Adjustable IR filter? How does this work?

[Andrea B.]

https://www.ebay.com/itm/223219705237

 

Have we discussed this before? How does this adjustable IR-pass work? Is it an IR-pass paired with a polarizer? How can that go from 530 to 750 nm ??

 

ADDED LATER:  Yes, this filter was discussed here:  Variable IR Filter

[Andy Perrin]

Yes we have discussed these kinds of filters, quite a lot this year. 

 

[Andrea B.]

But how can darkening the visible light affect the IR light?

[Andy Perrin]

I don’t think it does? It’s shifting the edge of the cut in the visible range and not affecting the IR, at least in enrico’s graphs. But since the overall bandpass is affected you would get different false colors, same way using Tiffen 8 versus 12 gives different false colors, even though both pass the same IR. 
 

There is a bit of hand-waving going on when we call a filter like Tiffen 12 an “infrared” filter, since plainly it also passes visible light up to 550nm. Same kind of thing with these variable filters I think. 

[enricosavazzi]

11 hours ago, Andy Perrin said:

[...] There is a bit of hand-waving going on when we call a filter like Tiffen 12 an “infrared” filter, since plainly it also passes visible light up to 550nm. Same kind of thing with these variable filters I think. 

Pretty much so. The "variable" part of the transmission curve is between 600 and 800 nm (mostly between 640 and 740 nm), so mostly in the VIS. At any one wavelength, transmission remains constant both above and below this interval.

[OlDoinyo]

12 hours ago, Andy Perrin said:

There is a bit of hand-waving going on when we call a filter like Tiffen 12 an “infrared” filter

It is considered such only because of its niche use in IRG photography as a blue-blocker (which is what it really is.) Of course it does pass IR, but so do most other filters.

[Andrea B.]

Thanks all for the info. 

[Andy Perrin]

I actually have one of these, a really really old one that I inherited from my grandma (my grandma literally wrote a book on special effects photography in the film days). I should try it out when the sun comes back...it may be some time...