Not really. It's based on the need to maintain the same negative density for any neutral tone, black, white or grey.
I make this point not to be contentious but merely to reinforce the points that 'middle grey' has no relevance to ISO speeds for negative film (which are of course based on shadow detail), and that 'middle grey' has no meaning whatsoever as applied to negative density.
Cheers,
R.
Roger, all smiles here, so I'll respond to your comment for further clarification, but I don't want to be part of a hi-jacked the thread, if possible. If you want, we can debate in PM's if you disagree so I won't respond further on the subject in this thread.
I'm not sure that I implied that there is a correlation between middle gray and ISO speed for negative film, or that middle gray has a meaning that is particular to negative density. I mean, the middle of the gray scale certainly does have density. I think we can agree that negative density is relative to the degree of exposure, filter or no filter.
I was trying to imply that: If one was to place a "neutral tone" such as an 18% gray card (middle gray) within the scene, then photograph using a filter and applying the filter factor, then it is presumed that the factor will supply the appropriate compenstation to provide a negative density to return a middle gray print value back to the gray card.
It should be apparent then, that if the factor is not applied when the gray card is within the scene, then the gray card (or any other neutral tone) will be rendered on the negative without enough density to faithfully return the appropriate print value back to the card as middle gray.
My original statement is meant to mean all that without actually saying all that, and I think that it does as I certainly don't want to confuse the person that I am responding to. Although, it is certainly possible for confusion to abound when the proper words are not chosen to convey the best of intentions.
Thanks,
Chuck