Editing Circle of confusion (section)

===T.H. 1866===
Circle-of-confusion calculations: An early precursor to [[depth of field]] calculations is the {{harvtxt|TH|1866|p=138}} calculation of a circle-of-confusion diameter from a subject distance, for a lens focused at infinity; this article was pointed out by {{harvtxt|von Rohr|1899}}. The formula he comes up with for what he terms "the indistinctness" is equivalent, in modern terms, to

<math display=block>c = {f A \over S}</math>

for focal length {{mvar|f}}, aperture diameter {{mvar|A}}, and subject distance {{mvar|S}}. But he does not invert this to find the {{mvar|S}} corresponding to a given {{mvar|c}} criterion (i.e. he does not solve for the [[hyperfocal distance]]), nor does he consider focusing at any other distance than infinity.

He finally observes "long-focus lenses have usually a larger aperture than short ones, and ''on this account'' have less depth of focus" [his italic emphasis].