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.
646. Lens for Enlarging - For professional work the regular lens employed for making portraits may be used for enlarging; all that is required is, that the lens be of sufficient size to cover the plate to be enlarged. Fortunately for the amateur who possesses only a hand or view camera fitted with a rectilinear lens, this is in every way suitable for enlarging; in fact, should be used in preference to others for the enlarging of groups, landscapes, mechanical drawings, etc., and it will work equally as good for portraiture, with the exception that it does not work quite so quickly. This is because it has not as large an aperture. For full and half-length figures it is quite as rapid, because, for this purpose, with a portrait lens it would be necessary to stop down considerably in order to get the entire image sharp. Any lens that will make a negative can be used for enlarging, and the proper size of lens depends almost entirely upon the negative to be enlarged from, and not at all upon the enlargement to be made. If the lens will cover the negative it will make an enlargement from it of any size.
647. A good rule to follow is to use for your enlarging lens the same lens, or the same size lens, that was used to make the original negative from which you are going to make your Bromide enlargement. For portrait enlargements to be made from 5x7 negatives, and under, a half-size portrait lens will be found suitable. This lens can be worked nearly wide open for bust pictures, but will have to be stopped clown for half or full-length figures.
Quality Of Negative Best Suited For Bromide Enlarging. Any negative from which a good, snappy, brilliant print can be obtained in contact printing is suitable for Bromide enlarging. By exercising a little care in selecting the proper grade of paper, using "hard" or "soft," as the case may require, almost perfect prints can be produced from harsh and contrasty, or very thin negatives. If only one grade of paper is used, then the difference in quality of negative must be overcome in the exposure and development. A hard, contrasty negative, for instance, must receive a longer exposure than a thin, snappy one, and vice versa. All negatives should be carefully spotted, and portrait negatives retouched before enlarging from them; for remember, all blemishes or defects will be very much more noticeable in the enlargement. Also bear in mind that a very badly discolored, yellow negative will necessitate a great deal longer exposure, as the yellow image will have but slight effect on the sensitiveness of the paper; hence the extra long exposure necessary.
Placing Negative In Position. We will now suppose that you have your enlarging apparatus prepared for enlarging. Having selected your negative, the next step is to place it in position for enlarging. The negative is placed in the negative-holder upside down, and inserted in the camera with the film side facing the lens, unless you want the image inverted, when, of course, the negative may be placed glass side to the lens. The quality of the enlargement is exactly the same, no matter which way you insert the negative.