Photography cheat sheet: what are circles of confusion?
Circles of confusion in photography are aptly named, and if you’re not sure what the term actually means, you’re not alone! We will try to explain the concept simply, as well as its link with depth of field (opens in a new tab).
According to Wikipedia (opens in a new tab)when we think of optics, “a circle of confusion is an optical spot caused by a cone of light rays from a lens that does not come into perfect focus when imaging a point source .”
When a lens focuses, each point of the subject is projected as a dot on the camera’s sensor. A single point on a subject is only registered as a point on the camera sensor if the lens is precisely focused at the correct distance. Otherwise, it will actually be recorded as a circle (called a “circle of confusion”).
Depth of field isn’t just a matter of blurring certain things. Areas of a scene can be slightly out of focus (a small circle of confusion) or completely out of focus. Things that are just outside the depth of field area are still recognizable, which can actually distract them.
To reduce or eliminate distractions, you may want to throw some areas (normally the background) so blurry that they are beyond recognition. This may mean doing everything possible to keep the depth of field to a minimum. This is why many pros prefer lenses with the widest maximum apertures, as these help minimize depth of field.
How to Use the Circle of Confusion in Your Photos
Unfortunately, there is no way to mathematically calculate the circles of confusion you will get in an image, although you can adjust the circle of confusion on your image plane by adjusting the aperture and focal length of the lens . Check out our depth of field reminder to help you.
Have you ever heard of depth of field maps? These can also be used to determine the hyperfocal distance of a lens (see hyperfocal distance and depth of field explained (opens in a new tab)) – the distance you need to be from your subject for it to be in perfect focus.
You can find DoF maps online, but of course many lenses also have depth of field markings on the focus rings themselves. Instead of trying to calculate the circles of confusion (because who wants to), you can just stick to the depth of field chart and use the hyperfocal distance it gives.
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