Understanding Aperture, F-Stop and Depth of Field + Reasons Why You Shouldn't Always Shoot Wide Open
Knowledge of aperture is essential for photo and video shooters. It is one of the three pillars that determines image exposure and is an important technical element and tool that helps a photographer decide how to best capture an image.
What is aperture?
Inside every camera lens are a series of blades. These blades are designed to open and close to create a bigger or smaller hole inside of the lens. This hole is called the aperture. The size of the aperture correlates to a number called an "f-stop". A photographer or video shooter sets the f-stop on the camera or the lens before taking a shot. To make it confusing, a smaller f-stop number refers to a larger hole/aperture and visa versa. For example, f/22 is a very small aperture, while f/2.8 is a very wide aperture.
How does aperture change the look of your image?
When you're on a shoot and you open up your aperture wide to an f/1.4 for instance, this results in a brighter image. When you close down your aperture to something like an f/5.6, this results in a darker image. Why does this happen? It all has to do with the size of the hole/aperture. The wider aperture at f/1.4 allows more light into the camera, resulting in a brighter image. The smaller aperture at f/5.6 allows less light into the camera, resulting in a darker image. Aperture is just one of three basic pillars to change how bright or dark your image appears along with shutter speed and ISO. It's important to understand all three pillars to fully understand the best settings to capture the image that you want.
Knowing that your aperture changes your image exposure is important, but there is another way that it effects your image. It also changes your depth of field. "Depth of field" is a term that refers to how "deep" the plane of focus is in your image, or how much of your image is in focus. Changing the f-stop/aperture on your lens changes your depth of field. Below are examples of the same image shot at different aperture settings. Note: The exposure for each image was adjusted with an ND filter.
Notice how the depth of field changes as the aperture changes. At a wider aperture, not only are you letting more light into your camera, you are also going to end up with a much more shallow depth of field than you would at f/22. When you are out shooting, it's important to consider how much of the scene you want to be able to see so that you can set your aperture accordingly. Keep in mind that it's not always best to shoot at your widest aperture despite the appealing look it can sometimes give.
Why shouldn't you shoot at your widest aperture?
For new photographers, and even some seasoned shooters, it can be tempting to keep your lens locked at the smallest f number. There are some circumstances where this will create spectacular results, giving you a soft background for your subject to stand out against, however, this doesn't work for every situation, and there are several reasons to avoid doing so.
Reason #1: You'll have areas of your image out of focus that you want to keep in focus
It's important to know when to use shallow depth of field and when to use long depth of field. When you're shooting gorgeous desert scenery, a shallow depth of field does not quite suit the situation when you're trying to show the beauty of the landscape. Refer to the image below.
Of the two images, I prefer the shot at f/22 where we can see all of the beauty of the landscape beyond just the dead bush.
This concept doesn't only apply to backgrounds. If you're photographing people or animals, even if you nail your focus on the eyes, your depth of field could still be too shallow to keep their ears or nose in focus! Experimenting with different apertures and knowing what depth of field looks best in different applications will make you a better photographer and help take your images to the next level.
Reason #2: Moving Subjects Are Much Harder to Track at Shallow Depth of Field
This primarily applies to video shooters and action photographers. When you're shooting at your widest aperture, it becomes much more difficult to keep your subject in focus when your subject is moving, when the camera is moving, or both. In some cases even an inch of variance can put your subject out of focus. In these cases, you might consider bumping your f-stop to a higher number to increase your depth of field and give you or your focus puller an easier time getting the shot.
Reason #3: Sweet Spot
The term "sweet spot" refers to the aperture that captures the best detail sharpness on a particular lens. This is never the widest aperture and is generally 2 or 3 stops down from the widest aperture on your lens.
When I say "sharpness" I'm not talking about the depth of field. I'm talking about the sharpness around the edges of the details in your image. If you zoom in to an image at f4 vs an image at f8 taken on the Canon 24-105mm f4L, you'll notice a difference in sharpness. That's because f8 is closer to the sweet spot of this lens where it performs best. See below.
Experiment with different apertures on your lenses to find the sweet spot. The best way to do this is to set up your camera on a tripod and take the same photo of a static object at different f-stops, starting with the widest aperture, all the way to the smallest aperture. A high-contrast subject works best, such as black words on a white page. After you take a series of images, load them onto your computer and compare the differences. You might be surprised!
When DO I use the widest aperture on my lens?
The correct answer to this question is to only use the widest aperture when you know you need the shallow depth of field. With that comes the importance of knowing which situations to use a shallow depth of field and which ones not to.
You can also shoot at a wider aperture in low-light conditions when your image is underexposed and you can't adjust your shutter or ISO. That said, I will add that in situations like this, the honest and best solution is to add more light so that you can continue to shoot at your current aperture without having to sacrifice the depth of field you need to get the shot you want. I understand that this is not possible in every situation, so if you need to adjust your aperture to achieve proper exposure, then, by all means, you should do so, but be sure you know what the limitations of that are so you know what to expect out of your final image.
Check out our Youtube tutorial on this lesson to further understand aperture and depth of field!
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