This section is from the book "The Sinclair Handbook Of Photography", by James A. Sinclair. Also available from Amazon: The Sinclair handbook of photography.

WATKINS' TABLE OF PINHOLE DISTANCES.

Wat kins' Power Number. | Inch. | Nearest Needle Number. | Most suitable distance. |

3 | •053 | No. 1 | 40 inches. |

4 ----- | •040 ... | 4 ...... | 20 |

5 | •032 ----- | 5 ...... | 15 |

6 ----- | •027 .... | 7 ...... | 10 ,, |

7 ----- | •023 ----- | 8 ...... | 8 |

8 ----- | •020 .... | 10 ...... | 5 |

10 .... | •016 .... | 12 ...... | 3 1/2 " |

12 .. | •013 ----- | 13 ...... | 2 1/2 „ |

TO CALCULATE EXPOSURE. Multiply the W.P. No. by the distance from pinhole to plate (focus), and use the result as an f value to calculate the exposure in the usual way. Then, whatever the exposure indicated in seconds (or fraction of a second) give the same number (or fraction) of minutes. Thus, W.P. 6 pinhole at 5 inches would be f/30 and if used at 10 inches would be f/60. A distance less than the length of the plate will give a wide-angle picture, a distance twice the length of plate or more, a narrow-angle picture. - The Watkins' Manual.

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