Calculating Reciprocity with a Red #25 Filter

aag_7 · Oct 31, 2019

I am new to this site (first post) so go easy on me if this isn't properly formatted. I have been shooting film for about a year and a half and I am starting to take it a little more seriously which has led me down a rabbit hole.

SO, my question has to do with the use of a red #25 filter while taking a long exposure in the ball park of 30 seconds using either Tri-X 400 or T-Max 100. I know that the red #25 filter has a filter factor of three stops. I also have been able to find charts that show the adjusted exposure times for reciprocity for each film without the filter (T-Max 30 sec increases to 49 seconds and Tri-X increases from 30 sec to 1:18 from what I have been able to find).

What I do not understand is how using a red filter that has a three stop filter factor affects these exposure times. Would I increase the initial 30 sec exposure to 240 seconds (3 stops) and then account for reciprocity based on those times or is that incorrect? Also, I know that images get more contrasty as the exposure time goes up and that development times may need to be adjusted in lieu of this. Would the red filter worsen that effect?

If you could help out with any of those questions I would greatly appreciate it. I have been scouring the internet but I can't find a good answer for my question.

GLS · Oct 31, 2019

aag_7 said:
Would I increase the initial 30 sec exposure to 240 seconds (3 stops) and then account for reciprocity based on those times or is that incorrect? Also, I know that images get more contrasty as the exposure time goes up and that development times may need to be adjusted in lieu of this. Would the red filter worsen that effect?

Yes, calculate the base exposure accounting for the filter factor, then further correct that figure to adjust for reciprocity failure.

The inherent contrast change from using a red filter will vary depending on the colours contained in the scene, but in more general terms anything which lengthens the exposure will create a more contrasty result, because the brighter and the darker areas in the exposed image will experience different levels of reciprocity failure (i.e. most in the shadows, least in the highlights).

Kino · Oct 31, 2019

You could try to tame the contrast with a compensating developer, but that might be too many variables in the mix to start.

Test, test and more testing; unfortunately you have to use a lot of film to learn in these uncertain backwaters of photography.

GLS · Oct 31, 2019

Situations like the above is where Acros really shines. It requires no correction at all for up to 2 minutes, and only +1/2 a stop beyond that. We can but hope Acros II (when it appears) will also have this property.

aag_7 · Oct 31, 2019

Thank you GLS and Kino for your answers! I am glad to hear that I was on the right track. I will just have to shoot it, see how the filter affects it and then adjust from there. During my research I also stumbled across Acros only to be let down when I saw it was discontinued. Not having to account for reciprocity sounded like a dream come ture. Hopefully Acros II will live up to its predecessor. Thank you for the help gents.

alentine · Oct 31, 2019

aag_7 said:
... and Tri-X increases from 30 sec to 1:18 from what I have been able to find) ...

Hello and welcome.
That's very close to HP5 film.
This page could improve your correct expectations about the total time, it includes old and new emulsion films.
https://www.ilfordphoto.com/wp/wp-content/uploads/2017/06/Reciprocity-Failure-Compensation.pdf

aag_7 · Oct 31, 2019

That's very close to HP5 film.

Just to make sure, the tri-x exposure length would be similar to HP5 and I could use their formula when calculating shots using tri-x?

GLS · Oct 31, 2019

aag_7 said:
Just to make sure, the tri-x exposure length would be similar to HP5 and I could use their formula when calculating shots using tri-x?

It will be pretty close. If you work out the correction for a 30 second exposure with HP5 it comes to 1:26.

240 seconds base exposure with your red 25 filter will mean an approx. 22 minute corrected exposure based on the HP5 data. You could round that down to 20 minutes to account for the slightly lower factor of Tri-X.

I really like that table of reciprocity factors Ilford recently provided. It makes calculating precise exposures very easy, and I wish all manufacturers would do the same.

alentine · Oct 31, 2019

aag_7 said:
Just to make sure, the tri-x exposure length would be similar to HP5 and I could use their formula when calculating shots using tri-x?

Could not be sure honestly specially in situations like this.
But, both films has more resemblance more than any other two films. Same era emulsion.
I think, for your scenario you may consider HP5 times, because 30sec is just the initial exposure(240sec as corrected for #25 red filter).
Then you need to apply ilford formula: 240^1.31 = 1312sec = 22 min.
Develop for N-1 and not to forget to pray much.
As far as I know, 4 minutes extended to 22min(2.5 stops) is not that much and you may discover that you do not need pull processing.

alentine · Oct 31, 2019

Plus,
Three other points may considered:
1. Total time of 22min is never be the final corrected time. At that point of time, your film has gone extremely insensitive. May or may not need extra correction!
Effectively, your 400 ISO film is acting around ISO 8 !
2. Subject lighting through a red filter, film response to light spectrum reflected by subject.
Generally with no filter, the contrast will increase. This is the reason for pull processing after enough or more than enough correction of time.
Then, depending on your subject, the contrast may increase further or retain the basic effect on contrast without filter.
I think pull processing is indicated.
3. I do not know any exposure meter that can go below -2 EV. Consider all times below this EV are just estimations, not measured.

aag_7 · Oct 31, 2019

Thank you so much for all of that information. Going to have to look into the things you mentioned, it sounds like I have alot to research to do!

GLS · Dec 24, 2019

alentine said:
3. I do not know any exposure meter that can go below -2 EV

Just noticed this. FYI there are some meters & cameras which can do this. For e.g. the Nikon D5 can measure down to -3EV, and the Sekonic L-858D can measure down to -5EV in incident mode.

Sirius Glass · Dec 25, 2019

Three stops. I have used multiple meters to read through the filters and they are just not accurate.

Calculating Reciprocity with a Red #25 Filter

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