How to test reciprocity failure?

[menglert]

How do you go about testing for reciprocity failure? I’m interested in doing some long daylight exposures (15-30min) with b&w films.

Thanks,
Martin

 

[Lee L]

There's a very brief description of one method here: http://www.robertreeves.com/filmtest.htm

Look about three paragraphs down the page. The filter Reeves recommends is the B+W 110 because of it's spectral transmission curve. Some brands of ND filters become uneven across the spectrum at this high a density.

For greater detail on this method, which involves matching film densities with and without the ND filter, see Reeves' book Wide-Field Astrophotography or Covington's Astrophotography for the Amateur (covingtoninnovations.com). Basically, you shoot a gray card/gray scale at 1/128 sec (i.e. 1/125) with no filter and then again at 128 seconds with the ND filter which requires 1000 times as much light. Bracket in 1/3 f-stops to about 3 stops wider open than the no-filter shot and then find the exposure that yields equivalent density to the no-filter shot. That's your speed loss in stops, which you can use to calculate a reciprocity factor to apply. You might find the Covington book at a local library.

If you have a target exposure length, it might be better do test with a filter that gives you a resulting test exposure nearer your target times. Just as information, I recently ordered a B+W 113 ND filter only to be notified that it was discontinued.

Lee

 

[gainer]

The firdt step is tp look for existing data. Howard Bond did a series of tests for an article in Photo Techniques which were meticulous and covered a wide range of exposures of Tri-X, HP5+, FP4+, TMX and TMY. I forget what issue, but I think PT has an index of back issues. I did a curve fitting analysis of his data which was published in a later issue. You can also find that in www.unblinkingeye.com.
Howard's article is valuable not only for the data but for the complete description of how he got it, so you could find your own data for films he did not cover.

 

[Lee L]

This thread: (there was a url link here which no longer exists)
has a lot of good discussion of reciprocity issues, including Gainer's formula for calculating reciprocity adjustments (calculated from the Bond info he mentions in his post above) and the Covington revision to the classic Schwarzschild formula. It also has pointers to other resources and some charts you might find helpful.

I don't know the issue of Photo Techniques that the Bond article was in, but it's probably well worth searching out and reading. You can look here: http://www.phototechmag.com/back_issues.htm and I'd guess Vol 24, either number 4 or 5.

Lee

 

[Roger Hicks]

How do you go about testing for reciprocity failure? I’m interested in doing some long daylight exposures (15-30min) with b&w films.

Thanks,
Martin

Dear Martin,

Remember that the penalties for overexposure are much smaller than the penalties for underexposure, and than many will cheerfully accept these penalties (bigger grain and reduced sharpness) in return for better tonality.

With 35mm, the penalties may (or may not) be significant. With anything bigger than 35mm, a stop or two (or even three) extra will probably not have signifcant adverse effects.

Start out with the manufacturers' times, and give +1 and +2 stops over these (or start at +2 and cut later).

Cheers,

R.

 

[menglert]

Thanks for all your input. Perhaps I'll just get out there and try some times and see what works best. I'm not sure if I have the time or money to put into properly testing for RF.

Regards,
Martin

 

[gainer]

For goodness sake, first look at the references to see if someone has already done that for you!

 

[menglert]

For goodness sake, first look at the references to see if someone has already done that for you!

Well, I've already checked to see if someone has done this for the films I'm working with, and no, there was not information posted for the films and times I'm interested in using.

 

[Lee L]

You've given the exposure times you're interested in, maybe if you mentioned specific films people here could offer something more.

There is an online pinhole forum, f:295.org that would be a good place to find people with experience using long exposures and a variety of films.

I'm not aware of manufacturers testing emulsions at 15-30 minute times.

Here's another link to a testing method under moonlight (about the exposure times you want) and test data for several films: http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/reciprocity.htm

Lee

 

[menglert]

I guess I'm paticuarly interested in EFKE 25. I have found times out to 120 sec, but beyond that haven't found any data.

Thanks again,
Martin

 

[gainer]

If you will tell me the published actual times corresponding to two measured times, I will give you an estimate of the actual exposure time for any other measured time and an equation for estimating any other exposure time.

 

[Lee L]

Martin,

Andrew O'Neill reported his Efke 25 tests in this thread: (there was a url link here which no longer exists) referring to this post: http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=008bb4

He found a lot lower failure rate than the published info at JandCphoto.com

If you go to the reciprocity thread I posted earlier, you'll find references to Gainer's formula and Covington's modification of the Schwarzschild formula.

I did a quick spreadsheet and matched the suggested adjustments from Andrew O'Niell's results closely out to the adjustment for 100 seconds in both the Schwarzschild and Gainer formulas and got the following factors:

Gainer multiplier = 0.0287
Schwarzschild exponent = 0.92

This gives recommended adjusted times that differ about 2/3 stop at your 30 minute target exposure time, but it should get you started. You might want to plug the formulae into a spreadsheet and use that to manage some fine tuning.

Gainer method:
Adjusted time = 0.0287*(metered time^1.62) + metered time

Covington/Schwarzschild method:
Adjusted time =(metered time +1)^(1/0.92)-1

Lee

 

[menglert]

Thanks a lot, those formulas should get me started.

Regards,
Martin

 

[Lee L]

Thanks a lot, those formulas should get me started.

Regards,
Martin

You're welcome Martin. This is extrapolation pretty far beyond the data, but both equations have matched observed behavior well (within a fraction of a stop) with a large variety of films. Fine tuning out in your target range should put you in good shape.

Please report back on your results.

What method are you using to get such long exposures in daylight?

Lee

 

[gainer]

Might be a pinhole camera with a deep red filter? Joking of course. I'd like to hear it too.

 

[menglert]

Right now I have an ND400 (9-stops I think), and a red filter, combined with stopping down. Although, I haven't tested it yet, so I'm not sure if I can get the long exposures I want during daylight, but will play around with it a bit.

Regards,
Martin

 

[menglert]

After reading a bit more, I'm doubtful I'll get my goal exposures of 15-30min, with what I currently have. Although, I'm open to any other suggestions of how to increase exposures.

Regards,
Martin

 

[Donald Miller]

After reading a bit more, I'm doubtful I'll get my goal exposures of 15-30min, with what I currently have. Although, I'm open to any other suggestions of how to increase exposures.

Regards,
Martin

Begin with a very slow film. If Iwere wanting to try something like this and were shooting large format, I would shoot
APHS (Ortholith film EI 3-6, Freestyle Photo)...see Jim Galli for his rodinal formula with this film.

Next stack two or even three polarizers. By adjusting them individually and alternating their orientation, you will come up with some pretty lengthy times. I would suggest metering through the polarizers to come up with a filter factor.

Next stop your lens down (F45 or even 64 if your shutter will accomodate) and after adjusting for reciprocity this should get you into the ball park of what you are wanting.

Good luck.