Lens__ Circle of confusion

[xkaes]

I have one of these charts printed out for each of my lenses. Here's one I marked off to quickly show hyperfocal criteria.

I started out using home-made, DOF graphs -- one for each lens with near & far lines for each f-stop (Y-axis) and distance (X-axis).

Once I discovered the DOFMaster, DIY, rotating scales, I switched to those. Very easy to make with card stock and laminate sheets. You can make them any size you want -- and includ any details you want, at whatever Circle of Confusion you want. Here's one for FIVE lenses. I've set mine up for only TWO lenses each -- much more manageable:



http://www.subclub.org/fujinon/dof.htm

[Drew Wiley]

For circle of confusion authenticity, view your calculation disc or chart with vaseline smeared over your glasses.

[reddesert]

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!

It's a filled circle. I don't think that is the confusing part for most people.

Other terms that could be used include "disk" (commonly used for the Airy disk as someone mentioned), and "spot size." Spot size is how optical designers sometimes talk about the theoretical size of the image of a point source that is produced by the design. But that's a different use, because that spot size is a property of the optical design, while the circle of confusion in a depth of field calculation is a property of focus: even a hypothetical perfect lens has limited depth of field.

[pgk]

In all honesty, a circle of confusion is a theoretical construct intended to illustrate what we see empirically. It rarely represents an actual reality. But it is helpful in keepig photographers happy whilst discussing what it might explain.

[Tin Can]

Good one!

Since my vision is very poor

I use DOF calculators on every shot

Tape measure, string, chart

Range of in-focus focus.

[phdgent]

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...

[phdgent]

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

I was told: twice the diameter was the good distance.

[xkaes]

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...

You say po-tate-o and I say po-tah-to
You like to-mate-o and I like to-mah-to
Potato, potahto, tomato, tomahto
Let's call the whole thing off

[pgk]

I was told: twice the diameter was the good distance.

But doesn't that depend upon how sharp you want the print to look?

[Dan Fromm]

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...

confusion. Also confusing, but that's a different idea.