Alfred Watkins made pinhole lenses with single holes of different sizes (W.P. No. 6, 8, 10 or 12) and a model with all four size holes on a rotating plate. The W.P. number was based on the standard size of needles, it originated with Watkins and was modified by Dr. D'Arcy Power. To calculate the exposure the W.P. number is multiplied by the focus (pinhole to plate distance) to give an f number e.g. W.P. 6 at 8" focus = f48, the exposure is calculated (perhaps using a meter) based on the calculated f number, the result is them multiplied by 60. Pinhole photography was also a subject explored in the Watkins Manual.