Quote Originally Posted by jdef
How is the film's spectral sensitivuty linked to the densitometer's? [...] The densitometer reads those densities based on the intensity and spectrum of the light passing through the negative. There is a complete disconnect between the two. How does the densitometer know what spectrum of light exposed the film? What am I missing?
A film's spectral sensitivity is not in any way linked to the spectral sensitivity of any densitometer. You're right. But depending on how you make the exposure onto the film, your choice of densitometer and filtration could make a difference.

If you make exposures in a camera by varying the amount of light using a shutter and aperature, it doesn't matter. As you say, there is a complete disconnect between the densitometer and the exposure.

But if you make the exposure through a step wedge, then your measurements of the step wedge can vary based on the filtration used in the densitometer. The actual exposure that is given to the film is not affected by your choice of densitometer/filter, only the numbers that you generate which will be used in your calculations later on will be affected. This variablity could be significant.

I'm not talking about measuring densities of the film to which the exposure was made when making these measurements, but the step wedge that was used to control the exposure in the first place.

So what I was trying to point out in that sentence, "But this may be a concern as you say if your film is more sensitive to red than blue and your densitometer is not measuring any red but is measuring some (albeit only a little) of the blue... " is that if you are controlling your exposure on your film (or paper as you pointed out) and the densitometer is not able to make measurements of your step wedge under conditions that simulate the actual conditions of use for your material, then you may have some concerns about the numbers that you are reading off the display of your densitometer.

Let me restate some more of what a said in the other post with a few changes to the wording that may make it more clear -

If you have a step wedge that is a relatively neutral material over the range of wavelengths that your film or paper is sensitive to, then it will not really matter much if our Visual filter does not cover the entire range. You would get comparable results if the material is fairly neutral whether the densitometer can see part of the range of wavelengths or the entire range of wavelengths. It is when we start making exposures through materials that have significant colors in them that this will come into play and then be a big issue.

Did that help?