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.