FP 4 reciprocity.

Dorothy,

Here's a formula derived from the chart on the Ilford Data Sheet for Ortho Plus. Please note that Ilford's reciprocity data from other products has been proven to be very much out of date.

adjusted exposure time = 1.306 * metered exposure time^1.228 + metered exposure time

Here's the data sheet: http://www.ilfordphoto.com/Webfiles/2011427119221450.pdf

In the formula supplied * is multiply and ^ is raised to a power

Hope you're doing well. It's been too long.

Lee

djkloss said:

I suppose there's a similar way of calculating for Ilford Ortho Plus IF the number were available? That would be so helpful. I'd much rather use the formula than a graph. fwiw, I have searched the internet and have found very little regarding this film and its' reciprocity

Thanks,
Dorothy

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philldresser said:

I have been told by a reliable source from Ilford that to get the correct exposure on FP4 when under reciprocity failure conditions that you multiply the selected exposure time (in seconds) to the power of 1.49.

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That contradicts the reciprocity correction curve published in the Ilford datsasheet by quite a bit.

Rel exposures for metered from 1 sec to 35 sec vary from 1 sec to 175 seconds, and it's not a straight line.

The datasheet can be downloaded from Ilford's website.



- Leigh

I find their data to be on the heavy side.

Andrew O'Neill said:

I find their data to be on the heavy side.

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That is of significance only to you, based on your shooting habits and processing methodology.

I dare say Ilford has spent more man-hours developing their data set than you've spent breathing.

- Leigh

Here's a plug for Ralph Lambrecht's Way Beyond Monochrome, 2nd Ed. There is a table on p.188 showing results of his FP4+ reciprocity tests (that he combined with results from Howard Bond & John Sexton). Also indicated are theoretical reductions in dev time to counter the increase in contrast that occurs with increasing exposure corrections. It's provided as a starting point for one's personal testing. A good data source for photography in general, to be sure.

Leigh B said:

That is of significance only to you, based on your shooting habits and processing methodology.

I dare say Ilford has spent more man-hours developing their data set than you've spent breathing.

- Leigh

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I'm using data that I generated myself and have been using for years. That is what people should do, rather than using manufacturers' charts. Curves don't lie. It's also a known fact that manufacturer's data is overly generous.

Andrew O'Neill said:

Curves don't lie.
It's also a known fact that manufacturer's data is overly generous.

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You realize you just contradicted yourself.
And to whom is their data overly generous? Not to me.

Personal curves are relevant to each individual because they calibrate out the errors in your equipment and process.

That does not in any way invalidate the manufacturers' curves.
It just quantifies your errors.

- Leigh

I most likely did contradict myself and I probably shouldn't write anything in haste and before morning coffee...but, the fact of the matter remains. Manufacturer's data is exceedingly generous. You are the only person I have heard of that uses them at face value. I'll just let you know that my curves were generated under an enlarger, bi-passing my camera, lens, then done again in camera for comparison. The curves are identical.

Check some of the older reciprocity threads here. Ilford's reciprocity data has long been out of sync with user experience. The charts in their literature are very much generic, as is the raising to the power of 1.49, which has been the same since the late 60's. The films have since changed. The last careful testing I've seen done (with methodology explained) was Robert Reeves in the late 90's in his book on Astrophotography.

Based on his info the following formula should be used:

adjusted exposure time = 0.5557 * metered exposure time^1.326 + metered exposure time

Ralph Lambrecht's conventional film formula suggested for FP4 yields:

adjusted exposure time = 0.3178 * metered exposure time^1.5215 + metered exposure time

Plot those out and you'll find they don't diverge significantly until after 240 seconds of metered exposure time.

If you check the older reciprocity threads, you may recognize that I'm using a modification of Pat Gainer's equation for reciprocity based on Howard Bond's very careful work with multiple films published in the 90's in PhotoTechniques. Two of my friends here on APUG requested FP4+ reciprocity info from me in the past, and the first formula I give above worked extremely well for them.

If you check Ilford's newly published (Aug. '17) reciprocity data, explained in this thread: https://www.photrio.com/forum/index.php?threads/ilford-film-reciprocity.153342/ , you'll find that the older Ilford charts were not very accurate. The newly published Ilford factor for FP4+ based on testing current emulsions matches the first formula in my post preceding this one well within 1/5 stop after metered exposure times of 8 seconds. The difference in exposure between Ilford's old and new factors is significant. Thanks to Ilford for doing the work and publishing up to date info.

Foma's film spec sheet has an excellent explanation in layman's term.