Editing Hyperfocal distance (section)

===Johnson 1909===
George Lindsay Johnson uses the term ''Depth of Field'' for what Abney called ''Depth of Focus,'' and ''Depth of Focus'' in the modern sense (possibly for the first time),<ref>{{cite book |first=George Lindsay |last=Johnson |title=Photographic Optics and Colour Photography |url=https://archive.org/details/photographicopt00johngoog |location=London |publisher=Ward & Co |year=1909 }}</ref> as the allowable distance error in the focal plane.  His definitions include hyperfocal distance:

{{blockquote|1=Depth of Focus is a convenient, but not strictly accurate term, used to describe the amount of racking movement (forwards or backwards) which can be given to the screen without the image becoming sensibly blurred, i.e. without any blurring in the image exceeding 1/100&nbsp;in., or in the case of negatives to be enlarged or scientific work, the 1/10 or 1/100&nbsp;mm.  Then the breadth of a point of light, which, of course, causes blurring on both sides, i.e. {{nowrap|1/50&nbsp;in = 2{{mvar|e}}}} (or {{nowrap|1/100&nbsp;in = {{mvar|e}}}}).}}

His drawing makes it clear that his {{mvar|e}} is the radius of the circle of confusion.  He has clearly anticipated the need to tie it to format size or enlargement, but has not given a general scheme for choosing it.

{{blockquote|Depth of Field is precisely the same as depth of focus, only in the former case the depth is measured by the movement of the plate, the object being fixed, while in the latter case the depth is measured by the distance through which the object can be moved without the circle of confusion exceeding 2{{mvar|e}}.

Thus if a lens which is focused for infinity still gives a sharp image for an object at 6&nbsp;yards, its depth of field is from infinity to 6&nbsp;yards, every object beyond 6&nbsp;yards being in focus.

This distance (6&nbsp;yards) is termed the ''hyperfocal distance'' of the lens, and any allowable confusion disc depends on the focal length of the lens and on the stop used.

If the limit of confusion of half of the disc (i.e. {{mvar|e}}) be taken as 1/100&nbsp;in., then the hyperfocal distance

<math display="block">H = \frac{F d}{e}\,,</math>

{{mvar|d}} being the diameter of the stop,&nbsp;...}}

Johnson's use of ''former'' and ''latter'' seem to be swapped; perhaps ''former'' was here meant to refer to the immediately preceding section title ''Depth of Focus'', and ''latter'' to the current section title ''Depth of Field''.  Except for an obvious factor-of-2 error in using the ratio of stop diameter to CoC radius, this definition is the same as Abney's hyperfocal distance.