Fomapan reciprocity failure

[Lee L]

Well, here's a fit to Maris' findings for Fomapan 200 with my favorite formula for reciprocity, the third formula mentioned in my earlier post #19 to this thread:

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

where
a=1.0728074
b=1.4366099

and the numbers compared:
metered.....Maris.....calculated
1..............1.5............2.1
2.................4............4.9
3.................7............8.2
4...............12..........11.9
6...............18..........20.1
8...............28..........29.3
10.............40..........39.3
14.............48..........61.5
20...........100..........99.4
30...........175.........172.1
50...........350.........346.0
100.........900.........901.2

Pretty decent fit.

Lee

 

[smn]

Based on my very unscientific "test" with Fomapan 200 under street lights. 4 seconds metered seems to correspond to 12 seconds of exposure.

 

[Iain Maclachlan]

Fomapan 100 reciprocity

I know this is an old thread but has it been determined what the true reciprocity is for Fomapan 100? Using the suggested adjustment for Fomapan 200 I find a Geiner coefficient of about 0.55. This implies an adjusted exposure time of 24 secs for 8 secs measured, compared with an adjusted time for FP4+ of 11 secs (ie. about double). For Tri-x it would be 13 secs. Any experiences to confirm?

 

[Xmas]

Fomapan 200 reciprocity characteristics are supposed to be easy to test for so I tried. Well it wasn't so straight forward but three 120 rolls later here are some results:

The exposure level I chase in landscape metering is Zone IV = shade side of tree bark, dark rocks, etc. The question becomes "what exposure time extensions must I give for long exposures so I get the same negative densities for Zone IV as at short exposures?" Answers:

Measure 1 second on the meter...give 1.5 seconds
Measure 2 ... give 4
Measure 3 ... give 7
Measure 4 ... give 12
Measure 6 ... give 18
Measure 8 ... give 28
Measure 10 ... give 40
Measure 14 ... give 48

Because I don't own a densitometer the above values are eye approximations arrived at by comparing actual pieces of film side by side on a light box. I am confident I can pick when two densities are the SAME even though I don't know their absolute values.

An experimental constraint that could have been a confounding factor is that the long exposures are made up of several shorter exposures. For example, a forty second exposure is generated out of four ten second exposures. I can't find anything in film technical literature to suggest that a "intermittency effect" is harming my experiment.

I have investigated longer exposures with preliminary results being:

Measure 20 seconds...give 100 seconds
Measure 30 ... give 175
Measure 50 ... give 350
Measure100 ... give 900
These numbers have a more approximate character (wider interpolations) and I want to repeat the experiment some time. The difficulty is achieving close density matches with widely spaced exposure times. Would you believe that doing a long series of consecutive 15 minute exposures is a tedious way to lose an afternoon. You bet.

For completeness I should get around to re-doing these tests with Zone VIII as a target density. Maybe the results will be different. Another thing not measured here is the tendency for reciprocity failure to stretch the contrast of a scene; the bright bits build negative density a lot faster than the lethargic dim bits!

Anyway I have some numbers I believe and I'm going to shoot lots of Fomapan 200 8x10 format at some longish exposures. And I won't be bracketing.

The Foma data sheet says sheet film has a different spectral sensitivity than 120 that may make a big difference to reciprocity, it may show wood effects with deep red filter.

 

[IanBarber]

Well, here's a fit to Maris' findings for Fomapan 200 with my favorite formula for reciprocity, the third formula mentioned in my earlier post #19 to this thread:

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

where
a=1.0728074
b=1.4366099

and the numbers compared:
metered.....Maris.....calculated
1..............1.5............2.1
2.................4............4.9
3.................7............8.2
4...............12..........11.9
6...............18..........20.1
8...............28..........29.3
10.............40..........39.3
14.............48..........61.5
20...........100..........99.4
30...........175.........172.1
50...........350.........346.0
100.........900.........901.2

Pretty decent fit.

