In my own experience from developing myself in Flexicolor, Portra 400 can work well at 1600, and pushing beats underexposing with no push imho.
I almost never underexpose my Portra, because I mainly use it for portraits. It excels @ box speed and @ 320 in this regard with normal processing.
Portra 160 did handle underexposure well (1-2 stops) without pushing as well... it was from when I was excited to shoot it as it had just become available and the light had faded too much for normal exposure.
Invest in a tripod if it's a problem.
Underxposure and pushing is useable, but make it your last resort after exhausting every other option.
In photo school, 16 years internet forum education and personal experience has taught me one thing: that color negative film does not like AT ALL to be underexposed - no matter how much later you "push" it.
For your assignment on hand (very low level light), shoot digital.
Having done both with and without pushing on Portra 160 and 400, I disagree.
It also works well on Portra 800, and is the designated C-41 push film from Kodak.
You might of learned it back then but they did not have the new portra 400 back the then.
Originally Posted by Andre Noble
cjbecker, Regardless of the film in question, as exposure is reduced, detail is lost. The new Portra is really good but Kodak hasn't found a way to manipulate the laws of physics.
At a certain point reducing exposure will start affecting the print we expect, period.
Where that point is for each of us may vary. I'd hazard a guess that you, Andre, Athiril, and I have different photographic sensibilities and expectations.
In the absence of testing for oneself deviating from box speed is a real gamble.
When I do shoot portra 400 I normally shoot it at 250, as that's where it looks best to me. But If I'm in a situation were I have to get the picture I will shoot portra 400 up to 3200. I know I'm going to be loosing detail, colors and adding grain, but getting the picture is better then no picture at all. And I have done this on paying jobs where it was dark and flashes where not allowed.