Agfa APX 100 reciprocity....

[rob3rt5]

Hello all:
I just purchased 150 rolls of Agfa APX 100 film that I plan to use @ 80 ISO processed in Rodinal. I was wondering if anyone has an exposure time compensation table for long exposure with this film?

 

[rob3rt5]

Agfa states that for exposeures of 1sec to add +1 to f/stop, 10sec to add +2, and 100sec to add +3. What do I do if I don't want to change the f/stop? What if my time is 30min?

 

[walter23]

If your time is 30 minutes, then with reciprocity you're probably getting into several hours. I mean even if it's a straight factor of +3 you're looking at 4 hours, but it's likely more (8+ hours).

In those situations I usually just expose as long as is made practical by other considerations (how long can I wait? is the camera unattended somewhere? Will condensation form on the camera because I'm outside somewhere? Will the sun come up?).

 

[Lee L]

Astrophotographers use a Schwarzschild exponent to characterize reciprocity failure, but before we go there, be forewarned that most people who actually test films in the real world find that the manufacturers' published reciprocity adjustments are just a "shot in the dark" (to use an English idiom) as opposed to a real photograph shot in the dark and checked for speed loss.

Since you have a loss of 3 stops in 100 seconds, the standard calculation gives a Schwarzschild exponent of 0.55, extremely low by any standard. Even the worst of the now common films are about 0.65, with Acros being close to 1.0, which is no reciprocity failure.

To calculate a longer exposure corrected for reciprocity failure using the Schwarzschild exponent, designated as p, use the following equation:

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

This is a Covington modified Schwarzschild formula to produce more accurate adjustments at shorter time exposures in the range of a few seconds to 60 seconds or so.

I'd bracket. Try exposures based on p=0.55, but also use a couple of times in the range of p=0.65 to 0.80 and compare the negative density to a standard fraction of a second exposure of the same object. Keep good notes. This will get you to the right correction range fairly quickly.

I'd also suggest trying Gainer's method, and testing to find the coefficient a in his formula:

corrected time = metered time ^ 1.62 * a + metered time

Instructions for that are here: http://www.unblinkingeye.com/Articles/LIRF/lirf.html

a=0.6 might be a good place to start with the Gainer formula for APX 100.

You can easily make a spreadsheet table of adjusted times for yourself using the above formulae. If you don't have one, openoffice.org has an excellent free office suite with a spreadsheet that runs on pretty much any computing platform.

Lee

 

[rob3rt5]

Thanks Lee for your reply. I rather suck at math and the equations are a little beyond me it seems. I keep getting these wildly off base answers.

 

[rob3rt5]

for those that might be interested in a table:
Reciprocity Table for Agfa APX 100

1 sec = 2 sec
2 sec = 4 sec
4 sec = 11 sec
8 sec = 29 sec
10 sec = 39 sec
20 sec = 1 min 47 sec
30 sec = 3 min 15 sec
40 sec = 5 min
50 sec = 7 min
1 min = 9 min 12 sec
1 min 10 sec = 11 min 38 sec
1 min 20 sec = 14 min 14 sec
1 min 30 sec = 17 min 3 sec
1 min 40 sec = 20 min

If anyone has a better table please let me know and post your findings for myself and others- thanks.

 

[Lee L]

Robert,

Where did you find that data? It's a Schwarzschild exponent of 0.65, approximately a Gainer coefficient of 0.59 (see my earlier post).

That's equivalent to a loss of 2 1/3 stops in a 100 second exposure, not the Agfa claimed 3 stops, and likely a lot closer to real world performance.

Lee

 

[RobertV]

http://www.aphog.de/downloads/agfa/apx100.pdf

 

[rob3rt5]

Lee the posted table was for Kodak TriX and should be the same for Agfa APX 100 according to a few sources.
I'm sorry man, but the math was way too much for me to do and set up a table- so it necessitated a rethinking of the problem i.e. find a film with the same tc.
Now, I admit that this may be a gross miscalculation on my part but, it sounded sound to me. I welcome any reworking of the data to come up with a more accurate table.

 

[rob3rt5]

http://www.aphog.de/downloads/agfa/apx100.pdf

Thank you so much Robert. I was aware of the Agfa data but find that the Gainer approach is more accurate for time values than anything else. It takes into account the non-linear nature of reciprocity.

 

[Lee L]

Well, the Agfa data is also non-linear, but you can be pretty certain that it overstates the amount of compensation needed, likely to a degree that it would lead to poor results at longer exposure times. There are lots of ways to be non-linear, and a number that work reasonably well for reciprocity corrections within the margins of error we typically work with.

APX 100 may or may not be like Tri-X as regards reciprocity failure. Tri-X isn't like Tri-X either when you're talking about different vintages and formulations, and as others have stated, experimenters have found significant variation from batch to batch with B&W films.

Rob3rt5, who came up with the data you give, and how? On the internet at least, "a few sources" are very often the same source, not attributed, and repeated in lots of places, which makes it appear to be commonly accepted and verified knowledge. And yes, you may call me a skeptic.

Testing and careful notes are what's needed, but there's very little of that to be found floating around.

Lee

 

[rob3rt5]

The calculations were from Gainer himself on another site. He provided the table listed here for TriX.
I thought it was funny that a table was provided for this film in a discussion originally posted for Agfa APX 100 reciprocity- but, there you are.
As I remember, you contributed to that discussion as well.

 

[Lee L]

Thanks for the reply Rob3rt5.

Let us know what your results are.

Lee

 

[emiliohm]

Hi rob3rt5, I have tested your table with some rolls and I think your times are a little bit short.
I will try to locate another table and publish it here.
Regards

 

[emiliohm]

I have found Dead Link Removed
I´ll try. Thanks