That's not it.Astrophotography books refer to the Wratten #13 as a "more efficient" version of the #11. The #11 and #13 pass the 480-580 nm spectrum, and the #11 has a filter factor of 4 and the #13 a factor of 5. I suspect that the "more efficient" comment means that the #13 has a sharper cutoff, i.e. is better at blocking unwanted wavelengths outside the passband.
In what way is that not "better at blocking unwanted wavelengths outside the passband", e.g. blocking more red and/or blue?That's not it.
The 13 absorbs more red than the 11. The behaviour of both at wavelengths shorter than about 590 nm is virtually identical.
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