Help from the old-timers! (Film question)

[OsmosisStudios]

Yes, I said film. Now pick your jaw up off the floor and pay attention.

I've been toying with the idea of some VERY long exposures. I got my hands on one of B+W's 10-stop ND filters recently and want to try it out. While on digital it likely wont be a problem (i've got a few things already selected for that), film is another issue. I'm worried mainly about reciprocity failure.

From what I've been able to find online, my 3 favourite films (Ilford XP2, Ilford Pan F 50 and Kodak Portra 400VC) have some rather odd-looking (at least to me) characteristics.

I just want to make sure I get this right before I go blowing money on film.

Ilford XP2, from all accounts, should be exposed as

Exposure Time = Metered Time ^ 1.48

... For shutter speeds 0.5s and up.

What I can understand from this is that, for instance, a 10s METERED time should actually be exposed for 10^1.48 seconds. That comes out to 30s. That SEEMS extreme, but may very well be correct.

Similarly, I shot a roll of PanF 50 a while back and have found similar characteristics (ET = MT ^1.49, in this case). This seems to support the results I got as my exposures ranged from 2-8 seconds and were all several stops under exposed.

Furthermore, my Kodak Porta 400 VC should apparently be "ballparked" as:

MT of 1-4 seconds: ET=MTx2
MT of 5-30 seconds: ET=MTx5
MT of 30+ seconds: ET=MTx12

This, too, sounds odd.

Does this all sound about right? I'm assuming it wouldnt change based on format (35mm, 120).

Back to your pixels now!

[BigVinnie]

Those sound about right.

The porta will probably color shift on you. Might be a good thing maybe?

I found with most of the ilford films the reciprocity failure was pretty dramatic. I never did big ND type shots, just nighttime stuff. I also used slower films. Kodachrome 25 was my choice for 35mm.

The last B&W film I used was Fuji Acros (100). Absolutely beautiful tonal range. You might think about giving it a shot.

What is your subject matter?

[BCampbell]

Os,

You are probably running into reciprocity failure (or you will soon). There's plenty of info out there on it, try looking it up. Good luck!

[OsmosisStudios]

Originally Posted by BCampbell

Os,

You are probably running into reciprocity failure (or you will soon). There's plenty of info out there on it, try looking it up. Good luck!

... Did you read my post? I said that I was worried about this exact thing and that I wanted to double-check my research findings.

[LeeR]

It's been a long time since I have done any long exposures with film. As I remember we usually just did a lot of bracketing and hoped for the best. However, a little checking confirmed that you were on the right track with a far more scientific approach. According to the Ilford XP2 Technical sheet, which I suspect you have already seen, modifications to metered times are required. They use a graph, but the curve looks standard enough that a formula should serve the same purpose. I remember Pan F being really slow and having some trouble with longer exposures. I see why now. It requires major adjustments. Shots metered at 20 seconds are going to require actual exposures over 75 seconds! That's a huge adjustment. That's nearly two whole stops! The Big Yellow God offers no similar information for the Portra 400, offering instead that you should "make tests under your conditions." I suspect that is a nice way of saying you should expect big shifts in exposure times and color balance. Good luck with that. Any film as saturated and contrasty as Portra is likely to show significant color shift. It may work for you, however, as I remember it usually shifted to the blue/magenta side.

[ravncat]

doing long night exposures - I recall when working with film the adjustments for reciprocity failure being very long indeed... such a huge adjustment does not seem "incorrect"

[OsmosisStudios]

Thanks to all

BigVinnie: Kodachrome, sadly, isn't being done anymore, and while I do like Arcos 100, I've found it to be rather finicky. I've always used Ilford so it has a sort of sentimental value. I also like XP2 because it's C-41 process, which saves on both time and money when it comes to processing.

Leer: I did in fact see the Ilford tech sheets, but some quick googling saved me a lot of trouble. Photo.net has a few excellent people that do the math for me

ravncat: Thanks again for you input.

[jdepould]

What are you planning to shoot? Might affect your choice of film stock. I loved the Tri-x when I was shooting film, but I'm a PJ guy.