Sharpening and micro contrast enhancement has been frequently used in film photography. This article briefly introduces similar digital and analog techniques.
Digital Unsharp Mask
Unsharp mask selectively increases contrast of edges and small objects by making dark areas of edges darker and light areas of edges lighter. Since this creates a sharpening effect, unsharp mask is frequently used to improve apparent sharpness in digital imaging. The following image shows enlarged detail of an edge with and without unsharp mask. The image consists of two gray boxes; the upper part is with USM applied.
If USM is used with narrower radius settings, it can effectively increase apparent sharpness. The following image shows effect of the unsharp mask on an image. The first character R is normal, the second is a copy of the first one with gaussian blur applied and the third one is a copy of the blurred one with unsharp mask applied.
If USM is used with wider radius settings, it can increase micro contrast.
Exposed molecules of silver halides react during development with developing agent and convert the white silver halide into black silver and creates visible photographic image. This chemical reaction breaks up the developing agent and creates certain byproducts that are capable of retarding further chemical reaction – development. Eberhard found that if developer is left absolutely still, smaller details and edges are developed more rapidly than larger areas, because smaller areas and edges can be easily surrounded by fresh developer, while silver halide particles inside large areas are surrounded by exhausted developer that does not have the vigor of the fresh one. Eberhard effect can be observed in still developers and diluted developers, but exists in conventional development as well and is frequently used by photographers to increase micro contrast and apparent sharpness. The effect can be replicated by digital unsharp mask. An extreme detail boosting technique, which turns photograph into graphics is based on copying or enlarging the photographic image to a large format high contrast film and developing with emulsion soaked in developer pressed on a glass sheet so that there is only thin layer of developer that can’t move. The film is copied and developed this way several times. I applied USM on black and white image in Photoshop four times to achieve visually identical effect:
As you can see, tonality is gone and the image is created only by local contrast.
Micro Contrast in Lenses
Camera lenses also exhibit micro contrast – or lack thereof. Lenses with good micro contrast have good MTF at frequencies around 10lp/mm. There is some consensus that Zeiss lenses consistently excel in micro contrast.
The following two images are taken at the same day and hour, identically processed, but with two different lenses. First was taken with Canon EF 35/1.4L and second with Zeiss Distagon 28/2 ZE. Both are sharp lenses, but one has bad micro contrast and one has good. Click on the images to see larger version.