film speed test and spectral sensitivity

I am in the middle of my first serious film speed test. I found it convenient to use a clear blue northern sky to represent a uniform surface to photograph, instead of a standard gray-card (which I don´t have). It now strikes me that the blue color/spectrum of the sky, together with the spectral sensitivity of my film (Ilford FP4+), might give me miss-leading results from the test. Does anyone have any comments on this?

You are right.

What kind of a 'serious film speed test' are you conducting? Be specific, a full blown speed and development test or a simple one-roll speed test for normal development?

Ralph,

By the way you put it, I guess it is more of a simple one-roll speed test. I shot a roll with {blank, -4, -3.5, -3, -2.5, -0.5, 0, 0.5, 1, 3, 3.5, 4, 4.5} exposures. I intend to use my enlarger meter (Darkroom Automatic) to find b+f and the frame/EI corresponding to an appropriate zone I.5 density. I will use the same measurement procedure to verify if current my development time renders an appropriate zone VIII.5 density.

henpe said:

... I shot a roll with {blank, -4, -3.5, -3, -2.5, -0.5, 0, 0.5, 1, 3, 3.5, 4, 4.5} exposures. I intend to use my enlarger meter (Darkroom Automatic) to find b+f and the frame/EI corresponding to an appropriate zone I.5 density. ...

Click to expand...

A Gray Card will be useful to do this.

henpe said:

... I will use the same measurement procedure to verify if current my development time renders an appropriate zone VIII.5 density.

Click to expand...

What do you do if it doesn't?

This is my plan: If the -3.5 exposure renders a zone I.5 density (approx 0.17 o.d.), then I am happy with the speed that I assumed while shooting the roll. Then I will proceed and check the density of the 3.5 exposure. If this exposure has an appropriate zone VIII.5 density (approx 1.37 o.d), then I will also be happy with my dev.time. If the negative is to thin I will have to increase my dev.time. If the negative is to dense, I will have to decrease my dev.time.

As regards my discussion about gray-card vs. the blue northern sky, I realize that a gray-card is more appropriate. I can however not stop myself from reason about the (in)-appropriateness of using a northern sky. Ilfords data-sheet indicates that FP4+'s sensitivity to blue (approx 450nm) is about average as compared to the sensitivity to other wavelengths. My test-roll should therefor be OK to use!?. I will probably develop my film and complete my testing procedure, for curiosity, and then do another test some other time, using a purchased gray-card. I guess the bigger underlying question is to which extent the lightning conditions used while testing for film-speeds should match a the lightning conditions normally used by a photographer...

henpe said:

... Ilfords data-sheet indicates that FP4+'s sensitivity to blue (approx 450nm) is about average as compared to the sensitivity to other wavelengths. ...

Click to expand...

Please read the headline to this graph in the data sheet!

SPECTRAL SENSITIVITY
Wedge spectrogram to tungsten light
(2850K)

That's hardly your northern sky, more like a tungsten light bulb.

I am aware that the wedge diagram was captured using tungsten light. I do not know the corresponding wedge diagram using any other light-source, but I guess any difference could be related to the spectrum of the different sources, thus I predict that a day-light diagram do not have such a strong sensitivity to the yellowish portion of the diagram. Anyhow, this is just speculations. As said, I will complete my test for curiosity and then maybe compare the results with a later new test. I gratefully appreciate the discussion, it made me realize a thing or two....

Look up the recent thread on APUG about Ctein's Post Exposure book being made freely available via download in .pdf format. Download the book. There's a section on testing film/meter spectral sensitivity using a MacBeth (now X-Rite) Color Checker.

Lee