The following table gives the conditions under which a reasonable degree of sharpness can be ensured without focusing.

To those who are using detective cameras, or taking "snap" shots, the table will be of use, showing the number of feet beyond which everything is in focus when the equivalent focus of the lens and the relative diameter of the stop are known.

 Equiv. Stop. Stop Focus equals f/5 f/10 f/15 f/20 f/25 f/30 f/40 f/50 f/60 2 in. 7 3 1/2 2 1/4 2 1 1/2 1 1/4 1 7/8 3/4 2 1/2 10 1/2 5 1/2 3 1/2 3 2 1/2 2 1 1/2 1 1/4 1 3 15 8 5 4 3 1/2 3 2 1 3/4 1 1/2 4 27 14 9 7 5 1/2 5 3 3/4 3 2 1/2 5 46 21 14 11 9 7 1/2 6 4 3/4 4 6 60 30 20 15} 12 1/2 10 1/2 8 6 1/2 5 1/2 7 82 42 27 21 17 14 10| 9 7 1/2 8 107 54 36 27 22 19 14 11 10 9 137 68 4* 34 28 23 18 14 12 10 167 84 56 42 34 30 21 18 15 11 202 101 67 51 41 37 26 21 18 12 241 121 80 61 49 41 31 25 21 13 283 142 94 71 57 48 37 30 25 14 328 164 109 83 66 56 42 34 29 15 376 189 125 95 76 64 48 39 33

As no two makers number their stops alike, and some do not number them at all, it is desirable to designate their stops in terms of the focal length of the lens As an example of how misleading is the information obtained by giving the number of the stop, we would refer to the Dallmeyer lenses: - Stop No. 3 in lens 1A is equal to f/32; stop No. 3 in lens 3 D is equal to f/128. Here are stops of the same number, in lenses by the same maker, and yet one requires eight times the exposure of the other. The small amount of trouble caused by designating stops in terms of will be amply repaid by the aid it will give in obtaining correct exposures.