F/8, F/11.3, F/16, F/22.6, F/32, &c.  Since the squares of those numbers, 31.4, 64, 127.7, 256, 510.7, 1024 are approximately each twice the preceding number of the series, the apertures, F(ocal length), divided by the successive numbers as denominators, are each half the area of the preceding and require twice the exposure, F/16 requires twice the ’exposure of F/11.3, and four times that of F/8, and so throughout the scale.

Stops are used to regulate either ‘depth of focus’ or length of exposure.  The ‘depth of focus’ means the distance before and behind the point in theoretically accurate focus, at which objects are sufficiently focussed, for the purpose the photographer has in view.  This length is greatest when only the central portion of the lens is in use.  It is greatest with a pinhole, and least with a full aperture.  Hence a small stop is required if the picture is to include near and far objects, while a large aperture may be used if all the subject is far enough away to be in clear focus—­say more than 25 feet—­or if it is a flat surface.  The small stop is also required when the rising front or the swing back is in use.  The power of regulating the time of exposure is convenient for shortening long exposures in dark interiors, or for lengthening inconveniently short exposures in a bright light.

In practice it will be best to become familiar with the use of about three stops, say the full aperture (perhaps F/5.6 or F/8), F/16, and F/32.

For judging long exposures, the use of an actinometer (issued in many inexpensive forms) is helpful.

A telephoto attachment increases the photographer’s power of rendering distant details on a large scale.  The results are greatly superior to enlargements of a small plate.  It is, however, useless in a wind, unless the camera is specially supported, and is otherwise rather tricky to use.  The traveller is strongly advised to master its management at home.  It should be adjusted by the maker to the camera for which it is intended.

Unless a photographer’s dark room can be had the developing of the bulk is best left until the return home, but tests should be made to see that the exposures are correct.  A piece of ruby fabric or ruby paper tied over an electric light will give a safe light after dark, and ‘Scalol’ or some such one-solution developer which requires merely the addition of water, will give all that is needed for developing.  For fixing use 4 oz. hypo to a pint of water.

In warm climates, use cold water.  If it is not cool enough, the gelatine of the negatives may give trouble.  In that case, get colder water, and use an alum bath.  If water is precious, plates can be sufficiently washed by moving them forward in succession, through half-dozen soup plates filled with water.

If habitual use is not made of tabloid developers, &c., it is advisable to have some in reserve, for use in the case of broken bottles and spilt solutions.