Alan Edward Klein
Member
Numbers too can be confusing. For example, B&W has a 22 product number which equates to light orange like 15, whereas with Wratten and Heliopan, 22 signifies a very deep orange. Hoya has an X0 filter which resembles 11 Yellow-green; but they're X1 is distinctly medium green. Reds come in 23, 24A, 25 (the most common), and deeper 29 (which can differ brand to brand in intensity). Then there are that old K-series numbering system sometimes still lingering around like a Coelocanth.
Here's a chart that compares and explains.


