Editing Circle of confusion (section)

===Dallmeyer and Abney===
{{harvtxt|Dallmeyer|1892|p=24}}, in an expanded re-publication of his father [[John Henry Dallmeyer]]'s 1874 {{harv|Dallmeyer|1874}} pamphlet ''On the Choice and Use of Photographic Lenses'' (in material that is not in the 1874 edition and appears to have been added from a paper by J.H.D. "On the Use of Diaphragms or Stops" of unknown date), says:

{{Blockquote|Thus every point in an object out of focus is represented in the picture by a disc, or circle of confusion, the size of which is proportionate to the aperture in relation to the focus of the lens employed. If a point in the object is 1/100 of an inch out of focus, it will be represented by a circle of confusion measuring but 1/100 part of the aperture of the lens.}}

This latter statement is clearly incorrect, or misstated, being off by a factor of focal distance (focal length). He goes on:

{{Blockquote|and when the circles of confusion are sufficiently small the eye fails to see them as such; they are then seen as points only, and the picture appears sharp. At the ordinary distance of vision, of from twelve to fifteen inches, circles of confusion are seen as points, if the angle subtended by them does not exceed one minute of arc, or roughly, if they do not exceed the 1/100 of an inch in diameter.}}

Numerically, 1/100&nbsp;inch at 12–15&nbsp;inches is closer to two minutes of arc. This choice of CoC limit remains (for a large print) the most widely used even today. {{harvtxt|Abney|1881|pp=[https://books.google.com/books?id=3QpNAAAAMAAJ&pg=PA208 207–08]}} takes a similar approach based on a visual acuity of one minute of arc, and chooses a circle of confusion of 0.025&nbsp;cm for viewing at 40–50&nbsp;cm, essentially making the same factor-of-two error in metric units. It is unclear whether Abney or Dallmeyer was earlier to set the CoC standard thereby.