The Following Article On Exposure Is Kindly Contributed By A. S. Platts, Esq;

To the beginner in Photography the question presents itself, "how long shall the cap remain off the lens during an exposure;" and, "what is the duration of the mystic period, known as 'correct exposure'?" So many varying elements enter into its composition, that it expands and contracts, grows and diminishes, and seems ever to elude the grasp of the panting neophyte. Correct exposure is the Will-o'-the-wisp of Photography. The many hued tints of changing nature, the bright-eyed sky, the sombre woodland, the stretching landscape, the solemn gloom of the cathedral aisle, the glamour of noon-day, the dying twilight, the sweet touch of spring, the golden richness of autumn, the cold shiver of winter, the tiny circlet that shields the glistening lens, the nervous sensitiveness of the quivering plate, all minister at the shrine of "correct exposure;" and all these the novice must conquer if he would ensnare the wayward sprite.

That the factors of subject and lighting, time of day and year, aperture and plate, may be considered each in due course, and not left to haphazard conjecture or doubtful inspiration, I have laid down rules for my own guidance in the following tables, which the beginner may do well to follow: -

Before making an exposure, I find out by a glance at Table I. what is nearest the subject in hand. 1 write down the figure I decide upon, and multiply it by a figure from Table II.; I next multiply the product by a figure from Table III. which agrees with the stop I am using, and the product I divide by a figure from Table IV. agreeing with the plate I am using. The answer is in seconds, and is the length of exposure the subject requires.

Table I. - Subject And Light

Compiled and slightly altered from Eder's and Burton's Tables.

Sunshine.

Diffused Light.

Dull.

Very Dull.

Gloomy

Sea and Sky...........

¼

Panoramic View..............

1

2

3

4

5

Do. with Thick Foliage, or strong foreground, or light buildings...................

2

4

6

8

10

Dark Buildings............

3

6

9

12

15

Heavy Foliage Foreground ...

4

8

12

16

20

Woods and badly lit River

Banks......

10

20

40

50

Living Objects outdoors........

4

8

12

20

30

Portrait near window.........

8

16

24

40

60

Interiors..Upwards of

100

Copying same size..................

6

12

20

Table II. - Time (Dr. J. A. Scott)

HOUR OF DAY

JUNE.

MAY.

april,

MARCH.

FEB.

JAN.

DEC.

a.m.

p.m.

JULY.

AUG.

SEP.

OCT.

NOV.

12

1

1

1

2

4

II

I

I

I

4

5

10

2

I

I

3

5

6

9

3

I

2

4

12

16

8

4

2

3

10

7

5

2

3

6

6

6

3

6

Yellow Sunset affects these figures.

5

7

5

6

4

8

12

TABLE III. - Lens and Stops.

U. S. Stops.

Intensity Ratio Stops.

Exposure.

F 8

2/3

6°25

F 10 Unit

I

F 11°31

F 12

12°25

F 14

2

16°

F 16

20°25

F 18

25°

F 20

4

32°

F 22° 62

5

36°

F 24

42°25

F 26

49°

F28

56°25

F 30

9

64°

F 32

10¼

81°

F 36

13

100°

F 40

16

128°

F 46° 25

20½

144°

F 48

23

182°25

F 54

29

225°

F 60

36

256°

F 64

41

306°25

F 70

49

400°

F 80

64

512°

F 90° 50

82

576°

F 96

92

Table IV. - Plates

Unit. - Very Slow Plate, Panoramic View in Sunshine, June Noon, F 10 Stop,

I Second Exposure.

Sensitometer Numbers.

Divide by

10

2

to

4

11

3

"

5

12

3

"

5

13

4

"

6

14

4

"

8

15

5

"

10

16

6

"

13

17

8

"

16

18

10

"

20

19

12

"

25

20

15

"

28

21

20

"

35

22

25

"

40

23

30

"

45

24

35

"

50

25

40

"

60

Unknown

10

"

30

Notes To Tables

Table I.

This table must be used intelligently. A panoramic view I take to be a stretch of country with nothing particularly prominent in it. If masses of thick foliage are present (not in foreground) I double the exposure, but this must be done with judgment, varying this and every other item as I think the subject demands. No rule of thumb adherence must be given to this table. Thus I photograph my friends in diffused light in open air, but in such a secluded built up spot, that I always set down 16 or double the table to commence with.

In copying it must be remembered that if, as invariably occurs, the focus is lengthened, longer exposure must be given. Thus I copy a print same size in diffused light with my W.A. lense of 6 inch focus.

To focus correctly, I must lengthen to 12 inches, which means 4 times extra exposure. Four times 12 (copying in diffused light) are 48, that is 48 times exposure of a new in sunshine. When focus is lengthened, ascertain the relative exposures by squaring the two numbers, and divide the greater by the lesser. Thus as above, 6x6 = 36, 12X 12 = 144. Divide latter by former; answer 4, that is, the 12 inch focus requires 4 times exposure of the 6 inches. By the same rule a portrait near window (about 1½ feet distance, camera outside window) requires longer exposure for every foot or distance from window, thus a given light at 1 foot distance will be 4 times weaker at 2 feet, 9 at 3 feet, 16 at 4 ft., etc. As however so much depends on size of window, and whether it has open view of the sky or not, together with distance from it, that I have refrained from giving a figure for "portrait in ordinary room."

Table II

This requires no comment, beyond giving all the credit for its compilation to Dr. J. A. Scott, of Dublin.

Table III

If the beginner knows the focus of his lens and the numbers of his stops, the table is ready to his hand. If not, however, it is imperative that he shall find them out. The length of focus is ascertained by measuring the distance betwixt the focussing screen, and the object glass of a single lens, or the diaphragms (stops) of a doublet lens. Focus sharply on some distant object, and measure accordingly. Next measure accurately the diameter of each stop aperture. Divide the length of focus by this diameter, and if the answer is - say 28, the stop is called F28. Thus 10 inch focus with 1 inch diameter of stop would be F 10. If the student desires to use the Uniform System Numbers (column 1), he must ignore column 3, and multiply by the figures in column 1. It is necessary however at the same time to use figures 6 times greater in Table IV. The Uniform System unit is F4.

Table IV

If the sensitometer number of plate is known, divide by a medium figure between the two given in column 2, and alter until the figure best suitable for the developer in use, and the exposure most desired (full or severely correct) is arrived at. In using an unknown plate the same plan must be adopted. Let it be understood that lower figures mean longer exposure, and vice versa. The plates I use myself for most work are of the cheapest, registering 18 sensitometer, and I divide according to subject with 1 5 to 20.

Example Of Exposure

Suppose a village scene in diffused light at 3 p.m. in April, F30 stop, plate sensitometer 18. Table I. light buildings, etc, 4 multiplied by 1½ (Table II.) = 6, multiplied by 9 (Table III.) = 54, divided by 15 (Table IV.) equals 3 2/3 seconds.

Diffused light, means bright sky without sun, or where no sun shines on subject. Dull - sky partially overcast. Very dull - much overcast. Half-points between any of these two may be used.