"Father & Son" is a collection of photographs by British fashion designer Paul Smith and his father, Harold Smith. Pocketsize and unusually bound as a two-volume paperback, this series of photographs sums up the humor and acute observation which Paul Smith has brought to British design--few other designers have so successfully combined wit, originality and the highest standards of craftsmanship. Smith the younger explains his collaboration with his father in this way: ''My father, Harold Smith, by profession a credit draper in Nottingham, was a passionate amateur photographer and a co-founder of the Beeston Camera Club. When he wasn't supplying clothes and household goods direct to peoples' homes, he thought about composition, light, witty juxtapositions, experiment--not only taking photographs but also developing and printing all his own work in his home-made attic darkroom. In contrast, my snaps with an instant camera are about observation, humor and caught moments. Very different from my father's photographs, but, oddly, strongly similar. What a pair ''