ND puzzle

[Franklin]

I have an Antares ND13 filter (thanks Thunderstruck) and from what I've read it has a 13% transmission, hence the name. I also have an Explore Scientific ND0.9 filter and this also has a 13% transmission. The puzzle is, why is the ND13 much darker than the ND0.9 if they have the same 13% transmission? Are they labelled up wrong? Do the suppliers not know what they are talking about? I don't really mind because it will be useful to have two different densities, it just seems quite odd. Has anyone else noticed this advertising blunder?

[johninderby]

Found this on the Telescope Express website which may explain the difference. It seems that the Antares ND13 filter is mislabelled. 

Edited September 20, 2020 by johninderby

[Louis D]

Photographic ND filters are rated by stops of darkening.  Thus, an ND13 should pass 1/(2**13)=1/8192 of the original light.  These are typically used for solar imaging.

A mislabeled ND filter could be very dangerous.