it has been suggested to me that i use Azo POP (or maybe centennial?) for contac t printing 4x5 b/w negs, rather than my normal multi-grade fiber base enlarging paper. what is POP, and why should it work better for contacts? if it is bette r for contacts, is it also better for enlargements? why dont i see azo or cente nnial papers in the B&H ad, or in the calumet catalog? does POP come in double- weight? thanks.