Lens__ Circle of confusion

[Doremus Scudder]

Correct me as a non native English speaker, but I thought that the correct English expression was: circle of diffusion and not circle of confusion?
I find it a bit confusing...

I guess we should be serious for a while and clear this up. The responses you've got so far are indeed humorous, but not really to the point

The Circle of Confusion (CoC for short) is a function of focus. At sharpest focus, a lens makes a spot, not a point, since there are always some aberrations and imperfections in the lens. As that spot is defocused (e.g., changing lens-to-film distance), it becomes a circle, which grows in size the more out-of-focus the spot gets (aperture affects the growth of the CoC, hence small apertures for more depth of field). This is the CoC. I'm not sure how the word "confusion" got attached to this, but that's the term.

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way. Diffraction is always present, caused by the scattering/bending of photons as they pass an edge. Smaller apertures cause more apparent diffraction because the diffraction quotient of the total light passed is larger at smaller apertures.

"Diffusion" usually refers to the spreading of a concentrated substance through a less concentrated carrying medium, such as water or air, till equilibrium is reached. Think of a sugar cube slowly dissolving in your tea until it is all equally sweet. See https://en.wikipedia.org/wiki/Diffusion

Best,

Doremus

[xkaes]

Well, now I'm confused. Are you saying our diversions and digressions were diffusions or diffractions?

[Drew Wiley]

They were differentially distracted diversions. Think of cone of confusion, and not just a circle.

[Mark J]

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way.

The Airy Disk came from Astronomy, George Airy. It's most applicable to resolving double-stars.
It is really only present at best focus - outside of this, a ring structure develops which may or may not have a spot or disk at the centre.
It's helpful when we're talking about a simple case where we are just looking at the image of a bright point or a star. Otherwise it's more useful to know the cut-off frequency associated with a given f/number, which is relevant to MTF on general subjects.

Note however that the CoC used to generate DoF tables does not consider diffraction effects like this.
In many cases ( especially 35mm format ) the airy disc structure is a lot smaller than typical CoC values, eg. at apertures like f/5.6 or f/8.
In LF, though , at apertures like f/45 or f/64, the diffraction structure gets quite big and fuzzy, so the best-focus is not as good. This does affect perceived DoF, in a way that John was saying, where 'out of focus' depends to some extent on what you have nearby to compare it to.

[reddesert]

The Circle of Confusion (CoC for short) is a function of focus. At sharpest focus, a lens makes a spot, not a point, since there are always some aberrations and imperfections in the lens. As that spot is defocused (e.g., changing lens-to-film distance), it becomes a circle, which grows in size the more out-of-focus the spot gets (aperture affects the growth of the CoC, hence small apertures for more depth of field). This is the CoC. I'm not sure how the word "confusion" got attached to this, but that's the term.

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way. Diffusion is always present, caused by the scattering/bending of photons as they pass an edge. Smaller apertures cause more apparent diffraction because the diffraction quotient of the total light passed is larger at smaller apertures.

"Diffusion" usually refers to the spreading of a concentrated substance through a less concentrated carrying medium, such as water or air, till equilibrium is reached. Think of a sugar cube slowly dissolving in your tea until it is all equally sweet. See https://en.wikipedia.org/wiki/Diffusion

Doremus, in your second paragraph you wrote "Diffusion is always present ..." but obviously meant to write "Diffraction is always present ..." - just for clarity.

"Confusion" in imaging can refer to when individual sources are unresolved and blur together into a background, like a double star that is not resolved by the eye (or the smooth-ish appearance of a highly out of focus background). That's probably where circle-of-confusion came from.

The Airy function is the point spread function of a ideal filled circular aperture (it's the Fourier transform of the filled circle with a sharp edge), so for example a telescope with diffraction-limited optics, in the absence of atmospheric turbulence, would render star images as Airy discs. Real telescopes often have central obstructions, which modify the Airy pattern slightly, and atmospheric seeing usually blurs the image out so you don't actually see multiple Airy rings, but they are there. For photographic imaging in pictorial situations, the image blur size is a combination of the effect of diffraction, non-ideal optics, and defocus, and the relative importance of these terms depends on the f-stop in use.

