THE SIMPLEST GUIDE TO EXPOSURE.
The simplest guide to exposure consists of tables calculated upon the table of the sun's altitudes given on p. 95, and for the sake of simplicity letters have been placed wits of considerable latitude in exposure. For example, if in Fig. 175 the plate were exposed only sufficiently to show the vary ing contrasts of the distant trees the figure would appear hard and black from under exposure. Therefore, in such cases the proper plan is to over-expose such parts and then development may be continued until the detail is out in the darker por tions. This has been put in the form of a rule which, although old, still holds good : " Expose for the shadows and let the lights take care of themselves." Strictly speaking, this should be : Expose for those parts in which detail is required. This is 7 to represent the varying intensities. A care ful study of Table I., p. 87, will show that the exposure for a distant view at mid-day varies between June and December in the proportion of about -1 to 1. The same pro portions apply to other subjects. At 9 a.m.
or 3 p.m., fifteen times as much exposure is required in December as is required in June, and eight times as much in Novem ber. This is in the case of distant views, such as sea or sky, but in the case of a near view with dark foreground at the same time of day-9 a.m. and 3 p.m.—the proportions are as 1 is to 20. These tables should he carefully studied, noting in this way the variations caused by the time of year, time of day, and nature of subject. Table B is calculated for fS, this being the largest aperture at which most lenses work ; but in the case of portrait lenses, which are occa is given, while with an " Ordinary " plate 1 four times the exposure will he required. 1 To make the tables of further benefit, a I list of proportionate speeds, showing the degrees by which the exposure indicated sionally used at apertures up to 2'5, the portionate exposure is ascertained as fore indicated by squaring the f numbers.