It is quite impossible to write down anything definite with regard to the length of time for which the lens must be uncovered. If we had only one particular subject to photograph, and this subject were always lighted by the same amount of daylight, it would be an easy matter to calculate the amount of exposure required with every diaphragm of a lens, provided that the necessary amount had been ascertained by experiment with but one of those diaphragms. For these stops or diaphragms, as furnished with modern lenses, have aperture3 bearing a definite relation to one another. As a general rule each diaphragm will require double the exposure needed for the next size larger. Or to put it in another way - suppose the smallest stop of a lens to require an exposure of 24 seconds with a given subject, with the next size larger the exposure will be 12, then 6, then 3, then 1½, until we come to the full aperture of the lens, by which the picture can be taken in ¾ of a second. But as a matter of fact, the exposure varies not only with the nature of the subject, but with the time of day, the time of year, and the state of the atmosphere. The old adage, "Experientia docet" cannot be more aptly quoted than in connection with this question of exposure.
As a rough guide to the worker a table is appended by which he may learn the very great difference there is in the exposure required for different subjects. Let us suppose that he is working with a gelatine plate or average sensitiveness, and is using a medium sized stop. For the reasons given above, such a table can only be regarded as a hint to workers - not as an infallible guide. It must be read in conjunction with what has been already said with regard to time of day, state or atmosphere, etc.
The medium stop of a rectilinear lens will require the varying exposures noted below : -
Sky and Sea.
I to 2 secs.
Landscape with trees close at hand.
3 secs, up to 3 or more minutes.
Under trees, in woods and forests.
Interior of rooms, well lighted.
½-an-hour to 2 hours.
Interiors badly lighted, or artificially lighted.
In making out this short table, the writer - who has worked for years with one description of lens, and one description of dry plate - has relied solely upon observations noted in his own practice. In the next Chapter we show how the exposed plates can be developed.