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What happens if I shoot a film with ASA 200 at ASA 200 but develop it at if it was rated ASA 400 (push 1 stop)?
Push-developing is used for compensating underexposed film, but what happens if I don't actually underexpose it? How will it affect the contrast and saturation?
In theory: if you shoot at 200 and develop at 400 you will get a 1 stop overexposed negative. Somewhat blown highlights, unexpectedly clear shadow detail. Plus general push process artifacts, like increased contrast and grain.
In practice: a lot will depend on details. Such as metering for highlights vs. shadows. And emulsion - classical grain, like TRI-X or HP5+ - can handle 1 stop difference in a stride; T grain is more fussy. Not to get into developers, some of which push better than others.
My suggestion would be to give it a shot - and try for yourself. You may (and may not) like the results better than shooting at regular speed...
Here is a good explanation of Pushing and Pulling Film. If you scroll down, there is a B&W photo of a woman playing guitar, which has the following description:
[Pushed +2] At a stop push is when you’ll really start noticing a large increase in contrast and grain. As you can see, the shadows are very dark and the highlight are very bright, very close to being blown out.
I agree with this statement. In the past when I've pushed film, it gets contrasty and grainy. This can be a good ro bad thing depending on your vision.
Another interesting site that gets deep into the theory is Contrast and Tonality Part 3: Characteristic Curves for Film and Paper. This goes into a lot of detail and talks about it in terms of Ansel Adam's zone system
In general, if you increase development, you increase contrast: the linear part of the H&D curve (log exposure vs. density) gets steeper. This generally has only a small effect on shadow details -- used in defining film speed -- but will push what would have been midtones to or near highlight density values.
This is commonly done for two reasons: one, if the film was (intentionally or not) underexposed; this can bring low values into a midtone, or two, if the scene was of very low contrast (gray on gray on a cloudy day, so no strong highlights or shadows). The latter case is the "legitimate" one; the former the one used by thousands of photojournalists and street photographers over the past century or more.
In either case, regardless of the exposure given, the separation between bright and dark tones will be increased -- as others have noted, this may result in highlights becoming unprintable or difficult to scan (especially if the film received normal exposure with a normal brightness range). Use of a compensating developer (like D-23, Pyrocat, or high dilution low agitation Rodinal) can help prevent highlights from blowing out, but will also reduce the level of contrast increase.
In my own darkroom, I use a Rodinal clone at 1:50 dilution, develop as if Push +2, but agitate only every third minute instead of every 30 or 60 seconds; I use this method to increase shadow detail (by giving maximum development) without much contrast increase (because the developer will exhaust locally in high exposure areas between agitation cycles).
