A single ‘straight’ printing exposure often fails to suit every part of the picture. The reason may be that the negative density range, while well matched to the paper for most subject tones, exceeds it at one extreme or the other.
Perhaps a patch of shadow becomes solid black or highlight detail looks ‘burnt out’, when mid-tones are correct for density and contrast. Perhaps the subject lighting was uneven, or you simply wanted to darken and merge some parts of a composition to emphasize others. Most printers would agree that most prints can be improved in some way during printing by locally reducing exposure (known as ‘shading’, ‘holding back’ or ‘dodging’) or extending it (‘printing-in’, or ‘burning-in’).
To lighten part of the picture (also known as dodging) insert your hand, or red acetate or opaque card, into the light beam during part of the exposure. Hold your shading/dodging device about halfway between the lens and paper to soften the shadow edge and keep it slightly on the move during the exposure. To darken a chosen area, follow up the main exposure with an additional period when you print in using a hole in a card, or the gap between your cupped hands, where you let light through onto a selected part of the print to darken it.
Feather the edge of your exposure-controlled area in the same way, as for shading you need to keep your hands or card constantly moving to get soft edges. Note that a soft edge to the shadow is essential in both cases to make the exposure change invisible, otherwise, you will have an unnatural-looking print with odd light or dark patches. It may be possible to use a more distinct cut-off on a harder edge such as a wall edge or a horizon, but it is seldom necessary to be this precise; it is also much more difficult to control.
To decide how long to shade or print-in, make the best possible straight print first. Then, place pieces of printing paper across excessively dark or light image areas and make test strip exposures at shorter and longer exposure times respectively.
If necessary, sketch out a shading ‘map’.
This will remind you where and by how much you must shade during your main exposure, and the same for extra exposure afterward. The harder the contrast of your paper, the greater these exposure differences will have to be.
Sometimes an area being heavily printed to black contains some small highlights that still appear as grey shapes, no matter how much additional exposure you seem to give. The best solution then may be to fog over these parts. Either print-in with the enlarger turned out of focus, or fog using a small battery torch fitted with a narrow cowl of black paper. Keep the enlarger on, red-filtered, to show you the exact image areas you are treating with white torchlight.
What is the local control of contrast?
By controlling the exposure locally on a print, you can change the contrast of the image; note that this can also change the composition of your image as our eyes are naturally drawn to areas of strong contrast. Shading and printing-in are like retouching: if done well, no one should know they have taken place. But when they are overdone you will find that shadows look unnaturally flat or grey, and burnt-in highlights veil over, again with a lost of contrast. The image might look flat because parts of the subject were exposed on the tone-merging ‘toe’ of the film’s performance curve, or near the top end where irradiation again destroys tone separation. Perhaps the cause is general under-exposure or over-exposure respectively, or just subject range beyond the capabilities of your film.
Either way, these tone-flattened areas need extra contrast, which you can best achieve with variable contrast paper using selective filtration. Imagine, for example, that you need to shade and contrast-boost a simple patch of shadow in a picture that otherwise prints with grade 2 filtration.
