Circle of confusion

Photoemulator · 2026-04-24T22:37:08-0400

I was aware of the idea of an image circle of lens and how they cover the format of desire. But the Stroebel book “View Camera Technique” 5th edition, has this (new to me) idea of “circle of good definition” which is not the same circle of coverage because it is likely smaller than coverage. Now, that makes sense. But I never considered it before it was mentioned. But what is meant by the second image, “substituting a longer focal length of conventional design”?

djdister · 2026-04-24T23:10:56-0400

Photoemulator said:
I was aware of the idea of an image circle of lens and how they cover the format of desire. But the Stroebel book “View Camera Technique” 5th edition, has this (new to me) idea of “circle of good definition” which is not the same circle of coverage because it is likely smaller than coverage. Now, that makes sense. But I never considered it before it was mentioned. But what is meant by the second image, “substituting a longer focal length of conventional design”?

He means substituting a longer focal length lens that is not a telephoto design.

wiltw · 2026-04-24T23:12:05-0400

Generally speaking, the longer the FL of the lens, the larger both the Circle of Good Definition, and the 'Circle of Illumination' both get.

What is not apparent is what is a lens of 'conventional design' vs. a lens of 'unconventional' design! I have Stroebel's book, and the 'unconventional' does not seem to be given a definition.

What is also not apparent is if the 'Image Circle' commonly defined in lens specs is what Stroebel calls the 'Circle of Good Definition' or the 'Circle of Illumination'.

...a bit of 'literary license'?!

I found this definition for 'Image Circle': "An image circle is the circular, light-projected area a lens creates on an image plane (sensor or film). It defines the maximum area of acceptable image quality, usually covering the diagonal of the sensor to prevent dark, vignetted corners. And so it seems that the common 'Image Circle' is Stroebel's 'Circle of Good Definition'

Kino · 2026-04-24T23:12:13-0400

I think it refers to a "standard" lens design, not retro focus or telephoto, etc.

In other words, the focal length is indeed the distance from the lens to the film plane when focused, not closer or farther.

djdister · 2026-04-24T23:26:26-0400

wiltw said:
Generally speaking, the longer the FL of the lens, the larger both the Circle of Good Definition, and the 'Circle of Illumination' both get.

What is not apparent is what is a lens of 'conventional design' vs. a lens of 'unconventional' design! I have Stroebel's book, and the 'unconventional' does not seem to be given a definition.

What is also not apparent is if the 'Image Circle' commonly defined in lens specs is what Stroebel calls the 'Circle of Good Definition' or the 'Circle of Illumination'.

...a bit of 'literary license'?!

I found this definition for 'Image Circle': "An image circle is the circular, light-projected area a lens creates on an image plane (sensor or film). It defines the maximum area of acceptable image quality, usually covering the diagonal of the sensor to prevent dark, vignetted corners. And so it seems that the common 'Image Circle' is Stroebel's 'Circle of Good Definition'

The text for the illustration does give an explanation that by inference you can determine that a conventional lens is neither a wide angle nor a telephoto design. This is from the 3rd edition, so when it says Figure 99 it is the same figure as 3-23 above).

Circle of confusion

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