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.