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.
Shading The Lens. In extreme shadow lightings, or with full length figure positions, when a larger lens is used than is necessary to cut the size negative being made, the camera is often located at such a distance from the subject that the flash is closer to the subject than to the camera. Then, even though the flash be out of direct range of the lens, it will cause a diffused or false light to be reflected in the lens, producing an even fog over the plate. In such cases it is necessary to shield or shade the lens, which can be effected, when making the exposure, by holding a large piece of cardboard in the proper position between the lens and flash, or by placing a screen between the two. No matter what device you employ in shading the lens always be careful that it does not come within the range of view. This adjusting is best done when focusing.
Lens Hood. When there is any danger of strong light striking the curved surface of the lens, and thus causing cross reflections and light fog on the plate, one should use a lens hood to cut out all side reflections. Especially in flashlight photography is it essential that a lens hood be employed. In fact, one should be used in all cases where the lamp is placed even with or in front of the lens, for the volume of the flash extends some little distance in front of the lamp, and although one might think that the light will not fog the plate, the chances are that it may; so precaution, when using flashlight, is important.
502. A hood may be easily made of heavy black paper or light weight cardboard, by cutting the sheet the shape shown in Illustration No. 38. The dimensions given for this hood are correct for the average hand camera lens of 1 to 1 3/16 inches in diameter. Before pasting the two ends together the cone should be placed around the end of the lens tube, so that it will be tight and fit perfectly. The two ends should then be pasted or pinned together.
503. In cutting the pattern for this hood the outside is the arc of a circle, the radius of which is six inches, while the inside edge is the arc of a circle having a two inch radius. This will give a cone practically four inches long.
The distance between the two extreme corners is l0 1/4 inches, while that between the two inside corners is 3 9/16 inches.
504. After the two edges of this cone have been fastened together, a band one-half to one inch wide should be pasted around the small end of the cone, so as to give a support, and have the cone fit snugly over the lens mount. The point where this band and the end join should be fastened by pasting a strip of black cloth around the inside and outside. Black paper or cardboard must, of course, be employed, in order that there will be no reflection of any kind, and care must be taken that the hood does not cut into the angle of view.
Illustration No. 39 See Paragraph No. 508.
505. To make a larger cone the dimensions should all be proportionately increased. For instance, doubling all the measurements will give a cone that would fit on a lens of approximately two inches in diameter.
Illustration No. 41 See Paragraph No. 509.
506. By using precaution in shading the lens in flashlight work, you will never be troubled with fogged negatives.