Editing Hyperfocal distance (section)

===Derr 1906===
Louis Derr may be the first to clearly specify the first definition,<ref>{{cite book |first=Louis |last=Derr |title=Photography for students of physics and chemistry |url=https://archive.org/details/cu31924031218674 |location=London |publisher=Macmillan |year=1906 }}</ref> which is considered to be the strictly correct one in modern times, and to derive the formula corresponding to it.  Using {{mvar|p}} for hyperfocal distance, {{mvar|D}} for aperture diameter, {{mvar|d}} for the diameter that a circle of confusion shall not exceed, and {{mvar|f}} for focal length, he derives:<ref>{{Cite book |last=Derr |first=Louis |url=https://books.google.com/books?id=AN6d4zTjquwC&pg=PA78 |title=Photography for students of physics and chemistry |date=1906 |publisher=Macmillan Company; London, Macmillan & Company, Limited |language=en}}</ref>

<math display="block>p = \frac{(D + d) f}{d}\,.</math>

As the aperture diameter, {{mvar|D}} is the ratio of the focal length {{mvar|f}} to the numerical aperture {{mvar|N}} ({{math|1=''D'' = ''f''/''N''}}); and the diameter of the circle of confusion, {{math|1=''c'' = ''d''}}, this gives the equation for the first definition above.

<math display="block">p = \frac{\left(\tfrac{f}{N} + c\right) f}{c} = \frac{f^2}{N c} + f</math>