Per Astrodon for filters to be parfocal they need to be very much the same thickness. Its dependent on the thickness of the filters.
I found Baaders not to be parfocal despite advertising they were. The only ones I have had that are parfocal are the Astrodons.
I routinely focus my luminance filter and its accurate for all of them including the narrowbands. Perhaps there is some slight deviation between them which I haven't bothered to chase down as they are workably close.
Greg.
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