F/stops—How They Work and Why You Want to Understand Them

by Kevin Shank | Jun 17, 2021 | 0 comments

F/stops sound high tech, and might even sound like something that will take all the fun out of photography. Stick with me, though, and you will learn they are not quite as scary as they sound.

The term “f/stop” refers to the size of the aperture opening inside the lens. It controls how much light will be let through the lens when you click the shutter. The f-stop of the lens works together with the shutter speed of the camera to allow the correct amount of light into the camera so your photo is properly exposed. When the aperture is made smaller inside the lens (referred to as “stopping down the lens,” or “narrowing the aperture,”) it will take more time for the light coming through the aperture to properly expose the picture. Therefore, the shutter speed is slowed down to compensate.

When the aperture of the lens is stopped down one full stop, the hole in the diaphragm is made enough smaller that twice the amount of time is needed to let an equal amount of light pass through the lens. It is much easier to observe this with the shutter speeds than it is to observe it by the various numbers that designate the f-stop in the lens.

Shutter speeds look like this: 1 second, 1/2 second, 1/4 second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, 1/125 second, 1/250 second, 1/500 second, 1/1000 second, 1/2000 second, and so on.

While not all lenses start and stop at the same place, full f-stops look like this, starting with the widest opening and going to the smallest: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Many cameras today jump in ½- or 1/ 3-stop increments.


Now let’s do a little exercise. If a scene calls for an exposure of f/16 at 1/125 second, what would the shutter speed need to be if the camera lens were changed to f/11? The lens is opening up one full stop, so the shutter speed needs to be only one-half the time. Therefore, 1/250 second is the correct shutter speed. Now let’s change the aperture to f/22. What happened to the shutter speed? Now the shutter speed is 1/60 second.

So, you might now wonder what difference this makes. Well, more happens than just changing sets of numbers. With a smaller aperture, the depth-of-field (amount of area in focus) increases. This is valuable to know, because sometimes you will prefer a shallow depth-of-field, or just the opposite.

Besides depth-of-field, think about what happens when the shutter speed changes. If something is moving through the photo, a fast shutter speed will stop the action, while a slow shutter speed will blur the action. Again, there will be times when you prefer a fast or slow shutter speed.
Maybe your camera has an automatic mode(s) and you wonder about just using that. It is fine to use an automatic mode; however, there are times it is nice to know how to shoot manually to take advantage of your artistic visions for the photo. For example, if you want moving water to blur, you will want to shoot slowly.

Personally, I combine this knowledge with shooting in an automatic mode—the AV mode, to be exact. Using Aperture Value means I select the aperture on the lens while the camera selects the corresponding shutter speed. If I want moving water to blur, I’ll choose a small aperture, knowing this will also slow down the shutter speed. If I want shallow depth-of-field or a fast shutter speed, I’ll choose a wide open aperture. If the light is low and I am hand-holding the camera, I’ll choose a wide aperture to keep the shutter speed high enough to not blur.

Some cameras have shutter priority as a choice for an automatic mode. I do not choose to use this mode, because the shutter speed range of the camera will easily surpass the working range of the lenses I use. Let’s say I’m on a hike and I see a deer running across a field in my direction. If I spin the aperture to the widest it can go on the lens that is mounted on the camera, I know I am shooting the fastest possible shutter speed that the current light levels, ISO setting, and the lens can handle at that moment. If I’m on shutter priority and spin the shutter speed to 1/2000 second, I may have a dark picture because the lens could not open wide enough to utilize a shutter speed as fast as 1/2000 second.

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