The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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JExposure
In connection with the above, Piatt's table of Subject and Light will be useful: -
Compiled and slightly altered from Eder's and Burton's Tables.
Sea and Sky ............
Panoramic View .........
Do. with Thick Foliage, or Strong Foreground, or Light Buildings ............
Dark Buildings .........
Heavy Foliage Foreground ...
Woods and badly lit River Banks ...............
Living Objects Outdoors
Portrait near Window ......
Interiors ...... upwards of
Copying same Size.........
Sun-
Diffused
Dull.
Very
shine.
Light.
Dull.
i
I
2
3
4
2
4
6
8
3
6
9
12
4
8
12
16
io
20
30
40
4
8
12
20
8
16
24
40
ioo
12
20
6
Gloom
10 20
50 30 60
Hurter and Driffield's actinograph (for the description of which see p. 306) gives, quite apart from its other functions, a ready means of finding equivalent exposures at all times and seasons. A separate instrument, however, or a separate cylinder, is re-quired for each considerable variation in latitude.
As bearing chiefly on the light factor, we may give the follow-ing useful tables for calculating the time of exposure due to A. de la Baume Pluvinel. For the time of exposure, /, of a photographic plate applies approximately the well-known and easily-proved formula -
n which the constant receives certain values, first empirically determined by Dorval. In this formula are now, however, as one perceives at first sight, several important factors totally dis-regarded, and it may be therefore desirable, on the ground of the laws of mathematical optics, and with the aid of certain necessary assumptions, to submit the problem to a deeper investigation, and to strive to bring its solution into better harmony with the physical facts.
In a meritorious brochure (" Le Temps de Pose," Paris, 1890, Gauthier-Villars), issued recently, the French investigator, De la Baume Pluvinel, made the attempt to determine a formula applicable to any possible case, according to which " the time of
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