Editing Unsharp masking (section)

== Photographic darkroom unsharp masking ==
[[File:Unsharp_mask_principle.svg|thumb|Simplified principle of unsharp masking]]

For the photographic darkroom process, a [[Large format (photography)|large-format]] glass plate negative is contact-copied onto a low-contrast film or plate to create a positive image. However, the positive copy is made with the copy material in contact with the back of the original, rather than [[Photographic emulsion|emulsion-to-emulsion]], so it is blurred. After processing this blurred positive is replaced in contact with the back of the original negative. When light is passed through both negative and in-register positive (in an [[enlarger]], for example), the positive partially cancels some of the information in the negative.

Because the positive has been blurred intentionally, only the low-frequency (blurred) information is cancelled. In addition, the mask effectively reduces the [[Dynamic range#photography|dynamic range]] of the original negative. Thus, if the resulting enlarged image is recorded on [[Photographic paper#Contrast control|contrasty photographic paper]], the partial cancellation emphasizes the high-spatial-frequency information (fine detail) in the original, without loss of highlight or shadow detail. The resulting print appears more acute than one made without the unsharp mask: its [[acutance]] is increased.

In the photographic procedure, the amount of blurring can be controlled by changing the "softness" or "hardness" (from point source to fully diffuse) of the light source used for the initial unsharp mask exposure, while the strength of the effect can be controlled by changing the contrast and density (i.e., exposure and development) of the unsharp mask.

For traditional photography, unsharp masking is usually used on [[monochrome]] materials; special [[panchromatic]] soft-working black-and-white films have been available for masking photographic colour transparencies. This has been especially useful to control the density range of a transparency intended for [[photomechanical reproduction]].