Actinometer

A simple actinometer for measuring the chemical intensity of light at a particular time may be devised as follows: Soak a piece of bromide paper of any good make in a 10 per cent. solution of potassium nitrate. This operation and the subsequent drying must, of course, take place in yellow light. Take a manilla thick envelope and cut a small hole about 1/2 inch square in one face of it. Expose a slip of this specially prepared paper to reflected light for a period sufficient to give it a definite blue tint, then with water-colour or pencil surround the square aperture with a border exactly matching this tint. A constant fraction of the time which the sensitive paper takes in reaching the tint will be the exposure required.

To use this actinometer we must first get a standard exposure. Suppose that on a certain day we find that the paper takes two minutes to reach this tint, and that the correct exposure at f/16 is found to be one second. From these data we can write a memorandum as follows in the outside of the cartridge envelope.

Time of exposure per minute taken by the paper in reaching tint, given in seconds : f/15.6..... 1/15 sec.

f/8..... 1/8 „ f/11 ..... 1/4 " f/16..... 1/2 " f/22..... I " f/32..... 2 "

This is a very rough-and-ready form of actinometer, but quite sufficient for ordinary practice. There are many excellent instruments of more convenient and durable form to be had at low prices, and they contain a very large amount of information helpful to the student. Actinometers should not be exposed to direct sunlight, because detail in the shadows is the chief aim in most exposures; just within the shadow of the operator himself will suffice.

Exposure Tables

These must always be calculated for the latitude of the place, the variation at different times of the year not being constant. For instance, according to Spitaler's Tables the intensity of the light during the month of January at 400 and 500 latitude differs as 80 : 22; in April as 361 : 288; and in June only as 469 : 432. The observer should keep an exposure diary in which the time, nature of subject, and character of the light are carefully noted down for future guidance.

The following tables are only intended as a foundation for the reader to work upon. They represent the experience of some years, and are calculated for a plate registering about 125 on the Hurter and Driffield scale. The time given is the lowest at which detail in the shadow can be secured at f/8 when photographing a landscape the foreground of which is well lighted. (Latitude that of Southern England, or 51° 30'.)

Except in dull wintry weather, half these exposures will often suffice for a passable negative. Subjects containing excessive contrast should have a good exposure; those having few shadows and plenty of half-tones require considerably less. The nearer the object is to the lens, the longer the exposure necessary to give it prominence.

Authorities differ as to the effect of east wind and rain upon the time of exposure. Some contend that a drying wind increases the intensity of the light, because it tends to remove the aqueous vapour which otherwise would intercept a portion of the rays; others insist on a shorter exposure when the atmosphere is very moist. In practice we have found that with a moist atmosphere the foreground requires a much longer exposure : the background gains in illumination owing to the light reflected by the moist particles in the air. This applies to sea and lake pictures as well as to lucid intervals in wet weather.

Lighting

Brilliant sunlight does not always produce the most artistic picture; the gradations are softer and clearer in diffused light. It is rarely advisable to take a picture:

Jan., Feb., Nov., Dec.

March, Sept.

April, August.

May, June, July.

October.

A.

B.

c.

D.

A.

B.

C

D.

A.

B

c.

D.

A.

B.

C.

D.

A.

B.

C.

D.

7 to 8 .

-

-

-

-

-

-

-

-

1/25

1/15

1/10

1/5

1/35

1/25

1/15

1/10

-

-

-

-

8 to 10.

1/5

1/2

I

-

1/25

1/16

1/10

1/5

1/30

1/25

1/15

1/10

1/50

1/35

1/25

1/15

1/15

1/10

1/5

1/3

10 to 2 .

1/15

1/10

1/5

1/2

1/40

1/30

1/25

1/15

1/50

1/40

1/30

1/15

1/75

1/50

1/35

1/25

1/30

1/25

1/15

1/10

2 to 4 .

uncertain

1/25

1/15

1/10

1/5

1/30

125

1/15

1/10

1/50

1/35

1/25

1/15

1/15

1/10

1/5

1/2

4 to 5 .

-

-

-

-

-

-

-

1/15

1/10

1/5

1/2

1/50

1/10

1/10

1/5

-

-

-

-

Midday

1/25

1/10

1/5

1/2

__

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

A. - Bright sunshine.

B. - Light clouds on blue sky.

C - Diffused light, or light heat-mist.

D.-Dull.

For very dull weather and dark clouds double the exposures for dull.

The afternoon light in winter is of small actinic value, and exposures in the afternoon should be rather longer than those in the morning between 8 and 10.

Detailed Foreground ...... Double the above exposure.

Dark Foreground ...... Four times the above exposure.

Seascapes........ About one-fourth the above exposure.

Clouds........ About one-eighth ,, ,,

Glens, and Woods with no open foreground . About eight to ten times the above exposure against the sun, i.e. with the lens pointing towards the sun in such a way that it has to be shaded to prevent the fogging of the plate. If the light is directly behind the operator, the shadows will tend to be tame and flat. Lighting at an angle is the best for the average picture. A very good general idea of the lighting of a scene, and of how it will look when the colouring is lost on a monotone silver print, will be got by examining it through a square of blue glass.

Maxims For Practice

Expose for the shadows; never mind the high lights. When in doubt, over-expose.

Cod Drying Within The Arctic Circle, Norway.

Cod Drying Within The Arctic Circle, Norway.