THE LATITUDE OF EMULSIONS The latitude of an emulsion is its capacity to render in perfect gradation different degrees of actinicity in the same subject. An emulsion is said to have great latitude when it will register greatly differing intensities and to be "hard" on contrasty when it will register comparatively little actinic contrast or when a little actinic contrast in a subject suffices to fill, or fully utilize, its gradation capacity.
The latitude of an emulsion is indicated theoretically by the relative length of the straight part of the "characteristic curve" of the emul sion as described in Chapter VIII (p. 105). By trial the fractional part of an emulsion's correct speed-time exposure may be found which will suffice to create a just clearly visible deposit in the emulsion after normal development. Such an exposure may be known as a one inertia exposure since by normal development it only suffices to overcome once, so to speak, the inertia of the emulsion. The number of times that the normal or speed-time exposure is greater than the one inertia exposure, may be used as a factor to express the latitude of any emulsion. For example should the normal speed-time of a certain plate be 2 minutes or 128 seconds, and an exposure of 8 seconds under the equivalent of unit conditions, suffice to show a least discernible deposit after normal develop ment (one of 4 seconds revealing no deposit) then 8 seconds may be known as the one inertia exposure for that emulsion and the latitude of the emulsion would be 16 since 128 seconds divided by 8 equals 16. Expressed in other words, that emulsion is found to endure nor mally a 16 inertia exposure since the normal or speed-time is 16 times the one inertia exposure. Such an emulsion would be contrasty, as the modern fast emulsions will endure a 32 or 64 inertia exposure (p. 118).
Exposure may be comprehended as the act of overcoming the inertia of an emulsion a greater or less number of times, an over-exposure doing so too many and an under-exposure too few times or not at all, while a normal exposure overcomes the inertia the correct number of times thus producing the desired effect in the negative with normal development. The appli cation of this idea of contrast in emulsions is obvious. A plate having say a contrast of 1 to 32 by this method would render in intermediate tones in the picture all surface actinicities in the subject from just under the maximum (which is treated as the objective point in the unit method) down to L of the value of that maximum.
On developing snap shots made by the novice in the shade or in any too dull light it fre quently happens that the brightest parts of the subjects have impressed themselves on the emulsion only just enough to develop a visible deposit. In many such cases those planes have received approximately but a one inertia ex posure and as an average emulsion was used, i.e., that of roll film of which none are made of
a very "hard" quality, it would have required from 32 to 64 times the exposure actually given to have prepared the latent image for develop ment into a normal negative.
The latitude 32 may be considered as the aver age and is that of the usual modern fast plate. Films on the whole have greater endurance probably by reason of the entire absence of glass in their construction, which makes them naturally of a "non halation" quality. The latitudes 16 and 8 are "hard" or contrasty and of 4, is extremely contrasty. It is doubtful however if the latter degree of contrast exists in any emulsion.
The average and soft emulsions are to be used for the average sunlight view and for portraiture and all subjects having ordinary contrasts. The hard emulsions are used more especially for photo-mechanical or process work in which it is required to get extreme contrasts in copying flat prints or to obtain effects in black and white as in the production of white lines on a dark ground or the reverse. However when the illogical desire for speed has run its course among photographic workers and emulsions are selected purely by reason of their quality or latitude, which is simply their adaptability for subjects of certain known contrasts, then these hard emulsions will be used much more for all those subjects which have but little contrast such as bird's eye views and copies in general.
On these hard emulsions a little more than the correct exposure seems, on development, quickly to clog the film and spoil the gradation steps completely, they having but little endur ance. These emulsions seem usually to be quite thinly coated and to develop much more rapidly than emulsions of more latitude or softness and care should be taken therefore not to over-develop them. The normal devel oper should be diluted by nearly an equal quantity of water or the time of development be reduced to nearly half. Since the latitude of these emulsions is so limited, it is evident that more than the usual care should be exer cised in order to arrive at the correct exposure.
As has already been explained, it should be known of any emulsion that its normal exposure is one which overcomes the inertia of that emul sion, 8, 16, 32 or 64 times as the particular emulsion may require. It will thus be known how much under-exposure may be given with any emulsion when it is desired in that manner to decrease the detail in the weak or shaded parts of a well contrasted subject or to increase the tone contrast when photographing a sub ject of minimum or under-normal actinic con trast as is explained in the following chapter.