Depth of Field Explained
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A well-known depth of field definition is the distance between the nearest and furthest parts of the scene that are sharp enough to be in acceptable focus.
Most photographers know that depth of field is affected by lens aperture. A small aperture deepens depth of field and a wide aperture makes it more shallow.
One of the more common depth of field mistakes is thinking that lens aperture is all that controls it. So, what else controls depth of field?
Depth of Field Explained
Original: GoliardicoVector: cmglee / CC BY-SA via WikiMedia Commons
Bear with me, it’s going to get a little technical in here for a little bit.
The Circle of Confusion is the smallest point of light hitting the focal plane, whether a sensor or film, which describes the point of focus. Planes both in front of and behind the point of focus are other circles which are larger than the point called the circle of confusion.
The circle of confusion is the point of exact focus, the sharpest the lens is resolving at that point. The larger circles fore and aft of the focal plane are not exact focus, but they will be in enough focus or sharpness to still appear to the viewer to be in focus.
It may sound extremely technical, but this is the most accurate depth of field definition. The image above helps us visualize the concept.
If you’re still unclear about the concept of the Circle of Confusion, check out the video above from Adorama.
In this video, Mark Wallace takes a deep dive into precisely what the Circle of Confusion is.
It’s always important to get the most specific reasoning behind the Science part of Photography so we can start working on the Art part of our fine craft.
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What Controls Depth of Field - Aperture
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Lens aperture or f-stop is the first factor we usually consider for controlling depth of field. There is still only one point of sharpest focus, described by the science we just examined. But there is a range of what is acceptable focus, what is sharp enough.
The transition from sharp to unsharp is gradual. The smaller the lens aperture is, the more gradual the transition and and larger the range of acceptable sharpness or focus.
Depth of field range generally extends from about ⅓ the distance closer to the camera to ⅔ the distance further. So that would be ⅓ in front of the point of sharpest focus to ⅔ behind it. The wider the aperture (i.e., f/2), the shorter that overall range is. Conversely, a smaller aperture (i.e., f/16) gives a longer range.
This ⅓ to ⅔ consideration is not entirely consistent when calculating all the variables, but it’s a good enough approximation to use in our decisions concerning depth of field. Especially when focusing at medium distances with lenses close to our chosen format’s normal focal length.
As an example, if we focus on a portrait subject’s eyes and use a very wide aperture, we will end up with a very shallow depth of field. This is called selective focus and is a great portrait technique for isolating the subject from their surroundings.
Photo byMarcus Aurelius
To get the deep depth of field landscape photography often uses, we stop the lens down to a smaller aperture. Extreme depth of field tricks can be created with small apertures and setting the lens focus control to the hyperfocal distance, another fantastic tool or technique photographers can take advantage of.
What Controls Depth of Field - Focus Distance
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Another depth of field variable we must take into account is how close or far the camera and subject are. The closer the subject is, the range of acceptable sharpness will be shallower. It gets deeper the further away from the camera the focused subject is.
This is another consequence of the circle of confusion. The circle of confusion, and thus the range of acceptable focus sharpness, changes much quicker the closer the lens is focused. This is why it’s difficult to get a close up subject in focus along with the background.
Photo by Oleg Magni from Pexels
Using extremely small apertures and focussing the lens to the hyperfocal distance will give you the greatest depth of field available for your combination of focal length, lens aperture, and focus distance.
What Controls Depth of Field - Lens Focal Length
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At the same subject distance and aperture, a shorter focal length lens will have more depth of field than longer focal lengths. However, the field of view is different if you keep at the same distance.
If you physically moved the camera closer while shooting the shorter focal length lens to make the subject the same size as what you saw with the longer lens, the depth of field at the same f-stop or aperture would actually be the same.
Generally speaking, though, we don’t usually do that. Our focal length choice tends to center on other considerations. Still, that’s a part of the equation and it again goes back to the pure science of the circle of confusion.
Photo by @thiszun (follow me on IG, FB) from Pexels
By the way, another definition of circle of confusion, a tongue-in-cheek version, is a group of photographers sitting around trying to explain depth of field. Cracks me up!
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What Controls Depth of Field - Sensor Size
The size of the film format or sensor format affects depth of field due to what is measured as the normal lens for different formats. Since focal length does factor in here, it belongs in our consideration.
