Depth of field: What is it, and how it relates to photography

what is depth of field

More people are taking an interest in photography and capturing images than ever before. So it stands to reason that many would be learning more about the medium, likely hearing new terms for the first time. One term that’s frequently mentioned is “depth of field,” but what exactly does it mean?

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 in a moment. Depth of field is measured from the closest, in-focus thing in the image to the farthest, in-focus thing. 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, indicated by a high f-stop, means there is more of the image in focus. An example of when to use a deep depth of field could be an epic landscape image where most, if not all, of the image appears to be sharp and in focus to the viewer. Something like the following image:

sony world photography awards shortlist 2017 winter fairytale
This is an excellent example of an image that has a deep depth of field. Achim Thomae, Germany / Sony World Photography Awards, Open, Travel

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 of someone where the person is in focus and the background is out of focus and blurry. Something like this image below would be a good example of an image with shallow depth of field.

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Notice how the subject and the grill are in focus but the chair and trees in the background are blurry? This is an example of an image with a shallower depth of field.

As we noted above, the pool analogy is a simplification of some very complex math and calculations that determine what the depth of field in an image is. We are not going to get super in-depth here, but we want to cover enough so that you have a good basis of not just what depth of field is, but how it works.

The dictionary definition of depth of field is the distance between the nearest point of acceptable focus and the farthest point 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 purpose of this post, this is what you need to know. 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, while technically out of focus, appears to still be in focus to the naked eye. If you want more information about this we highly recommend researching the circle of confusion and how that works, which will explain the math behind all of this. This quick video is also a great primer on the subject.

Connecting the dots and circling back around to our original question, what is depth of field? The depth of field in your image is how much of your image is in focus, measured from the nearest spot in your image that appears to be in focus to the farthest point in your image that appears to be in focus.

How Do I Change My Depth of Field?

Depth of field is determined by the relationship of the image magnification on the sensor of your camera, the aperture (f-stop) of your lens, and the size (format) of the sensor. For a given f-stop, increasing the magnification of the image on your sensor will decrease your depth of field, while decreasing the magnification increases the depth of field. You can change the magnification of your image on the sensor by either changing your distance by moving closer or farther away, or you can change your focal length, by changing your lens or using a zoom lens.

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Scyther5 / 123RF

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. In some cases you may not be able to adjust your aperture or change your magnification to get the depth of field that you are looking for. If this is the case, then this is where you would investigate other photographic techniques like exposure stacking, which involves taking multiple images with focus set to different points, and then merging the images with Photoshop to create a single image with the combined focus of all the separate images. But that is a topic for another day.

Even this more advanced definition we have provided you is a simplification of a simplification. 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 it will clear things up. However, If you would like some more resources on the math and concepts that go into depth of field you can find a more expansive explanation from Canon, and Wikipedia also has a great breakdown of the math involved.