We received an e-mail from FPP listener Carlos about how to shoot when you intend on "push processing." For those who do not know about "push processing," check out the Wikipedia page here.
So let's say I shoot with the intent of push processing the film (400 - 3200 for example).
1. You underexpose by 3 stops, right? If no, then what kind of exposure adjustments do you need to do.
2. If yes and you somehow mess up on the exposure (ex fingers accidentally rotate the aperture ring or shutter speed dial) and you push process as expected, is there a way to "save" the photos in that roll of film?
To shoot a 400 ISO film at 3200 ISO, yes, you are underexposing
by 3 stops. The only four films I'd personally recommend under-
exposing this much, however, are Kodak Tri-X, TMax 400, Portra
400, and Ilford Delta 3200.
In B&W shooting (the first three films listed), it's always best to
push process/over-develop by the number of stops underexposed.
Chemically speaking, a push of 1 stop is ~25% of the total processing
time added to the standard developing time.
Shooting and pushing color? The whole game changes.
When shooting Portra 400 with the intent of exposing at ISO 3200,
it's typically best to push the film at least 1 stop, and up to a full
3 stops. From my own experiences, pushing 2 stops is a happy
medium, with moderate mid tone and highlight retention, with shadows
that can be "pulled" back to black in Photoshop/Lightroom. I've pushed
a roll the full 3 stops before, but hated the resulting tones and grain.
Pushing only 1 stop is tricky, and should only be done once you're
comfortable playing around with push processing.
Regarding your "messing up the dials" scenario, accidentally over-
exposing the film to be push processed is fine, but even more under-
exposure will typically yield an unsatisfactory image. Highlights can
also easily be "pulled" back in post production. If you do this with a
B&W film such as Ilford Delta 3200, there is a slight chance that
pushing the film even further (4+ stops) could yield an image.
With any kind of push processing, remember, you can't bring up
shadow details that were never there. All pushing will do is over-
process the midtones and highlights in an attempt to bring the shadows
up to an acceptable level. So if the film never actually recorded any
detail in a certain shadow area, all the overprocessing in the world
won't make a shadow appear.
Hope this answers your question, and keep on shooting film!