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.
Testing The Vignette. Place a piece of proof paper on the negative, clamp the back in position, and place in the window to print, slanting the frame at an angle with the sun. Print to the depth you would have the finished picture. Then remove from the printing frame and note the effects of the vignette. Should the vignette spread too far there will be need to paint the tissue closer to the opening.
1143. If the negative is a bust portrait and too much of the waist shows, place a loosely tufted piece of cotton (preferably absorbent cotton) at the bottom of the opening, between the negative and vignetting device. This cotton spread loosely will not only prevent the light from spreading, but will act as a filter and give a gradual blend. If, on the other hand, the vignette is too close, either arrange the cotton farther away or remove the painted tissue paper over the opening, replacing it with plain tissue, using the cotton for adjusting the size of the opening.
1144. An important consideration is the distance between the negative and the opening in the vignetter. Usually this should be at least one-half inch. The greater the distance, however, the smaller should be the opening, because the greater the distance the more the light will spread. With a little practice you will be able to produce almost any effect desired.
1145. With a variety of these box vignetters cut to different shaped openings, you can, in a few minutes, attach the device and produce a correct vignette that will necessitate no alteration. In fact, many minor miscalculations may be corrected by a simple variation in the angle at which the frame is presented to the light.
1146. While the methods described are intended for printing in bright sunlight, the printing can be done in the shade, or when the sky is overcast, or cloudy. In order, however, to gain speed in the printing on cloudy days, it would be best to remove the tissue paper, or onion skin. The notched teeth in the cut-out will filter the weak light, and you will find there will be sufficient diffusion or blending of the vignette. Where the tissue paper is removed, however, it is advisable to use the absorbent cotton loosely arranged around the opening, thus insuring against sharp lines.