Here are two of the four sheets of Kodak Portra 400 that I shot recently on my Toyo 45AII large format camera. I had received a box of Portra 400 for Christmas and was curious how it would handle long exposures. I normally like B&W for these types of shots, especially the foolproof Fuji Neopan Acros 100, but as I said curiosity got the best of me. I was also a little annoyed that the Kodak data sheet for 45 Portra read: "No filter correction or exposure compensation is required for PORTRA 400 Film for exposures from 1⁄10,000 second to 1 second. For critical applications with longer exposure times, make tests under your conditions." Super helpful, Kodak. Thanks! So you didn't test the film for anything longer than 1 second? You would rather let the consumer make their own tests (which I agree to some extent makes sense)? It is discouraging that a box of 10 sheets of Kodak Portra 400 costs about $30 and each sheet is $3-6 to develop depending on which lab you use. Mine is only $3, so my testing consisted of loading two film holders with four sheets of film and blowing $24 in fifteen minutes.
Yes, I'm being a little hard on Kodak. However, Fuji and Ilford do a fantastic job of documenting the change needed in exposure (due to reciprocity failure) for times longer than 1 second. Kodak should do better.
Anyway, I shot four sheets from my usual test location (in Dumbo underneath the Manhattan Bridge looking at the Brooklyn Bridge and Lower Manhattan) for long exposures and didn't see a bit of difference. The first shot was 8 seconds for f22, but I gave it 15 seconds as a starting point.
The second one was taken right after the first and I gave it 30 seconds. I couldn't tell a difference between the two.
I repositioned my tripod slightly for the second film holder and repeated the meter reading. As it was getting darker, the reading called for 15 seconds. I exposed one sheet for 45 seconds and the other for 90 seconds just to see if it would matter. It didn't, both negatives were pretty much the same.
Here's the 45 second exposure at f22.
The bottom line is that Kodak Portra 400 handles long exposures nicely. I got great results between 15 seconds and 90 seconds. Some of that was obviously due to the rapidly changing light conditions, but as its been well documented, this film is VERY versatile and forgiving. I wouldn't hesitate to use Kodak Portra 400 for exposures between 1 second and 90 seconds. Next time I would probably just give the shot double the time that the meter reading calls for. Please note, this was in no way a scientific method. I didn't keep notes, but I recall the exposure times and which film holders were which times. For critical paid use, I guess I would do as Kodak suggests and "make tests under your conditions."