Confusingly, the Fotga ND IR Fader is not an ND filter: it's an infrared filter that can be adjusted for wavelengths from 590nm to 750nm. It combines a pair of polarizers (like a variable ND filter) and a red filter, which somehow combine to magically adjust the filter strength. Some of you reading this will be saying, possibly out loud, "this guy is an idiot, the explanation is very simple, why didn't he do his homework." Well the answer to your question is that I have a lot of filters to catalogue today, so I'm not going to pack up to the library and write an optics dissertation, because the last time I tried that, I bombed out of physics class and ended up here writing descriptions for filters. Anyway, if you want to play around with different IR cutoffs, this is a handy way to do it. Be advised that, like variable ND filters, the filtering effect may be uneven across the frame, so these filters are best used as a sampler before buying a proper fixed IR filter.