What is depth of field? The secret to perfect backgrounds

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One key attribute of image quality that helps DSLRs and mirrorless cameras stand out is depth of field control, and shallow depth of field is such a good look that smartphones are trying to imitate it using dual lenses and computational photography. But what really is depth of field, and how do you control it?

Explained in the absolute simplest way, depth of field refers to how much of your image is in focus. This is an oversimplification, but we will get more in-depth (no pun intended) in a moment. The depth of field of an image is measured from the closest in-focus point to the farthest in-focus point. A popular analogy here is a swimming pool: At the deepest, indicated by a high number in feet, there will be more water; a deep depth of field means there is more of the image in focus.

An example of when to use a deep depth of field could be a landscape, where the entire image is essentially the subject. This is particularly true if there are both foreground and background elements that you want to keep in focus, as in the following photo:

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On the flip side of the pool analogy, a shallower pool means there is less water. So, if you guessed that a shallow depth of field means that there is less in focus in the image, then you would be correct. An example of this would be a portrait, where the subject is in focus and the background is out of focus and blurry. This can be especially useful if you have a busy, distracting background, like in the photo below:

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The dictionary definition of depth of field is adds a word to our above description, calling it the distance between the nearest and farthest points of acceptable focus. Seems simple enough, but you may be wondering to yourself right now — what is acceptable focus?

Acceptable focus is based around the “circle of confusion” and some other advanced topics, but for the sake of simplicity, you can use depth of field in your images without digging into those terms. True focus on a point in an image can only be achieved at one distance, and everything else in a shot is at varying levels of out of focus. However, our eyes can only see so much detail, and a point in your image does not look out of focus to your eye until the blur from being out of focus gets to be big enough that our eyes can pick it up.

So, acceptable focus is when a given point in your image either in front or behind of the true plane of focus appears to still be sharp to the naked eye. If you want more information about this we highly recommend researching the circle of confusion and how it works, which will explain the math behind all of this. This quick video is also a great primer on the subject.

How to control depth of field

Depth of field is determined by the relationship of the aperture (f-stop) and focal length of your lens, the distance of the subject in the photo, and the size (format) of the sensor.

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The most common way to change your depth of field is by adjusting the aperture of your lens, which determines how much light your lens allows through it and on to your camera sensor. The narrower the aperture is, the deeper the depth of field. A wide open aperture creates nicely soft backgrounds with a shallow depth of field. Here, the depth of field is sometimes so narrow that your subject’s eye could be in focus but the tips of the eyelashes may be soft. These wide apertures (f/1.4, f/2, etc.) draw more attention to the subject by blurring out the background, but larger or close-up subjects may not be entirely in focus.

On the other hand, a narrow aperture (f/8, f/11, etc.), keeps more of the image in focus. A small aperture is often recommended for landscape photography because you may need to maintain sharpness from very close foreground elements all the way out to distant elements like skylines or sunsets.

You may have noticed that smartphones often have bright lenses with seemingly wide apertures, like f/1.8 on the iPhone XS — and yet, the depth of field is still very long. Why? This is because sensor size also plays a role in the depth of field. The technical explanation behind this is a bit, well, technical, but the takeaway is this: The larger the sensor, the easier it is to get those nice soft backgrounds. Phones have to make do with very small sensors. A large sensor requires a much bigger lens, which simply wouldn’t fit in a phone-sized package.

Smartphones can, however, imitate a blurred background using software, the results of which can be impressively realistic in the right situations, and even allow you to change the depth of field after the fact since it’s as simple effect. However, computational portrait modes still fail in many instances and don’t always work with every type of subject, such as ones that are too close or too far away.

Remember, though, that depth of field is the range of distance that appears acceptably sharp. That means if you’re shooting at your widest aperture with a large sensor camera and the background still isn’t blurry enough, there’s still more that you can do. Move the subject farther away from the background, and that background will appear softer without changing any camera settings.

In a similar manner, the closer the camera is to the subject, the softer the background appears. Macro photographs are often shot at smaller apertures, even when that soft background is desired, because the camera is so close that the depth of field is shallow regardless of aperture. Some macro photographers even use focus stacking to get more depth of field, which involves taking several shots at different focal points and merging them together in an editing program for a sharper shot.

Telephoto lenses also create a shallower depth of field compared to wide-angle lenses. This is one reason why telephoto lenses are often preferred for portraiture, while wide-angles are used in landscapes — this is, of course, not a hard rule.

Conclusion

Depth of field and the related math is all part of an incredibly complex system of equations and rules that for most people will cause more confusion than understanding. But while the math (and reasoning behind it) is complex, the techniques are not. To create a softer background, use a wider aperture (lower f-number), use a large sensor camera, move closer to the subject, or move the subject farther from the background — or maximize the effect with a combination of all those factors. For sharper images with more details intact, use a narrower aperture, a smaller sensor camera, move farther from the subject, or move the subject closer to the background.

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