[Alan Klein]

I guess we should be serious for a while and clear this up. The responses you've got so far are indeed humorous, but not really to the point

The Circle of Confusion (CoC for short) is a function of focus. At sharpest focus, a lens makes a spot, not a point, since there are always some aberrations and imperfections in the lens. As that spot is defocused (e.g., changing lens-to-film distance), it becomes a circle, which grows in size the more out-of-focus the spot gets (aperture affects the growth of the CoC, hence small apertures for more depth of field). This is the CoC. I'm not sure how the word "confusion" got attached to this, but that's the term.

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way. Diffusion is always present, caused by the scattering/bending of photons as they pass an edge. Smaller apertures cause more apparent diffraction because the diffraction quotient of the total light passed is larger at smaller apertures.

"Diffusion" usually refers to the spreading of a concentrated substance through a less concentrated carrying medium, such as water or air, till equilibrium is reached. Think of a sugar cube slowly dissolving in your tea until it is all equally sweet. See https://en.wikipedia.org/wiki/Diffusion

Best,

Doremus

Don't forget "refraction" when using underwater cameras: Light bends as it passes from one medium to another. This bending of light is known as refraction, and can occur when light traveling through water passes through the glass and air of your underwater camera. Refraction underwater causes objects to appear approximately 25% larger/closer than they actually are causing a magnification effect.

https://www.scuba.com/blog/understan...r-photography/

[Mark J]

Hopefully refraction takes place when we use cameras in air - otherwise we'd all be in big trouble.

[phdgent]

I guess we should be serious for a while and clear this up. The responses you've got so far are indeed humorous, but not really to the point

The Circle of Confusion (CoC for short) is a function of focus. At sharpest focus, a lens makes a spot, not a point, since there are always some aberrations and imperfections in the lens. As that spot is defocused (e.g., changing lens-to-film distance), it becomes a circle, which grows in size the more out-of-focus the spot gets (aperture affects the growth of the CoC, hence small apertures for more depth of field). This is the CoC. I'm not sure how the word "confusion" got attached to this, but that's the term.

Diffraction causes a similar phenomenon, but the defocused areas in this case are called Airy Discs. Again, I'm not sure why the nomenclature ended up this way. Diffusion is always present, caused by the scattering/bending of photons as they pass an edge. Smaller apertures cause more apparent diffraction because the diffraction quotient of the total light passed is larger at smaller apertures.

"Diffusion" usually refers to the spreading of a concentrated substance through a less concentrated carrying medium, such as water or air, till equilibrium is reached. Think of a sugar cube slowly dissolving in your tea until it is all equally sweet. See https://en.wikipedia.org/wiki/Diffusion

Best,

Doremus

Thank you Doremus, at last a serious answer!

Now I have to 'transfer' this as I have the impression that the therms and expressions you quoted can change during translation.
"Confusion", for instance, has so many different equivalents through translation in Flemish/Dutch all having a completely different (contradicting) contents, even in physics, hence my initial question...

[pgk]

Don't forget "refraction" when using underwater cameras: Light bends as it passes from one medium to another. This bending of light is known as refraction, and can occur when light traveling through water passes through the glass and air of your underwater camera. Refraction underwater causes objects to appear approximately 25% larger/closer than they actually are causing a magnification effect.

Except with dome ports which produce a virtual image at 4r from the centre of the dome's sphere.

Actually its a wonder that we ever get 'sharp' images with all these complications. I would suggest that photography is, in practice, an essentially empirical activity and (although in my case my original background was in photoscience) we gain experience and as we doo we start to understand many of these and other interactions intuitively because we assess the images produced and learn from them. Digital has speeded this process up.

In generalist photography I am unsure just how much of the science behind image creation is relevant, although it is often interesting and can at times provide an insight into observed phenomena.

Still, what would photforums be without something to discuss and argue about?

[Tin Can]

We used to have angry drunks

arguing

they few were deleted