Unsharp Masking in Black and White Printing by Donald Miller
The use of unsharp masking procedures used in the production of a print brings with it certain distinct advantages. These benefits are not available through other means. Yet the process is relatively simple. It works most effectively with variable contrast printing paper. Graded papers may also be used. However, if graded papers are used then provisions must be made to ensure control of negative contrast and also of the unsharp mask density.
Unsharp masking can be used with all film formats of 35 mm and larger. However alignment (registration) becomes more difficult below 4X5 inch sheet film.
These advantages of unsharp masking are:
Enhanced apparent sharpness through “edge effects”.
Enhanced local contrast within the print tonal range.
Reduction of contrast on negatives of excessive contrast.
Improved print shadow detail.
The materials required will be the following:
Half tone ortho lithographic film of appropriate size (available from Freestyle Photographic or Photo Warehouse.
One sheet of Duratrans or other uniformly opaque diffusion material.
One sheet of black paper of 8x10 inch size.
One sheet of glass (3/16 or ¼ inch thickness) of sufficient size to cover the dimensions of the film with an excess of an additional inch beyond the film dimensions.
Dektol or equivalent paper developer.
3M Scotch lithographic tape (1/4 inch width)
One sheet of fixed, washed and dried unexposed film.
The process involves the production of an un-sharp, low-contrast, and low-density film positive of the camera negative. It is accomplished by contact printing the camera negative with an unexposed sheet of half tone lith film (graphics art film). There are certain very distinct differences from a normal contact print.
These differences are:
The contact process is performed with the two pieces of film separated by their respective bases. In other words the emulsions are apart from each other.
The use of an additional unexposed and fixed sheet of film acts as an additional spacer separating the camera negative and the half tone film.
The mask is exposed through diffusion material. This can be a material such as Duratrans or any other uniformly diffuse thin material such as sheet acrylic.
The process of producing the mask involves raising the enlarger head height to the height that would project an 8X10 enlargement with no negative in the negative holder. The first step is to place a sheet of black paper down on the easel in the area that the enlarger projected light image will exist. This sheet of paper serves as an anti halation measure.
One may do the following steps of the mask production using a safelight for illumination since the half tone film is an orthochromatic material and has the same sensitivity as photographic paper. The next step is placing an unexposed sheet of half tone film on the black paper--emulsion side down. Since lithographic film does not have notches to identify the emulsion side, this will be determined by viewing the sheet of film in the safelight illumination. The emulsion side will appear lighter in color then the base side. On top of this half tone film one then places the unexposed and fixed sheet of film. The next step is to place the camera negative on top of this clear sheet of film--emulsion side up. On top of this is placed the sheet of diffusion material followed by the sheet of glass to hold the materials in contact with each other.
The next step involves exposing the glass, diffusion material and film bundle. I find that with my normal camera negative density my exposures are normally in the range of 14-22 seconds at F16 with my enlarger light source. (Saunders 4550 XLG). Once the mask has been exposed, the lith film is next developed in highly dilute Dektol developer (1-30) at 70 degrees for 1 ½ to 2 ½ minutes. Fixing and washing the mask film in the same manner that film would normally be processed then follow the development. When dried and viewed this will be a very thin positive image of the camera negative. In other words the areas of high density on the camera negative will have no density on the mask and the areas of low camera negative density will exhibit the greatest mask density. The mask is best evaluated by placing it on a sheet of white paper. If you have access to a densitometer, the ideal mask peak density will be in the range of .15- .35. If you do not have a densitometer, compare your mask peak density to a .30 ND filter. Ideally the peak density should be lower then the .30 ND filter except in those cases when a stronger mask is desired (.35). In that case the comparison between the mask density and the ND filter should be quite similar.
The manner that the mask is used in producing a print is to align it with the camera negative. This alignment can be accomplished with enough precision on a light table by aligning the camera negative details with the mask details. When this is aligned I then tape the two pieces of film together at an end by using 3M lith tape. The actual printing of the negative/mask sandwich is accomplished in the same manner as the camera negative by itself. The changes required would be an increase of the contrast filtration and an increase in exposure time to compensate for the increased negative density. This increase in contrast filtration is required because the mask is compressing the overall negative density by the measure of it’s peak density. The compensating increase in filtration contrast will increase the local contrast within the print. Additionally, an apparent increase in print sharpness will be noted.
Unsharp masks are also very useful in more advanced sharp masking techniques in that they blend demarcation points that may exist with sharp masking. These sharp masking techniques require precise registration means and are beyond the scope of this article to cover.