PROPORTIONAL LIGHT INTENSITIES.
It will be admitted that pictures exist only by contrast ; therefore, the truest im pression of any subject can be imparted only by a thorough study of its contrasts in tensities which have to be registered by the photographic plate ; and this is pro-' bably the most important principle con cerning exposure. For example, the plate to be capable of registering differ-' epees of the light's action varying from 1, to 32, and representing them by densities, or deposits of metallic silver varying from 1 to 8—which is as much as the average, correct proportion. When the contrasts are extreme—such, for example, as a view taken from the south coast pine forest look ing seaward—the brilliancy of the contrast between the sea view and the shaded trees can only be suggested by extreme contrasts in the photograph. Such subjects are not suitable for photographic representation, as, except in very skilled hands and with a lot of dodging, the negative is almost sure to possess such extreme contrast as to appear "hard." This emphasises the fact that it is the pro portional and not the actual light in _ print is capable of showing—and suppose': the subject to be such a one as described above. If a contrast of 1 to 22 is
to 1 to 8, then, roughly speaking, a con-. trast of 1 to 2 will be reduced to 1 to 111. And, further, if the varying intensities . are extended from 1 to 61, then the con trasts of 1 and 2 are reduced to 1 to 11, and so on, until the contrast is practically nil. So that, supposing the varying inten sities of the dark trees to be represented by numbers of value 1, 2, 2, and the contrasts of sea, sky, and cloud by 200, I 201, 202, then it will be seen that if suffi cient exposure is given to obtain the con trasts of 1, 2, and 3, the contrasts 200, 201, and 202 must be completely buried ; whilst, on the other hand, if IJO, 201, and 202 are exposed for the contrasts, 1, 2, 3 must be longer the exposure the greater the con trast ; but directly that limit is reached, a reversing action takes place, and les, photographic effect occurs where the light has acted most, the ultimate result bein omitted, as these will have failed to im press the plate at all.