1. It appears earlier versions of the Formatt Hitech ND filters had strong casts to them, no doubt.
2. Formatt's new IRND filters are claimed to be color cast free and have significant benefit from the IR filtering during long exposures - is this true? Does this mean the Lee Big Stopper or other big name ND filters do NOT filter IR, or have significant issues from lack of IR filtration?
3. Where do the Singh-ray mor-slo filters fit in the quality continuum? They are EXPENSIVE. Are they worth the price premium? The 15 stop mor-slo option is attractive.
4. Just how important, overall, is the IR filtration capability of an ND for long exposure photography? The Formatt literature places a strong emphasis on IR filtration, as does Joel TJINTJELAAR, who claims better blacks.
Sorry for all the naive questions.
Unfortunately, I own one of the old Formatt Hitech filters...
Without the foam gasket, light can enter through the space between the filter and the filter holder; that light will illuminate the front element of your lens, and in front of your lens there is a black and polished surface, that acts as a mirror. In some situations, I could clearly see the front element from my lens when looking through the viewfinder. Not pleasant to see in one of your long-exposure photographs.
This filter also does not properly block IR light, and that is another real problem. To the naked eye, this filter seems quite neutral or even greenish; but most cameras have some sensibility to IR light. Under normal circumstances, your sensor receives lots of visible light and some IR light, that you cannot even notice; when using this filter, your sensor receives very little visible light and some IR light. The result is that photos turn out with a heavy red cast.
And about colour casts:
I guess that some filters are made from a dark material, while others are transparent with a dark coating. In the former type, light rays that cross the filter with an oblique angle have to cross a larger path inside the filter, an thus get more affected by the filter. This is noticeable when using wide angle lenses, because those filters produce a very gradual but noticeable vignetting.
Both vignetting and colour casts can be (easily) corrected during post processing; however, vignetting with a colour cast is very hard to correct.