Table showing distance of object upon which to focus with any stop in order to secure the greatest depth (hyper focal distances).

To Find the Hyper Focal Distance the formula is : - Focal length squared, - ratio of stop X circle of confusion. Now when making these tables it is generally assumed that a point represented as a circle of confusion not exceeding 1/100th inch in diameter will be considered satisfactory as regards definition in the negative. Then by the above formula suppose we have a lens of 5-inch focus with stop f/8 the hyper focal distance will be

Lens Facts 79

If we, therefore, withsuch a lens and stop focus on 26 feet we shall have objects from half that distance (13 feet) to Infinity in good focus.

Mr. Welborne Piper, in his useful book on " The Lens," has pointed out the interesting fact that if we divide the hyper focal distance by 1,2, 3, 4, 5, etc., we shall get a set of distances such that, if we focus on any one of them, the next two distances on the scale are the distances of the furthest and nearest objects in focus. Say the hyper focal distance is 30 feet, then 30/1, 30/2, 30/3, 30/4, etc., will represent 30, 15, 10 and 7 1/2 feet, and if we focus, say, on 10 feet then the 15 and 7 1/2 ft. will be the near and far depths, or on 30 ft. then on Infinity and 15 ft.

To Calculate Near Depth when Focussing on near objects.

Formula. - Distance of object in focus X Hyper focal distance. Distance of object in focus + Hyper focal distance.

Example. Hyper focal distance 26 feet object at 8 feet,

Lens Facts 80

For far depth the formula is

Distance of object in focus X hyper focal distance. Distance of object in focus - hyper focal distance.

If the sharp focus is secured at Infinity, then, with the stop indicated, all objects are in focus from the above distances up to Infinity. If, however, the pointer is set to the above distances then, with the stop indicated, all objects are in focus from half the distance focussed up to Infinity. These figures are based on a circle of confusion of 1/100th of an inch, and the distances must be reduced if greater sharpness is desired.

Focal length

Lens in inches.

Apertures marked on Stops.

f/4

f/5•6

f/6

f/7

f/8

f/10

f/11

f/15

f/16

f/20

f/22

f/32

f/44

f/64

BEST POINT ON WHICH TO FOCUS IN FEET.

4

33

24

22

19

17

13

12

9

8

7

6

4

3

2

4 1/4

38

27

25

21

19

15

14

10

10

7

7

5

34

2 1/2

4 1/2

42

30

28

24

21

17

15

11

11

8 1/2

7 1/2

54

4

3

4 3/4

47

34

31

27

24

19

17

12

12

9 1/2

8 1/2

6

5

3

5

52

36

35

30

26

21

19

14

13

10 1/2

94

6 1/2

54

34

5 1/4

57

40

38

33

28

23

21

15

14

11 1/2

10 1/2

7

54

34

5 1/2

63

45

43

36

31

25

23

17

15

12 1/2

11 1/2

74

6

4

5 3/4

68

50

46

38

34

27

25

18

17

13 1/2

13

8 1/2

64

4

6

75

54

50

42

38

30

28

20

19

15

14

9

7

44

6 1/4

81

58

54

46

40

32

29

22

20

16

15

10

74

5

6 1/2

87

62

58

50

44

35

32

23

22

17 1/2

16

11

8

54

6 3/4

94

67

63

54

47

38

34

25

24

19

17

12

84

6

7

101

72

68

58

51

40

37

27

25

20

18

12 1/2

9

6

7 1/4

109

78

73

62

54

44

39

29

27

22

20

13 1/2

10

64

74

117

83

78

64

58

47

42

31

29

24

21

14 1/2

10 1/2

7

7i

124

90

83

71

62

50

45

33

31

25

22

15 1/2

11

74

8

132

96

88

76

68

52

48

36

32

28

24

16

12

8

8 1/4

141

102

94

80

71

56

51

37

35

29

25

17 1/2

12 1/2

84

8 1/2

151

107

100

84

76

60

54

40

38

30

27

19

13 1/2

9

8 3/4

156

114

104

89

78

63

57

42

39

32

29

20

14

10

9

168

120

112

96

84

67

61

45

42

34

31

21

15

10 1/2

9 1/4

180

127

116

101

90

71

65

47

45

35

32

22

16

11

9 1/2

190

133

125

107

95

75

68

50

47

37

34

24

17

12

9 3/4

197

141

131

113

99

79

72

52

50

39

36

25

18

12 1/2

10

208

148

140

120

104

83

75

55

52

42

38

26

19

13