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.
Dodging. If you have some dense highlights in the negative that require printing more than other parts, and this oftentimes happens on faces and white drapery in which no detail appears, you can readily overcome this by extra printing-in other words, extra exposure on these parts. To do this, cut a 2 to 3 inch round hole in a cardboard, and holding this in front of the lens you can direct the concentrated light coming through this opening to any part you desire, thus enabling you to produce an even print. The nearer you hold it to the lens the larger the surface you expose, and vice versa.
678. By keeping the spot of light moving almost any amount of additional detail may be obtained locally. Remember, however, that this cardboard must be large enough to prevent the light striking the paper on any part other than where the hole is cut in the cardboard. Occasionally, in full-length portraits a hand requires less exposure than the rest of the picture. In this case a small piece of cardboard, cut to the proper shape and stuck on the end of a piece of wire or knitting-needle, can be used to screen that part of the image. Or, by sticking a round piece of cardboard (about the size of a half dollar) to the end of a glass rod, and adjusting this before the lens, over the portions to be held back, you can even the tone nicely. The glass rod, being transparent, will not affect any other portions of the print.
679. The paper, you will note, lends itself to innumerable dodges which may be practiced in a similar manner, the operator being able to see just what he is doing from his position near the easel.
Correcting Distortion. The application of the following method for correction of views already distorted will be found of considerable interest to the serious worker: There are few photographers who have not, at some time or other, obtained negatives of street scenes with the houses looking to each other for support, or architectural studies with columns out of plumb, caused by the absence or misuse of the swing-back or rising front.
681. You may have an architectural negative or view of a building to enlarge, the lines of which, however, are faulty. The building appears, for instance, broader at the bottom than at the top. To obtain a print with the lines corrected, provide a white card large enough to cover the size enlargement you expect to make. Rule this cardboard for different size openings. If your easel will receive a 20 x 24 card, rule one opening 16 x 20, another 14 x 17, another 11 x 14, etc. Tack this card to the easel; place the negative in the enlarging camera so the lines to be corrected are in a perpendicular position, and obtain a focus. By tipping the enlarging easel or board forward or backward, you will find it possible to correct these lines. Tip the board or easel until the perpendicular lines coincide with the perpendicular lines on the board or easel.
682. You must be careful, however, when you are placing your negative in the holder for enlarging, that you place it in upside down so that the image will appear right end up on the enlarging board. By tipping the top of the easel toward the camera it brings the top considerably nearer to the negative than the bottom, and it acts like a swing-back on the camera. You will, however, notice that either the top or bottom of the picture will be very badly out of focus. To overcome this out-of-focus effect you must slightly divide the focus and use an extremely small stop and, of course, give a correspondingly longer exposure.