Lens__ Circle of confusion

[joho]

first of all best wishes for the year 2024!!!

Now to the qustion WTF is meant by Circle of confisoin of a lens ???????????

is it the circle of the fstop??? focused ight to unfocued light???

[wikipedea does not count]

joho

[joho]

Forum: Lenses & Lens Accessories would be better for this ---ops!! I am cluless how to move it ...

[xkaes]

[Dan Fromm]

The circle of confusion is the diameter of the largest tolerable blur circle. What's tolerable depends on the photographer's and viewers' preferences and on how much the negative is to be enlarged. It is subjective. There are arbitrary rules of thumb, but arbitrary means subjective.

[John Layton]

Somewhat tangential (sagittal? axial?) musing here...but I also find that lenses of higher resolving power generally have less acceptable depth of field "tolerance" than do lenses of equivalent focal lengths which otherwise resolve somewhat less, a characteristic which can present a paradox in seeking to balance needs of sharpness and depth.

This (visual) effect can be somewhat mitigated by knowing where on the aperture scale diffraction begins to become evident, as this can be used to "smooth out" the transition between sharp and increasingly unsharp ("confused") areas in a given image.

The trick is finding a balance which still allows the "sharper" lens to remain visibly sharper (than the less sharp lens) while also providing a close to comparable acceptable depth of field. Kind of like knowing just how much of that puffer fishes' liver one can consume before it becomes fatal. (Jeesh...where did That come from).

[Mark J]

The blur circle than Dan/Xkaes refer to is the spot image that the lens produces from a perfect point in the scene.
The whole concept of acceptable blur circle depends very much on what size of print you are going to make, and how close you are going to view it.
It can hence be the basis of a lot of argument.
For a lot of DoF tables for 35mm lenses, the circle of confusion was based on what would be only just perceptible softness on an 8x10" print from the neg when viewed at arm's length so that make sense as 0.025mm on the neg as per the snip that Xkaes provided, I have also seen 0.03mm quoted.
For a contact print from an 8x10" neg, the circle of confusion could be a fair bit bigger, maybe 0.15mm.

John, you make a good point, and in fact some aberrations will help to extend the perceived depth of focus. Also, the perception of something going out of focus depends quite a lot on what you have nearby for comparison as 'sharp' . If the in-focus image from eg. an Apo lens is really crisp, it's more obvious that the subject is drifting out of focus.

[pgk]

There is also the confusion circle which is the one in which photographers have a circular argument about the others.

[John Layton]

Another definition would be a board of directors sitting around a circular table. The bigger the table....

[Drew Wiley]

It means you get confused trying to figure out why it's even important. Dan explained it best. It's a malleable rubber-band concept, one which I personally ignore. It might just mean it's time to clean your glasses, or magnifying loupe, or lens. Mr Magoo would understand.

[lassethomas]

Well sharpness is not absolute, its a gliding scale from from perfect sharp towards unsharp. Circle of confusion is a way to define when sharpness becomes unsharp. The larger circle the more unsharpness is defined as sharp. If that makes sense.

So with a larger circle you get a a larger depth of field, since you allow more unsharp distances to be defined as sharp.
All very subjective as stated above and dependant on image goals, film size, printed magnification
and more…