Question on stain and density

In doing some film and developer tests I am shootin a gray card at zone I, then trying to establish which exposure yields a density of 0.08 to0.10- the problem is the densitometers I have access to are RGB - I seem to remember one uses the blue channel for this.
So far so good. I am using pyrocat. HD, what effect would the stain introduce ??

Second question- why not use the scanner for a densitometer? How to interpret. The scanner's readings, I.e. 0-255?

It's hard to say what the effect of the stain will be at the brink of Zone I. From what I have read and seen, the effect is much more pronounced the further up the exposure scale you go. Stain is produced in proportion to the amount of silver developed. At the speed point of your film, there is not a lot of silver present, so there will not be a lot of stain. What proportion of the total density is the stain? If you can measure visual density against blue density, it may give you some clue.

Peter Gomena

herb said:

I am using pyrocat. HD, what effect would the stain introduce

Click to expand...

Probably wrong here (I don't use staining developers), but it seems it will add density to the reading. Which channel should be used may be the better question, but I don't really know.

There is a wealth of information about Pyrocat and the density from the stain in this article by Sandy King:
http://unblinkingeye.com/Articles/PCat/pcat.html

The blue channel is recommended for measuring the silver plus stain density.

It is quite possible to use a scanner as a densitometer. The absorbance, or density, is defined as:

D = -log(I/Io)
where I is the intensity of light passing through the material, and Io is the intensity without the absorbing material. For some scanners and software, it is possible to get a linear signal such that the pixel value is proportional to the light intensity. This usually requires turning off auto exposure and any non-linear curve adjustment. I have gotten good results using a Minolta Scan Dual III (with the Minolta software) and with an Epson V500, using VueScan. I haven't figured out how to defeat the autoexposure on a Nikon scanner, however.

To get absolute absorbance values, you will probably need to have a calibration standard, such as a step wedge from Stouffer:

http://www.stouffer.net/TransPage.htm

The other thing that is helpful is the program ImageJ:

http://rsbweb.nih.gov/ij/

This is an excellent open-source image analysis program, and it makes it easy to measure the average values for a patch of pixels.

I can provide more details on using a scanner as a densitometer, though I don't have the information at hand right now.

David

You mention stain but don't state whether you are refering to overall base stain or to image stain. The value of the base stain should be measured and subtracted from any image stain values to get the true value.