What was the significance of 35mm film?

The 35mm format combines compact size with a negative that is large enough to produce good quality prints up to 8 x 10" and sometimes bigger. Its size and the ability to build small, easily-carried and -handled cameras around it, were the impetus for the first truly portable high-quality cameras.

Since the basic stock is also used for commercial movie film, it is readily available, and in fact some companies sell movie film loaded into 35mm cassettes for still Photography. When you send it in for processing, you get both prints and slides back.

35mm is still the medium of choice for many serious photographers, including the writer.