I have no commentary to add to the main subject, but...(posted 8424 days ago)"Ideal format" used to mean 6x9cm (2-1/4 x 3-1/4in), either on 120 rollfilm, or on sheet film as in small Speed Graphics, Lihhofs, etc. I think the idea was that that negative size was a reasonable compromise between small-camera convenience and large sheet-film neg quality. Also, it could crop nicely to a variety of aspect ratios.
I believe the term was popular back in the 50s, when most pros (apart from those photojournalists using Rolleiflexes and Leicas) were still using 4x5 cameras for nearly everything important. Of course, with today's films and lenses, a 6x9cm negative contains the amount of detail you needed a big 5x7 or 8x10 negative for back in those days...