By C. Welborne Piper, from " The First Book of the Lens."

Focal Length. Inches.

Distance of Focussing Screen behind Principal

Focus.

1/100 in.

1/250 in.

1/500 in.

1/1000 in.

I

3 yds.

7 1/2 yds.

15 yds.

30 yds.

2

11

28 „

55 "

110 ,,

3

25 „

63

125 „

250 „

4

45 „

113

225 ,,

450 "

5

70 „

175 "

350 „

700 „

6

100

250 "

500

1000 ,,

7

136 „

340

680 „

1360 „

8

178 „

1/4 mile

1/2 mile

1 mile

9 3/4

264

660 yds.

3/4 „

1 1/2 miles

11 1/4

351 „ .

1/2 mile

1 ,,

2 „

12 3/4

434 „

1085 yds.

1 1/4 miles

2 1/2 „

13 3/4

525 „

3/4 mile

1 1/2 "

3 "

16

700 „

1

2 ,,

4

17 3/4

875 "

1 1/4 miles

2 1/2 "

5 „

19 1/2

1056

1 1/2 „

3 "

6 „

21

1225

1 3/4 „

3 1/2 „

7

22 1/2

1406

2

4 "

8 „

24

1600 ,,

2 1/2 „

4 1/2 „

9 ,,

25

1 mile

2 1/2 »

5 "

10

28

1 1/4 miles

3 1/4 "

6 1/2 „

13 „

30

1 1/2 "

3 3/4 „

7 1/2 „

15 "

33

1 3/4 "

4 1/2 "

9 „

18 "

35

2

5 „

10 ,,

20 „

By focussing accurately on distances not less than those given, we ensure that the focussing screen is within 1/100, 1/250, 1/500, or 1/1000 in. from the true principal focus.