I was looking thru some old kodak publications and they made mention of their panchromatic emulsions being classified as either Pan B or Pan C. Films such as Panatomic X, Plus X and Super XX sheet films were classified as Pan B. Pan C films were among others, Super XX roll and Tri-X.

In most cases the filter factors were identical but a major exception was with red and deep red filters.

I suppose with the advent of the new emulsions these old categories don't exist, but I was wondering if people still see a major difference in filter factors between Plus X and Tri X when using a red filter.

What's your experience? How about FP4 and HP5?