Size of circle of circumference for 35mm was dependent on a 36mmx 24mm format, the diagonal of which was 50mm ,so the circle of confusion was 0.033mm (50/1500 = 0.333333......recurring.The circle of confusion is determined by sensor's/film format's diagonal length divided by 1500-lens has nothing to do with it.
Excerpt from Luminous landscape:
http://www.luminous-landscape.com/es...t-Lenses.shtml
What this means is that the CoC for a given sensor is taken to be sensor diagonal length divided by 1500. For the FF sensor (or 35 mm film) this gives a CoC of about 30 micrometers while for the PS sensor the CoC is only 5 micrometers. There is nothing sacred about this choice of CoC. If very large prints are to be made, perhaps the CoC should be reduced; and vice versa if only 4" by 6" prints are needed one can get away with a larger CoC. Note that for an 11" by 14" print the CoC is about 0.3mm. .
Ken