I have shot two rolls of 35mm ISO 160 film last week. The camera was set to ISO 400, should I trash the two rolls? Is sending it to the lab a waste of money?
In my experience, unless it's slide film, it should be fine. 1-2 stop underexposure won't kill color negative film...in fact, might make it quite saturated depending on the film itself.
I join the choir here, get them pushed +1 in development and you'll probably be happily surprised how good it looks!
Some time ago I shot a cpl of rolls APX 100, thinking it was HP5+ 400, on EI 1600 (they came from an unmarked bulk loader and the previous owner told me it was HP5+ in it)..
When I found out I had not yet developed the rolls and managed to actually at least get something on almost all frames, I'm not saying I'd recommend this, but I'm sure there's a bunch of people who'd call the pictures great modern art....
I've been shooting Potra 160 in 35mm at ISO 800 and let the lab do only +1 push development, meaning development at ISO 320. So underexposure was comparable. I took shots of a music events and some on an automotive exhibition. The negs came back a bit dark, but nothing that a good scanner could not handle.
Your shots will probably be fine even if you do not compensate for it in development.
You underexposed by a full two stops. (I consider 160 film to REALLY be 100 film.) Thus you need to over develop by about 25% - 33%, or a bit more. You will attain decent highlights but shadow detail will be pitiful. But, honestly, you should have little problems with color-crossover and might be surprised with how great your highlights look. Midtones will be OK but not ideal. - David Lyga
A push development will increase the contrast, but won't make any meaningful difference with respect to shadow detail.
If the nature of the light and the subject are such that an increase in contrast might help, then go ahead and order a one stop "push".
If, however, your shots include a lot of highlight flesh tones and human features, you may prefer to leave the contrast unchanged, knowing that the "near shadows" will lack contrast.