The Dictionary Of Photography

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

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Lens
article P*ocuS will be found numerous rules and tables, which may be of some service. The lens is the most important and at the same time the most expensive part of a photographer's outfit; too much care cannot be taken in choosing or in keeping it.
Care of Lenses. Lenses should always be kept, when not in use, in a leather case, or else in a tin box padded with wool or washleather to prevent the access of light, air, and dust. The inside of the lens-tube and the diaphragms should be occasionally re-blackened to avoid disturbing reflec-tions. If the lenses become dusty or somewhat dim, they should be most carefully and tenderly wiped with a piece of soft silk or washleather, and when it is necessary to clean the internal surfaces of lenses (doublets, portraits, etc.), it is advisable to remove one combination and clean it, and then replace it before unscrewing another combination to clean ; by this means displacement of the combination cannot take place. With some old lenses a peculiar tree-like marking makes its appearance, which, to uninitiated eyes, appears to be in the middle of the glass. This is due to the balsam, which is used in cementing the glasses of the combination together, becoming old and starring. In such a case it is advisable, if the lens is worth anything at all, to send it to an optician, who will unset the lens and properly re-cement and reset it: if the lens is not of much value, and the owner is desirous of trying his hand at a practical remedy, the lens should be placed in some methylated spirit or turpentine in a water-bath and gradually heated, when the cement will be softened. The two glasses can then be taken apart, well wiped and cleaned, and re-cemented by a drop or two of Canada balsam, and gently warming. (For fuller directions see Bal-saming, Re-, of Lenses.) If by accident one of the lenses should be scratched, it is preferable to fill the scratch with black varnish, as the loss of light is. in this case preferable to the dis-turbing reflections of the scratch.
Selecting a Lens. The following notes on choosing and testing a lens are written from a practical standpoint, and if an accurate scientific examination is required, it would be advisable to send the lens to the Testing Department at Kew; but if a lens answers successfully to the tests described below, it may be accepted as practically satisfactory. The first point to decide in choosing a lens is to determine for what class of work
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