Lee

I have just acquired a box of FomaPan 200 for a trip I am going on Monday and don't really have sufficient time to start doing some serious testing for the reciprocity.

Having read this entire thread more than once, I am now starting to get quite confused

The chart above shows two different results from what I understand to be two different forms of calculations but looking at them side by side, they are pretty close.

Can anyone confirm that the times above are fairly accurate please

Ian

 

[awty]

I have just acquired a box of FomaPan 200 for a trip I am going on Monday and don't really have sufficient time to start doing some serious testing for the reciprocity.

Having read this entire thread more than once, I am now starting to get quite confused

The chart above shows two different results from what I understand to be two different forms of calculations but looking at them side by side, they are pretty close.

Can anyone confirm that the times above are fairly accurate please

Ian

Hi Ian
I use a phone app simply titled "Reciprocity "
It has most common films, allows for filters and ballow extention among other stuff. Even comes with a timer.
Simply work out your exposure time and it does the rest.
Accurate on all films I have used. Provided my initial time is accurate.
Found most other ways to be hit or miss.

 

[IanBarber]

Hi Ian
I use a phone app simply titled "Reciprocity "
It has most common films, allows for filters and ballow extention among other stuff. Even comes with a timer.
Simply work out your exposure time and it does the rest.
Accurate on all films I have used. Provided my initial time is accurate.
Found most other ways to be hit or miss.

Hi Paul

Thanks for the information, I have just installed it and will go with the times it mentions.

 

[David T T]

Wow, this program looks amazing, thanks! Installed!

 

[chris77]

indeed!!

 

[radiant]

I try to find reprocity for Fomapan 400. The film spec says:

From 1-10 seconds multiply the time by 1.5 - so times ranging 1.5 seconds to 15 seconds.
From 10-100 seconds multiply by 6 - what? That means 11 seconds become 66.

So 10 seconds -> 15 but 11 seconds to 66? Quite a jump.

Any smarter formulas welcome

 

[Andrew O'Neill]

Doesn't take much time to conduct reciprocity tests yourself, where none exist.

 

[radiant]

Doesn't take much time to conduct reciprocity tests yourself, where none exist.

Any tips how this can be done without densitometer is a reliable way?

 

[DREW WILEY]

Old thread. About all I can add to this, if it already hasn't been stated numerous times before, is that Foma 200 has wretched recip characteristics. It's also nowhere near 200 realistic speed. That makes it a very bad candidate for even modestly long exposures. The 400 speed product isn't good in that respect either. If you're smart, don't use a Smartphone app - pick a different film !

 

[Andrew O'Neill]

Any tips how this can be done without densitometer is a reliable way?

If you have a stepwedge, like a Stouffer 31 (third stop increments), you can contact print it to a piece of film under an enlarger... or in camera. I shoot large format, so that is feasible. Expose four sheets. One for about 1/4 sec (this will be your reference neg), one at 1 sec, 10 sec, and 100 sec. When I use the enlarger for a light source, I use ND filters instead of closing the aperture. You can use your lens' aperture, if they are truly accurate. After the film is developed and dried, I compare the long exposure negs to my reference neg. Match the step on the slow shutter speed negs which shows about .10, to your reference neg's .10. If you made your reference negative correctly, .10 is probably about three steps in. Once you've matched all three to your reference, count how many steps different. One step equals a third stop. That's how much more exposure you need to give. To get compensation for all times up to 100sec, make a graph. Go out in the field and test you data.

 

[revdoc]

I use the attached formulae.
I precalculated metered time vs. actual time, then printed it out for field use.
BTW, the rated speeds for Foma films tend to be too high. I generally halve them.

 

[radiant]

I use the attached formulae.
I precalculated metered time vs. actual time, then printed it out for field use.
BTW, the rated speeds for Foma films tend to be too high. I generally halve them.

Huge thanks for the formulas! I calculated few values with Foma 400 with those values, see attachment. Do you mean you measure the original exposure with ISO 200 setting and then calculate reprocity from the time you got from ISO 200 exposure?

