On the exposure the whole of our after success depends, and consequently too much consideration cannot be given to this portion of our subject. Fortunately for the photographer the photographic emulsion has a very considerable measure of latitude and allows to some extent for defective judgment, so that the problem is not so insuperable as might at first sight appear. Indeed, with exposure meters and exposure tables the photographer has many excellent guides which will keep him within the limits of permissible error.

The time of exposure depends on a great number of conditions, the most important of which are : - The rapidity of the plate. The time of the year.

The aperture of the lens. The nature of the subject.

The latitude. The altitude.

The meteorological conditions.

The earth being surrounded with an envelope of atmosphere which absorbs those rays which specially affect our photographic plate it will be seen that the nearer the sun is to the horizon the more this envelope intercepts the actinic rays, and consequently exposure is prolonged. This accounts for the difference in exposure needed in summer and winter, or between morning or evening and noon. In the same way in high altitudes there is less air intervening between us and the sun and photographic exposures in such altitudes are greatly reduced.

Moisture and dust also act as light filters and both modify our photographic exposures. To show how the chemical activity of the light varies in different places at the same time Mr. Chapman Jones in " The Science and Practice of Photography," gives the following table showing its proportional value in Iceland, Manchester and Cairo on the 25th March for each hour during the day.

A.M.

P.M.

Iceland.

Manchester.

Cairo.

6

6

...... 0

...... 0

.........0

7

5

...... .02

...... .22

...... 1.74

8

4

...... 1.53

...... 5.85

...... 20.13

9

3

...... 6.62

...... 18.71

...... 50.01

10

2

...... 13.27

...... 32.91

...... 78.61

11

1

...... 18.60

...... 43.34

...... 98.33

Noon.

...... 20.60

...... 47.15

...... 105.30

In order to gauge the actinic value of the light such exposure meters as Watkins' or Wynne's are exceedingly useful. Both consist of slide rules in the form of a watch and contain a special sensitive paper which darkens on exposure to light. By setting the stop used on the scale against a number supplied by the meter maker to represent the make of plate used, then noting the time for the paper to darken, the exposure for all ordinary subjects may be read off against this time where engraved on the meter. This is the Watkins' method and the Wynne is practically the same in practice. For special subjects the user may multiply or divide the meter results according to a table supplied. Even if the photographer purchases one of these excellent actinometers we should recommend the further acquisition of Wellcome's Photographic Exposure Record and Diary. This is published each year and is crammed full of most useful information. It is arranged with tables for different latitudes, and there are editions for both Northern and Southern hemispheres

The following table will, however, be useful for reference and might with advantage be copied out and pasted in the lid of camera case.

Outspanned For The Heat Of The Day

Outspanned For The Heat Of The Day. By Major S. Evans.

Taken with the Sinclair " Una."

Approximate exposures with ultra rapid plates on all ordinary subjects, such as street scenes, in towns, etc., in fractions of a second. 520 North latitude. Aperture of Lens, f/8.

Brilliant sunshine.

Sun through light clouds.

Diffused light.

Dull.

Very dull.

a.m. p.m.

November, December or January............

. . 11 to 1

1/25

l/l6

l/l2

1/8

1/6

" "

. . 10 or 2

1/I6

l/l2

1/8

1/6

1/4

February and October

. . 11 to 1

l/35

l/25

1/14

1/12

1/8

" "

..10 or 2

1/25

l/l6

1/12

1/8

1/6

" "

. . 9 or 3

1/I6

l/l2

1/8

1/6

1/4

March and September

.. 10 to 2

1/SO

l/35

1/25

1/16

1/12

" "

. . 9 or 3

1/35

l/25

1/14

1/12

1/8

" "

. . 8 or 4

1/25

l/l6

1/12

1/8

1/6

April and August

. . 9 to 3

1/SO

l/35

1/25

1/16

1/12

" "

. . 8 or 4

l/35

l/25

1/14

1/12

1/8

" "

. . 7 or 5

1/25

l/l6

1/12

1/8

1/6

May, June and July

.. 9 to 3

1/75

l/50

1/35

1/25

1/16

" "

. . 8 or 4

1/50

l/35

1/25

1/16

1/12

" "

. . 7 or 5

l/35

l/25

1/14

1/12

1/8

" "

. . 6 or 6

1/25

l/l6

1/12

1/8

1/6

Most special rapid plates would require double these exposures and ordinary brands three or four times these exposures.

The exposures must also be multiplied or divided for all such special subjects as follows : -

Open scenes in villages, such as village greens and where there are no very near objects, cricket matches, etc. ..

1/2 the indicated exposure.

Distant landscape, beach, river and

Shore scenes.......

1/4 " "

Open seascapes

1/8 " "

Clouds

1

Subjects with heavy foregrounds, architecture, etc....

double ,, ,,

Portraits and very dark near subjects

4 times the indicated exposure.

Tree-covered lanes, etc..

.8 to 16

Indoor portraits ................

32 times

The exposures given are for use with f/8 stop. Other apertures on the lens would require as follows :-

f/4.5

f/5.6

f/11

f/16

f/22

f/32

1/3

1/2

2

4

8

16

times the indicated exposure. Consequently it will be seen that the great value of a lens with large aperture is for photography when the light is bad. With a f/4.5 lens for example, snap-shots might be made of street scenes in December in about 1/48th second - an important matter to the press photographer.

In Southern Latitudes the table would, of course, be reversed, i.e., June exposures would be altered to December and December exposures to June, and so on with the other months.

Alpine climbers quite recognise the importance of this factor. As we ascend the actinic value of the light increases and at 5000 feet exposures may be halved, and at 7500 only one-third of the exposures indicated should be given. The increase in ultra-violet rays also makes itself manifest by causing sunburn.