This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Foci Of Condensers. When two are combined the focus is one-half that given in table, measured from center of combination. These foci are considered correct for the size of plate which the condensers will cover. Variations from these foci would mean the purchase of a variety of lenses and generally sufficient variation in cone of light can be obtained by adjusting the light to or from condensers. A ground-glass either between the condensers or between the arc and condensers diffuses the light in enlarging. A sandblasted watch glass of large size placed between arc and condensers has been recommended by Mr. Earle. The ground-glass should not be placed near the negative as it would be in focus and therefore show in the enlargement.
Enlarging. In the matter of photographic lenses, we have been asked several times for lenses especially corrected for bromide enlarging. The lens that will do good photographic work is suited for bromide enlarging, as well as for other work. The lens, of course, should be of good quality and have flat field and covering power sufficient for the negative which it is to enlarge. As the image is formed at a comparatively short distance from the lens the working distance is increased and therefore a somewhat larger plate can be covered but the speed is also reduced. It is better, however, to use the same focus lens as is used in making the picture. Some workers recommend the Portrait Unar for enlarging. Just as in other photographic work the better the lens the better the result, hence the anastigmat with its flatness of field, its great covering power, its even illumination, its superior correction for spherical and chromatic aberration, gives the best possible results. Stopping down the lens produces better results, just as in other photography.
Illustration No. 101.
944. In the matter of projection lenses, you may have occasion to determine what focus lens is suitable for certain work. I give the formula which I use:
Then L=(DXF)/S D=(LXS)/F F=(LXS)/S
945. Thus, if we have a hall in which we wish to work at 50 feet distance, with image on screen 10 feet square, with regular 3 inch slide, what lens shall we use?
F=(LXS)/D L=50 S=3
D=10 therefore (50X3) /10 = 15 inches which would be the equivalent focus of lens required.
946. In Illustration No. 100 we illustrate the Bausch & Lomb Projection Lens. Illustration No. 101 gives a sectional view, showing the optical construction of this lens. This lens utilizes an exceptionally large amount of the light of the lantern and renders sharp, clear, beautifully illuminated pictures. The field is extremely flat and the illumination is uniform over the entire screen. The mounting of the lens is of superior finish, the movable tube being nickeled and working in a cloth sleeve. The adjustment by "a spiral pinion and diagonal rack" works with great accuracy.
Where a small picture is to cover a large screen, at a short distance, the Special Wide Objective No. 6 is recommended.
Showing Distance from Object to Screen, Diameter of Picture, in
Feet, and Focus of Lens Which Must be Used When Object is Three Inches in Diameter.
Distance from Object to Screen, in Feet
Diameter of image on screen
Used through courtesy of Bausch & Lomb Optical Co., Rochester, N. Y.