I think Foma 400 blows the highlights pretty easily so I have not been too keen to overexpose it. Probably it's just not possible to prevent the highlight blowing so just save the shadows!

 

[RalphLambrecht]

Look up:

www.unblinkingyey.com/Articles/lirf

There are curves for a number of modern films from data by Howard Bond. You will see that all curves have the same slope within experimental error, and are straight lines on log-log paper. If you know the amount of exposure time to be added to any measured exposure time to correct for reciprocity, you can draw the line through that point on the chart parallel to the basic line. That line has 1.62 inches of rise per inch of run.
Somewhere on this forum I saw a link to a site where you can download log-log and other types of graph paper.
I advise not messing with f-stop corrections. For one thing, it messes up your depth of field. For another, it messes up your brain. Dedicate a roll of film to the task. Set up a low light situation and start with the nominal exposure, increasing the exposure time each frame by the same ratio. 2 is a good starting ratio. Develop the roll and find the frame that satisfies you most.

fully agree; it's best to correct for lo-intensity reciprocity failure by extending time not opening the aperture.as Pat explained; testing for it is fairly easy. any graphing program will allow for extrapolation.

 

[DREW WILEY]

An enlarger and step wedge test can be misleading unless you've balanced the colorhead to daylight color temp. I'd rate the 200 version at 100, or none of this will add up realistically. Actual shooting tests in advance will yield a better indicator than mere formulas. Add strong contrast filters and exp times will be glacial. You need to test for that kind of thing specifically too, with each respective filter you might potentially use. Published filter factors can vary between films, and can significantly change between themselves at long exp times anyway. Night photography introduces special problems due to the probability of unrelated light source spectra in the scene, so there is no substitute for experimental testing in the field, under analogous conditions.

 

[Ian Grant]

An enlarger and step wedge test can be misleading unless you've balanced the colorhead to daylight color temp. I'd rate the 200 version at 100, or none of this will add up realistically. Actual shooting tests in advance will yield a better indicator than mere formulas. Add strong contrast filters and exp times will be glacial. You need to test for that kind of thing specifically too, with each respective filter you might potentially use. Published filter factors can vary between films, and can significantly change between themselves at long exp times anyway. Night photography introduces special problems due to the probability of unrelated light source spectra in the scene, so there is no substitute for experimental testing in the field, under analogous conditions.

Agree it's al; misleading, and also that the Box speeds are way off.

Reciprocity isn't as simple as some complex equations suggest. Films have two ISP speeds Daylight and Tungsten anbd the difference depends on their spectral responce, so FP4 is 125 ISO in DAylight and 8o ISO in Tungsten light, Ortho Plus is 80 ISO in Daylight and 40 ISO in Tungsten light.

Practical tests and experience are the only sure way to get a correct exposure in low light levels. So a low light level daylight sot is a world away from a low light interior shot and although Tungsten bulbs are now rare their better replacements have similar spectral output.

Fomapan practical tests

Just did some practical tests and the reciprocity with Fomapan 200 was not at all like the published data.

At 1 second it only only needed around half a stop (recommendation is 1.5 stops) and at lower light levels 10 seconds it was about a stop (not the 3 stops recommended). These test were made in poor daylight 1 second @ f8 100 EI and very low interior lighting 10 seconds @ f8. These are the conditions the film will be used in.

Still need to test how the film behaves with nigh shots, but it seems to be only a little worse than HP5 / Tri-X for reciprocity failure.

Ian

This was my 10 year old reply from earlier in the thread and those tests have proved correct, over the years however I should add this is low daylight levels.

So say the Foma data refers to long exposures with Tungsten lighting, their box speed is off by 1 stop (which most people agree with) then perhaps their Reciprocity table is fine

You (by that I mean we) need to know your films, anyone else's recommendation has to be evaluated carefully. I'm confident enough to trust my own tests and buy and use Foma films, in fact will shortly be buying Foma films in35mm to larger formats, and in particular 7x5 and 10 x8.

Ian