In several posts on this forum I've seen the term "local contrast" refered to when prints are being discussed. What exactly is meant by this and how does it differ from the overall contrast. How is it achieved and what are we looking for, high or low local contrast?
Thanks in advance
For me the term local contrast involves the localized contrast within a given tonal scale on the print. Local contrast is what others and myself consider necessary to give a print the glow possible with black and white materials. So to answer your question yes one would want high local contrast.
We can achieve local contrast by increasing the paper grade or increasing the filtration in the case of VC materials. However when we do this the overall contrast can and very often does exceed the scale of the paper. This is why printing methods such as flashing the paper and contrast masking are utilized. These methods allow one to contract the overall contrast while expanding the local contrast.
Printing of fine images requires a balancing act of achieving local contrast while not exceeding the overall contrast of the paper on which one is printing.
contrast is the "difference" between the brightes and darkest parts of a print. You may apply this to the overall print, to a part of a print or even to tiny deatails. This is not to be meant "densitometrically", i.e. if there is a tiny spot with Dmin and another tiny spot with Dmax on "landscape in fog", the overall contrast is still low.
Local Contrast is often associated with "mid tone separation", i.e. how simple things, sturctures, textures etc. can be differentiated.
Detail Contrast on the other hand, is directly associated with "sharpness", i.e. how clear the eye can seperate tiny stuctures. This is why higher paper grades usually look sharper.
To add to the above posts, here are a few other considerations. The film and developer combination you use can affect overall contrast, local contrast and "micro contrast" within any given print. These terms were, at least to me, somewhat nebulous when I started reading some posts in the forums, but are now starting to make more sense.
I had been using PMK Pyro for all of my black & white prints, but found that others said it was too "muddy" in the shadows for contact printing. PMK is a staining developer which surrounds each silver particle with a "fog" of stain and masks grain very well for a smooth image, especially with finer grain films for enlargement. I decided to switch to ABC pyro to see what they were talking about and, yes, it does give a completely different look. The negative is "tack sharp" because it does not have the same staining properties. It shows up more in highlights and does not impart a "general stain" like PMK does. This general stain reduces contrast in shadow areas because the masking effect tends to reduce contrast in local areas of fine detail. ABC shows each grain of silver as if it were etched with a razor or fine stylus, which increases shadow separation.
Another developer, which I have not yet tried, is Pyrocat HD. It has similar properties to ABC pyro, but without the significant loss in film speed (1/2 the listed asa for PMK and 1/4 for ABC) of other pyro developers. It seems to be the best of both worlds, speed, acutance and tonal range are all well balanced, no small feat of chemical juggling.
All of these factors come into play when printing, so try a bit of testing to see where you are with your film and developer combinations. I'm not urging the use of pyro here, only trying to explain what variations are possible on so many levels.
Contrast is often a matter of individual taste in printing. A good print should have the ability to convey overall contrast from light to dark, as well as the subtle textures and tones in any given area of the print. Francesco has posted some excellent scans of prints showing very fine local contrast and examples of this topic. His rendering of snow textures would be one example of local contrast. Take a look at some of his prints and some of the others to see what variations are possible.