It has to do with how much you have to pour in to cover a whole roll in the cylinder. 4 120 reels will be about as high as 5-1/2 35mm. For instance, a Patterson System 4 Multi-Unit 2 Tank will hold 5 35mm reels, but only 3 of 120. It makes a big difference if you use once-and-dump developers. You are correct if you are talking about replenishing or when using something like regular t-max developer that is made to be re-used, where all that matters is surface area.
Originally Posted by Leigh B