Once upon a time - about a century ago in Europe - it was common to mark lenses not with focal length, but a number. One of my lenses is an Emil Busch Weitwinkel-Aplanat No.3 f:14; another an O. Simon Anastigmat No.4 f:7.2. Others have focal lengths, but I haven't yet seen any with both markings. A No.0 would have been made for the smallest plate format, about 6.5x9cm. The No.1 is 9x12cm, or 4x5" if made in an "inch" country. No.2 covers 5x7" or 13x18cm, those are close enough to be considered the same. But from there on it breaks down: I know that the Busch illuminates the full 8x10", but not whether it's made for that, 18x24cm or some intermediate plate size. And then there are the larger plate sizes... The German sizes were 18x24, 24x30, 30x40, 40x50. Same sizes as enlarging paper is still sold in, but noone really knows why I'm bidding on a No. 6 Aplanat, hoping that it covers 30x40cm. If not, it will be a nice portrait lens for 8x10"! Anyone?