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MISCELLANE< >US TABLES
393
AMATEUR CINE PROJECTION.
Projection Distances from Projection Lens to Screen.
ze.
Distance required with
Screen Si
1 inch Lens.
1^8 inch Lens.
2 inch Lens.
10" X
13"
3 ft.
4 ft.
6 ft.
Hi1/' X
22"
5 ft.
6H ft.
10 ft.
X
30"
6% ft.
9 ft.
13^ ft.
30" X
40*
9 ft.
12 ft.
18 ft.
39" X
52"
11'.. ft.
15% ft.
23 ft.
DIAPHRAGM NUMBERS.
Exposures at Different Apertures.
2 9
F. Numbers
1-8
2
2 2
2 5
2 7
3
3 16
3 4
Rel. Exposure
Required —
Fractions
2/3
3/4
1
IH
m
1%
2
2
Decimal
0 83
1
1 33
1 66
2
2 33
2-66
3
Seconds
1/1200
1/1000
1/750
1/600
1/500
1/428
1/375
1/325
3 9
F. Numbers
3 5
4
4-5
5
5 6
6
6 3
8
Rel. Exposure —
Fractions
2H
3
4
5
6
7
8
12
Decimal
333
4
5 3
666
8
93
10-6
16
Seconds
1/300
1/250
1/190
1/150
1/125
1/100
1/95
1/60
The above table gives the relative exposures with lens apertures.
The Fraction line gives a series of F/Nos. each requiring double the exposure of the preceding one. This series is F/2 -2, 3 16, 4.5, 6 3. The decimal line gives a similar series, beginning with f/2, viz., F/2, 2 7, 4, 5 6, 8. The last line gives the relative Speed of any lens, in comparison with another lens of different aperture.
Equivalent F/ and Uniform System Numbers
Rel. Exposure Req'd
F. Numbers +
U.S. Numbers
1 1
2
4
8
16
32
64
4
5 6
8
11 3
16
22 6
32
1
2
4
8
16
32
64
128
i;. 2
128
Among Continental opticians at the present time it is usual to adopt a different series of F/Nos. each requiring double the exposure of the preceding one. This series is: —
F/Number
3 16 4 5 6 3 9
12 5
18
25 3
36
Note. — Most lenses are now marked with the F/numbers. according to one or other of the above two series, although the U.S. numbers are used on Kodak lenses. Also the actual diameter of the diaphragm aperture in millimetres is marked on some Continental lenses.