Editing Hyperfocal distance (section)

===Abney 1881===
Sir William de Wivelesley Abney says:<ref>{{cite book |first=W. de W. |last=Abney |title=A Treatise on Photography |edition=First |location=London |publisher=Longmans, Green, and Co. |year=1881 }}</ref>

{{blockquote|1=The annexed formula will approximately give the nearest point {{mvar|p}} which will appear in focus when the distance is accurately focussed, supposing the admissible disc of confusion to be {{val|0.025|u=cm}}:
<math display="block">p = 0.41 \cdot f^2 \cdot a</math>
when
* {{math|1=''f'' =}} the focal length of the lens in cm
* {{math|1=''a'' =}} the ratio of the aperture to the focal length}}

That is, {{mvar|a}} is the reciprocal of what we now call the f-number, and the answer is evidently in meters.  His 0.41 should obviously be 0.40.  Based on his formulae, and on the notion that the ''aperture ratio'' should be kept fixed in comparisons across formats, Abney says:

{{blockquote|It can be shown that an enlargement from a small negative is better than a picture of the same size taken direct as regards sharpness of detail. ... Care must be taken to distinguish between the advantages to be gained in enlargement by the use of a smaller lens, with the disadvantages that ensue from the deterioration in the relative values of light and shade.}}