A Beginner's Guide to Aperture and Depth of Field

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If you've spent any amount of time reading photography tutorials or talking with photographers, you have likely heard the phrase that "photography is all about light."

Sure, there are other elements to bear in mind - composition, framing, and so forth - but light is the most basic and most essential component of any photograph.

While that's an easy enough concept to understand, how your camera records light and how you control that light can be a little confusing, particularly for a beginning photographer.

At its heart, light is controlled by three camera functions - ISO, shutter speed, and aperture - where ISO controls the sensitivity of your camera's sensor to light, shutter speed controls how long the sensor is exposed to light, and aperture is responsible for the amount of light entering the lens.

Each of these settings works in a different way to help you use light to create a more compelling image. But beyond that, each of these settings also controls a different creative element of the photo:

  • ISO controls the amount of digital noise in an image.
  • Shutter speed controls whether the subject is frozen in time or blurred.
  • Aperture controls the depth of field or the amount of the image that's sharp.

These elements comprise the Exposure Triangle, which is a much more in-depth topic that we explore in depth in this guide.

For our purposes here, we tackle aperture and depth of field to help you build a solid understanding of this most fundamental aspect of using light.

Aperture Defined

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Think of the aperture as the camera's eye.

Much like the pupil of your eye expands and contracts to control the amount of light allowed in, so too does the aperture.

So, when you walk outside on a bright, sunny day, the sheer amount of light available to your eyes is often too much. As a result, your pupils contract to restrict how much light enters your eyes. The same principle applies to aperture: when there's a lot of light, you want to reduce its size to limit the amount of light entering your lens.

Conversely, when you walk outside at night, your pupils expand to maximize how much light is collected. Again, aperture works in the same way: you open the aperture wider to let in more light into the lens.

The size of the aperture is controlled by a diaphragm, which is located in your lens, as you can see in the image above.

The size of the hole created by the diaphragm is measured in f-stops, which are illustrated below:

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There are many more f-stop values than what's pictured above, but you can still get a feel for the relationship between the f-stop and the size of the corresponding aperture.

As you can see, a larger opening is denoted by a smaller number. Conversely, a smaller opening is denoted by a larger number. That inverse relationship can be on the confusing side at first. However, if you think of them as fractions, you'll have a much easier time remembering this relationship.

For example, if shooting at f/8, and you can't remember if it's a bigger or smaller opening than f/11, just replace the "f" with a "1," like, so:

  • f/8 = 1/8
  • f/16 = 1/16

As a fraction, it's easier to see which one is larger - f/8 - because 1/8 represents a larger number than f/16.

Although there are variations in the f-stops that a particular lens has, meaning, some lenses have much larger maximum apertures and much smaller minimum apertures than others, the same formula always applies: the smaller the f-number, the larger the opening, and the larger the f-number, the smaller the opening.

Aperture and Depth of Field

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Now that you have a solid understanding of what aperture is and how it's measured, let's look at the creative aspect of photography that's controlled by aperture - depth of field.

As noted earlier, depth of field simply refers to the area of an image that's in focus.

A shallow depth of field indicates that a smaller area of the shot is in focus, as seen in the portrait above.

The benefit of a shallow depth of field is that with the background blurred out, our eyes retain focus on the primary subject.

In the image above, you can see how the background is nicely blurred, yet the man remains in sharp focus.

But, the depth of field can also put foreground elements out of focus as well.

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Here, you can see how the people in the immediate foreground are blurry, as is the background.

That's because depth of field refers to the area both in front of and behind the subject. That is, depending on the depth of field, you might have blurred out elements in the foreground and the background at the same time.

This is one of the ways you can add depth to your photos. Using the example above, we have a better sense of the depth of the scene with the inclusion of the blurry people in the foreground. And, as an artistic element, it adds interest to the shot that gives it more impact.

Controlling Depth of Field

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Though there are many elements involved in controlling depth of field, but the size of the aperture you use is one of the most important.

In a nutshell, the larger the aperture opening (the smaller the f-number), the smaller the depth of field. Conversely, the smaller the aperture opening (the larger the f-number), the larger the depth of field.

In the sample portraits we looked at earlier, a large aperture (i.e. f/2.8) was used to get a shallow depth of field.

But in the landscape image immediately above, a small aperture (i.e. f/11) was used to get a much larger depth of field.

Note in the landscape image how everything in the scene is nice and sharp, from the flowers in the foreground to the distant mountain peaks. This is beneficial when photographing landscapes such that the viewer gets the full scope of details of the landscape they are seeing.

In short, the larger the f-number, the larger the depth of field. The smaller the f-number, the shallower the depth of field.

If you still find yourself struggling to remember how aperture and depth of field are related, have a look at the video below by TechQuickie, in which they explain all the concepts above in under three minutes:

Problems With Aperture

There are a few caveats about aperture that you need to know that will help you get the best shots possible.

Though it might be tempting to use the largest aperture your lens can handle to minimize depth of field, or conversely, using the smallest aperture possible to get the largest depth of field, that's often not advisable.

The reason for this is simple: your lens - even if it's an expensive, professional-level lens - does not produce as good results when it's used at its maximum and minimum apertures.

Specifically, your images can be on the soft side when shooting at the maximum or minimum aperture, which simply means that they aren't as sharp as they could be.

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This has to do with the lens's sweet spot, which is the aperture at which you get the sharpest results.

Each lens has a different sweet spot - for some, it might be at f/4; for others, it might be f/8. But what is common to all lenses is that the maximum and minimum apertures will not be the sharpest.

So, no matter what sort of photo you wish to create - a portrait, a landscape, or something in between - remember that avoiding the extremes of aperture, even by just one stop, can make a huge difference in the sharpness of your photo. The landscape above, for example, shows excellent sharpness throughout because the photographer avoided using an extreme aperture in favor of one closer to the lens's sweet spot.

Also remember that as you manipulate the aperture, that not only is depth of field and sharpness impacted, but so too is the amount of light entering the lens.

When you shoot in full auto mode, your camera makes all the necessary adjustments such that the aperture, shutter speed, and ISO work together to get a well-exposed image.

But, full auto isn't the most advantageous mode to take pictures because you have no say regarding the camera's settings.

Fortunately, you can take more control by shooting in aperture priority mode.

Aperture Priority Mode: A Quick Explanation

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Basically, aperture priority mode is a semi-automatic shooting mode that lets you decide the aperture while the camera selects a shutter speed that results in a well-exposed image.

It's a great way to learn to take more control over your camera settings but without having the stress of shooting in full manual mode.

For example, if shooting a portrait and you want a shallow depth of field, you might choose f/2.8 as your aperture. Doing so locks that aperture value in and tells the camera to select a shutter speed to match, such that the resulting image, like the one above, has the shallow depth of field you want and is well-exposed too.

If it sounds scary, it's not! Here's a quick summary of aperture priority mode and depth of field by Phillip McCordall:

As the old saying goes, practice makes perfect, so let's practice getting to know aperture and depth of field.

All you need is your camera and lens, with the camera set to aperture priority mode. Select a large aperture opening, say, f/4, and choose a subject to feature in the shot. A stationary object will work best - like a toy, a teddy bear, or maybe even your child if they'll sit still!

Frame up a shot, positioning yourself a couple of feet away from your subject, with the subject a greater distance from the background (say, 10 feet) and press the shutter.

Review the image, and you should see that the background is nicely blurred.

Try the same shot a few more times, each time at a different aperture, noting the difference in the blurriness of the background.

There you have it! With a better understanding of aperture, depth of field, and aperture priority mode, you're all set for taking improved photos with more control over how they are presented.

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