Reciprocity failure for Rollei IR400?

[olwick]

Hi,

I'm trying to determine the reciprocity characteristics for Rollei IR400 film. I finally found the data sheet on the Freestyle site, but it only goes down to 1/2 second. I'll be taking minutes-long exposures.

Has anyone done any testing on this or have any information to share?

Thanks,

Mark

 

[keithwms]

Minutes long? With and IR filter in place, or without?

Absent a chart, I think I'd use the reciprocity chart for something like hp5+ or tri-x and bracket +/-2 stops. It isn't gonna be pretty. Offhand I'd guess 1 minute goes to 5 or so.

 

[2F/2F]

The data sheet might say something like: "For exposures longer than 1/2 sec, testing is required to determine the proper adjusted exposure time." That is what I suggest. As far as I can see, even if you will be using the film filtered, you should be able to do the reciprocity failure test unfiltered, just to see how the emulsion responds to long exposures, and determine what adjustments you need to make from your calculated exposure.

 

[olwick]

Without an IR filter, but possibly a 10 stop ND filter.

I like night photography

 

[polyglot]

A 10-stop filter during the day? Or at night?

And if without an IR filter at night, why are you using IR film? Is there some particular effect it gives you that a normal panchromatic film won't? Because the ratio of IR to visible light will be even lower at night than during sunny conditions unless I'm mistaken or you're scene-painting with IR LEDs or something.

You have me intrigued.

 

[2F/2F]

Why do you want to use ND at night? I have easily got 15 minute to one hour exposures with a fast film at night without using any ND. Are you looking to use very narrow depth of field or something?

 

[keithwms]

10 stop filter eh. Well you are going to find yourself doing hour+ exposures. If that's what you want...

Frankly I would not waste the film on this.... keep it for IR. If you want long exposures without an IR filter then why not just use hp5+ or tri-x. If you want less recip. failure then try acros.

 

[olwick]

There's a particular look I'm going for. The 10 stop filter would likely be used during the day/evening.

The Data sheet does not say Testing Required, by the way. It just says:

Reciprocity effect
Reading of exposure in sec.: 1/10,000 - 1/2
Exposure compensation in steps: N/A Compensation of development time: N/A

 

[keithwms]

Like I said, it's not a 'modern' t/epitaxial-grain film, you're probably going to get something like tri-x.

I have easily got 15 minute to one hour exposures with a fast film at night without using any ND.

Agreed.

What do you want in terms of highlight/shadow rendition? Let me suggest POTA. It's going to cost you several stops of sensitivity (which apparently you would like) plus retain a very wide range of tones.

 

[2F/2F]

If there is any currently-made b/w IR film to use at night for unique effects, it is Ekfe IR820 Aura, or even plain IR820. The Efke will be unique in its grain and in its IR sensitivity, and the Aura will be the same, plus unique in its halation. Anything else won't do anything notably different from a regular panchromatic film at night...and certainly not unless filtered.

The Rollei IR will not look like Tri-X when shot unfiltered...not at all. It will look more like Neopan or T-Max. It is a very "modern-style", technically amazing film. It's very sharp and contrasty. Beautiful...but nothing you can't get with Neopan or T-Max, with much better reciprocity to boot.

By all means; try it, and then let us know. I am sure the shots will look good...but do be aware that it really won't be that unique, and it is expensive.

 

[Lee L]

Since you already have a 10 stop ND filter, testing for reciprocity failure is easy.

Set up a shot under the type of illumination you'll be using for the photos you'll be taking, and at a level that gives you a 1/8 second (0.125 second) exposure. Make an exposure at that setting. Now put on the 10 stop ND filter and make a series of 125 second exposures (10 stops more exposure, 1000X) while you bracket for increased exposure by opening the lens aperture over a range of up to 3 stops or so. Keep notes. Process the film and find which bracket with the ND filter in place matches the density of the 1/8 second exposure. A gray card or gray scale in the shot is helpful.

I won't do all the math here, but the simplified version is this:

Find the Schwarzschild exponent p, where

p = 1 - (number of extra stops exposure needed/7)

p should be less than 1.

Example where you need 2 stops extra exposure with the ND filter and 125 second exposure to match the 1/8 second density:

p = 1 - (2/7)
p = 1 - 0.2857 = 0.7143

p is then used to calculate reciprocity adjusted exposure time with:

corrected exposure time in seconds =( (metered time in seconds + 1)^(1/p) ) -1

You can plug this formula into a spreadsheet using your value for 'p' and make a table of adjusted times.

A couple of caveats with this method.

Some ND filters pass significantly more IR light than they do at visible wavelengths. B+W glass filters and Kodak Wratten gels have been found to work well out through about 650nm or so.

Be sure to use the same type of lighting for testing that you would for the real shots. Testing under tungsten and then shooting under daylight won't work.

This method is described in Michael Covington's book on film Astrophotography, but there is a typo in the equation for the more complex generalized equation for deriving 'p', so be sure to check the errata page at covingtoninnovations.com for this particular book before using that method.

Lee

 

[Lee L]

Now put on the 10 stop ND filter and make a series of 125 second exposures (10 stops more exposure, 1000X) while you bracket for increased exposure by opening the lens aperture over a range of up to 3 stops or so.

Typo. Should be:

"...make a series of 128 second exposures..."

otherwise you'll be off 2.4%.

Lee