[Illustration: FIG. 85.--CAMERA WITH OPERA GLASS ATTACHED.]
During the last few years many so-called telephotographic lenses have
been placed upon the market. These instruments enable one to
photograph objects in the distance and obtain images very much larger
than those given by the ordinary photographic lens. These lenses are,
however, very costly. In an article by Mr. O. G. Mason, published in
_The Photographic Times_ for June, 1895, that gentleman described a
simple method of obtaining telephoto pictures by replacing the
ordinary lens with an opera glass. He says: "Several devices have been
brought forward with a view of decreasing the expense of telephoto
lenses, but I have seen no others so satisfactory, cheap and simple,
as the utilization of the ordinary opera glass for the camera
objective, which was described, figured and finally constructed for me
about a year ago by Mr. Alvin Lawrence, the horologist of Lowell,
Mass. An opera or field glass is a convenient and useful instrument in
the kit of any touring photographer; and when he can easily and
quickly attach it to his camera-box as an objective its great value is
at once made apparent. Mr. Lawrence's method of doing this at little
cost is a good illustration of Yankee ingenuity. It is not claimed
that such a device will do all or as well as a telephotographic lens
costing ten times as much; but it will do far more than most people
could or would expect. Of course the field is quite limited, which, in
fact, is the case with the most expensive telephotographic objective,
and the sharpness of the image depends much upon the quality of the
opera or field glass used. The accompanying views show the relative
size and character of image by a forty-five dollar rapid rectilinear
view lens and a four-dollar opera glass attached to the same camera
and used at the same point. The other illustrations show the camera as
used and the method of opera glass attachment to the lens-board. It
will be seen that the eye end of the opera glass is placed against the
lens-board, one eye-piece in a slight depression around the hole
through the centre, and by a quarter turn the brace between the two
barrels passes behind a projecting arm on the board, the focusing
barrel resting in a slot in this arm, where it is firmly held in
position by friction alone.
[Illustration: FIG. 86.--CAMERA SHOWING ARRANGEMENT FOR OPERA GLASS.]
[Illustration: FIG. 87.--VIEW TAKEN WITH OPERA GLASS.]
[Illustration: FIG. 88.--VIEW TAKEN FROM SAME SPOT WITH AN ORDINARY
VIEW LENS.]
As opera glasses are usually constructed for vision only, no attempt
is made by the optician to make correction for securing coincidence of
foci of the visual and chemical rays of light as in the well-made
photographic objective. Hence, it is often found that the actinic
focus falls within, or is shorter than, the visual. When this is the
case, the proper allowance is easily made after a few trials.