Lens__ Circle of confusion

[Louis Pacilla]

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

That's pretty funny John!

[pgk]

Rule of thumb; prints become shaper when viewed from further away.

[xkaes]

WTF is meant by Circle of confisoin of a lens ???????????

joho

Lenses don't have circles of confusion. People have circles of confusion. The C-O-C is a personal choice -- but most people just choose to ignore it.

It's basically what you accept as in focus or not in focus. That's why it's related to depth-of-field.

Just as you choose the depth-of-field, by selecting the f-stop, you can choose a circle-of-confusion -- what you accept as "in focus" at any f-stop. At a given f-stop, as YOUR PERSONALLY CHOSEN C-O-C increases the D-O-F will increase -- and vice-versa. On the other hand, a narrow D-O-F means a smaller C-O-C -- there's less in focus.

DOFMASTER lets you create your own D-O-F scales for each of your lenses -- BUT you have to tell it what C-O-C you want to use in order for it to compute the D-O-F of each lens at each f-stop. It has a default setting, of course, but it's completely up to you to decide how much you consider to be in focus. The smaller you set the C-O-C, the narrower the D-O-F will be -- so less will be in focus. It's all a matter of preference.

Here's the link:

https://www.dofmaster.com/custom.html

[Alan Klein]

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.

So on a Nikon lens for example, where they stamp onto the lens barrel the f stop range for each f stop, what size in numerals does that equate to?

[Dan Fromm]

So on a Nikon lens for example, where they stamp onto the lens barrel the f stop range for each f stop, what size in numerals does that equate to?

If you're asking about the lenses' depth-of-field scales, the diameter in the circle of confusion used in calculating the scales is an unpublished arbitrary number, usually larger than a picky photographer would select.

In other words, if you have a question about Nikon's practice, ask Nikon.

[Eric Woodbury]

Perceived sharpness is a combination of resolution and contrast. Without one there is no other.

In other words, don't worry about it. Do the best you can and it's fine. Many a great photograph created with less than optimum sharpness.

[Drew Wiley]

The circle of confusion began once people dove into it. Try to find a way out.

[John Layton]

Yes indeed...when a previously finely focussed point becomes visibly softer and visibly "enlarged," we see this as "confusion," and this in increasing increments as this "circle" gets larger.

I guess where this terminology fails for me is in the use of the word "circle."

True a circle can be used to define the outer visible "diameter" of an AREA of "visible confusion," but there is no actual visible circle, excepting perhaps for certain optics like mirror-based lenses, or in certain instances where certain types of light sources (in concert with optics) can begin to acquire a circular affect as they become less focussed in an image.

But I cannot right now think of another term which so handily refers to this optical principle. ("area of confusion,?" "zone of unsharpness,?"). Hmmm...maybe when the morning coffee kicks in I'll have an epiphany!

[Alan Klein]

Yes indeed...when a previously finely focussed point becomes visibly softer and visibly "enlarged," we see this as "confusion," and this in increasing increments as this "circle" gets larger.

I guess where this terminology fails for me is in the use of the word "circle."

True a circle can be used to define the outer visible "diameter" of an AREA of "visible confusion," but there is no actual visible circle, excepting perhaps for certain optics like mirror-based lenses, or in certain instances where certain types of light sources (in concert with optics) can begin to acquire a circular affect as they become less focussed in an image.

But I cannot right now think of another term which so handily refers to this optical principle. ("area of confusion,?" "zone of unsharpness,?"). Hmmm...maybe when the morning coffee kicks in I'll have an epiphany!

Range of focus.

[xkaes]

If you're asking about the lenses' depth-of-field scales, the diameter in the circle of confusion used in calculating the scales is an unpublished arbitrary number, usually larger than a picky photographer would select.

In other words, if you have a question about Nikon's practice, ask Nikon.

Exactly. Whether the DOF scale is on a Nikon, Minolta, or Yashica/Tomioka lens, it's just the manufacturer's personal decision. If you want less "blur", just use the next-smaller-f-number scale. For example, after focusing, instead of using the scale for f8, use the scale for f5.6 -- but keep the aperture at f8.