# Ilford formula or Geiner formula for hp5 reciprocity?

Discussion in 'Black and White' started by jos__garese, Jul 23, 2013.

1. ### jos__garese

I am planning some night shooting and did my homework before posting a question on how to calculate reciprocity failure for hp5: I
searched the photo.net and the apug forums. However, there were two answers:

Some people recommend Ilford's formula: http://www.photo.net/film-and-processing-forum/00JC3f?start=10
Other people recommend Geiner's formula: http://www.apug.org/forums/forum37/95877-reciprocity-failure-correction-gadget-gainers-formula-05-efke-pl100m.html

Apparently, Ilford's formula, and the chart they provide in the data sheets, are old and have not been corrected for contemporary Ilford
films. Is that so?

The resulting adjustment recommendations vary a lot. For example, if my metered time is 30 seconds I would have to expose for:

According to Ilford: 30 ^ 1.48 = 154 seconds
According to Geiner: 30 + 0.101 * (30 ^ 1.62) = 55 seconds

Those figures are WAY apart.
Of course, I have to shoot and test and blah, blah, blah. And I will. But the whole purpose of this is going out with some point of
departure. Forgive my poor maths (and do point out where I went wrong should that be the case)

Opinions?

2. ### NHSN

It is not way apart, it is about one stop when considering the failure. I'd suggest you trust the
official Ilford HP5 plus datasheet as the starting point.
I have never managed to overexpose a evenly lit night scene (to the point where it is not
printable), and it is better to err on the "over exposure" side than under exposure.

4. ### john_stockdale|2

That's a good article, Sal. Thanks for posting it.

5. ### chris_waller

By the formula I use the Ilford figure is a little high and the Geiner figure is low! Using my formula, I get 120 seconds actual exposure time for a metered reading of 30 seconds.

6. ### NHSN

I don't think it is beneficial to look at this as too much of an exact science. I do understand what
Howard Bond presents, and I believe it is valid in an evenly lit scene, and if that is what you are
shooting, you'll probably benefit from following it.

However if shooting eg. a city night scene with illumination ranging from brightly lit billboards to
pitch black shadows, the task is more that of scene evaluation of what exactly to measure and to
have knowledge of your personal skills in post processing to capture the important parts within printable range.

Even with Mr. Bonds formular, the difference between HP5+ at 50 and 150 sec is only 2/3 stop
making the scale less important than good scene evaluation.

7. ### jos__garese

Thanks for the inputs!