A smaller format (i.e., MFT, or micro four thirds, like the Panasonic GH5) will have a shorter focal lens as the normal lens in terms of field of view and normal perspective. A full frame format camera (i.e., like the Nikon Z7) has a generally accepted normal focal length of 50mm, the Nifty Fifty.
It should be noted that this focal length does not meet the strictest definition of a normal focal length. That would actually be 43.6mm for the 24mm X 36mm sized format. But the Nifty Fifty was a simple design and it worked well for a 1:1 ratio of naked eye view to the apparent field of view in the camera viewfinder.
In crop sensor cameras (like the Canon EOS M6 Mark II), the crop factor helps us compare lens focal lengths and apparent field of view of lenses for the various formats. On APS-C format cameras, to get the same field of view and apparent perspective of the Nifty Fifty, we would look for a 35mm focal length. On MFT cameras, that would be 25mm more or less.
Since these are actually shorter focal lengths, all other things considered, the depth of field of a 50mm lens would be less at a given aperture and distance than a 35mm lens or 25mm lens.
Photo by Migs Reyes from Pexels
While it doesn’t mean that crop formats have inherently greater depth of field, what it does sort of mean is the opposite. We can get shorter depth of field with the same aperture and distance using a larger format. The technical reasons can be found on our website in other articles, so we’ll leave the in-depth discussion alone here.
This is part of the reason why it’s somewhat difficult to get selective focus effects with smaller formats. In the super tiny sensor formats of smartphones, developers had to create apps to artificially create selective focus.
Depth of Field Mistakes
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You may have noticed how many hedge words I’ve used in this discussion. Generally speaking, apparent, acceptable, and so on. That’s because there is not always a cut-and-dried answer or a decisive depth of field definition. So many variables play a part.
One major depth of field mistake I see regularly is thinking that more depth is always the answer. The technique of selective focus and making use of the lens quality known as bokeh actually rely on shallow depth of field.
Photo by Joshua Abner from Pexels
We have discussed selective focus a lot already. Bokeh is a term used to describe the shape and quality of out of focus lights in our image. If everything is in focus front to back, there is no discernable bokeh. A world with no bokeh is not my favorite choice.
Another depth of field mistake related to stopping down is the problem of diffraction from aperture blades. All lenses have a sweet spot, an aperture or aperture range that corrects optical issues such as distortion and aberrations but before introducing sharpness diminishing diffraction effects.
If we must maximize depth of field in order to satisfy our artistic previsualization for the image we desire, making use of the hyperfocal distance is a great option.
Hyperfocal Distance
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Hyperfocal distance is the focusing distance that provides the greatest depth of field for any given f-stop or aperture. In order to find it on older lenses or current manual focus prime focal length lenses, we would use the depth of field scale on the lens barrel.
dicklyon / Public domain via WikiMedia Commons
In the above example, using an aperture of f/8.0, which is probably close to this lens's sweet spot, if we focus at 30 feet, everything from 15 feet to infinity will be in acceptable focus. It’s a neat trick and it works with any lens, just figure out the variables.
Those variables will be the camera format, the lens focal length, the aperture, and the focusing distance, as described in this video by Pull My Focus:
Since many of our autofocus lenses, either prime focal lengths or zooms, do not have the depth of field scales like the one shown above, we can use a smartphone app or a hyperfocal calculator chart to employ this technique.
What Controls Depth of Field - Your Vision
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As with every deep subject we discuss about photography, we came up with a lot of examples, definitions, and techniques. Making use of this knowledge is up to us as a photographer.
Just as with rules of exposure such as the Exposure Triangle or rules of composition such as the Rule of Thirds, the rules surrounding depth of field are variable, though not arbitrary. We need to know what’s going on in order to control it all for our creative vision.
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Take a look at some of your favorite photographs. Yours and from other photographers. Take note of what works or didn’t quite work in each image.
Was the intent of the photographer to isolate a subject from the foreground or background? Was it to have extremely deep depth of field? Was it to make use of the sweet spot of the lens while maximizing depth of focus?
All of these are excellent depth of field examples. Knowing what is happening and why it happens that way will give you more control over your images.
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