It’s a simple experiment—gathering my data didn’t take me more than 15 minutes—but your understanding of your lens will be improved massively. Be sure to share your results, for the good of the photographic community! When does your lens look sharpest, edge-to-edge? What f-stop lets you get someone’s whole face in focus at your minimum focusing distance? I look forward to answers to these questions and more; leave a comment on our Facebook page or on Instagram tagged with #mymikescamera #halfweekhomework to get the word out.

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