Quote Originally Posted by Helen B
"If the step wedge densities were truly neutral it wouldn't make any difference."

If... the question of which mode to use would not arise. It appears from Sandy's measurements that some step wedges are not truly neutral. However, I agree with you that they should be neutral.

The acetate base exhibits some UV absorption, in my densitometer the readings are uniformly higher by about 0.09 units.

IMO, one should use the readings that the film "sees", given that panchromatic film is more affected by visible spectra, I use the readings from the visible channel (blue light) when taking readings from the step tablet. I have not seen any difference in using either readings, since using the UV readings means the curve is shifted 1/3 of a stop to the right. If one is to take readings from stained negatives to "fit" to a given paper curve, then IMO one should read with the channel that the paper "sees" and fit the spectra so that the readings from the densitometer are equal to the spectra to which the paper is sensitive. For example, pt/pd actinic response is in the 290 nm range, my densitometer reads in the 390 range, this means my densitometer is less "sensitive" than the paper so I have adjusted the exposures of my negative to give 1/3 more exposure since my densitometer is reading 1/3 less density than what the paper "sees".

It has worked for me. YMMV.