A recent magazine article mentioned that pushing film has a negative effect on both saturation and sharpness. Can anyone expand on the reasons this might occur? I shoot both T-max 100 and color transparency 4x5 film.
This answer will refer only to reversal films. B&W does respond similarly, but at a much lower level.
Let's take a simple case, E6 film exposed to magenta light. Since the film is made of layers sensitive to yellow, magenta and cyan light, we expect that the magenta layer only contains exposed silver halide crystals (the latent image). During normal development only those exposed silver grains will develop to form a magenta dye cloud around each grain; the cyan and yellow layers will have no developed dye, and will be transparent.
Now we will underexpose, then push process our film. Push processing means to leave the film longer in the first developer (which acts much like a B&W developer), about 15% longer than normal. Since the film is in the first dev a longer time, silver grains that would normally not respond to the developer during normal development now have additional time, and some of them will respond. This means that some silver grains in the yellow and cyan layers will develop and form dye clouds around the sensitive but unexposed grains. In the fully-developed film, instead of seeing pure magenta, there will also be a bit of yellow and cyan 'polluting' the pure color. Saturation is the dominance of one color (be it a single-layer color like M, C, or Y, or a dual-layer color like red, green or blue), and any diminishing of that dominance, by mixing in other colors, is a loss of saturation.
But if it's only a single-stop push, the effect isn't very noticable.
The increase in grain during push development comes (I think) from one exposed silver grain influencing unexposed grains nearby, so that they develop in a cluster instead of individually. This will form big dye clouds that we see as grain.
Push processing has different effects on B&W, Color Negative and Color Slide Film.
Pushing does always mean: longer development times. A longer development time produce coarser grain and thus reduces resolution slightly. Resolution might have an impact on sharpness, depending on the enlargement factor and the viewing distance. However, it normally does not produce less sharp B&W negatives. Coarser grain might even increase the perceived sharpness of an image.
Color Negative and Color Slide Film do not have grain at all. The bleach-fix-process (whether in one or two process steps) eliminates all silver grain. What is left are dye clouds produced during color development. These dye cloudes do have less sharp contours than silver grain. And larger dye clouds might look more fuzzy.
Color Negative Film is not suited for push processing, since the development of silver grain and dyes take place in one step. Pushing a Color Negative Film causes crossover effects that cannot be filtered properly during printing. This is because the color development process has different effects on the different (color) layers of the film.
You may push Color Slide Film, because the development of the silver grain and the color dye development take place in two different process steps. However, pushing a Slide Film too far increases the base density in the first development process and will lead to less black shadows in the reversal process. Reduced saturation is a problem of color side densities. Pushing a film increases contrast and thus increases the effect of color side densities.