And now for a few words as to the chief difficulty in telephotography, namely, the exposure. I am far from saying that this is not a real difficulty, and I do not mind confessing that I sometimes go rather badly astray myself, but in such cases I generally find that through pre-occupation I have missed some condition which should have been fairly obvious. The accepted rule is to multiply the exposure which would be required if the positive or ordinary lens were used alone by the square of the magnifications, and I believe that, in the case of a near object which does not alter in character by being telephotographed, this rule is absolutely correct. This last sentence sounds a little cryptic, perhaps, but is easily explained. You have, let us suppose, an open landscape with a light foreground and a building in the distance. This is a subject which with an ordinary lens in a decent light would require a very short exposure indeed. But, if you telephotograph the building at, say, 10 magnifications, it may fill the plate and so become, from the exposure-meter point of view, quite another sort of subject, and you must make allowances accordingly. Then again, you have to consider the distance, especially if it is the least bit hazy. In such cases extreme allowance has to be made, and you will often find that a quarter of the exposure as ordinarily calculated will be sufficient. You will comprehend the reason for this when you reflect that in taking a similar view with an ordinary lens you would probably over-expose the distance at least four times for the sake of the foreground.
As a rule it is better not to use the shutter release for telephoto exposures, as even the slightest vibration tells. A deep cap is usually preferable.
No special instructions are needed under this head. But in case of under-exposure perhaps a little more assistance can be gained from the use of the Uranium Intensifier than in ordinary work. In some cases in my experience the improvement has been very remarkable.
The Lawn Tennis Championship.
Dr. C. D. Somers.
Jno. R. Keeble, Esq.
These prints show what can be done with actual speed of 1/100 second. The upper one was taken with the N.S. Reflex, the lower with the Sinclair " Una."