Can those lover nm filters also be used for IR photography? I mean, are they passing spectrum above the nm they block + IR, or only pass through red or whatewer color they are supposed to pass?
Judging by the curves, it should be fine - as transmission is cut below nm but it does not block IR which is further above.
I wanted to obtain b+w 091 for start (which is not called IR), however, got confused if it is a right thing to do.
Also, is red-blue channel swap in post required for all IR filters?
There are a couple of potential end points for IR. Keep in mind that the filters you mentioned were around in the film era, long before digital and could be used either to increase contrast for BW (IR or not) film or color IR film.
In the digital world, >750nm will only really allow for BW work or at least a single color. 720nm is still good for BW but also can permit some amount of color to get through beyond red that can be nursed to give you blue skies and beige to white leaves after a channel swap. Below 720nm, increasingly more color is there to eventually around 590nm allow for red (or any color you like) leaves and blue skies, all after a channel swap. The more permissive the filter, the less you have to push saturation and WB in post, but you always have to channel swap.
Kolari makes a filter called IR-Chrome that gives you more or less red leaves and cyan to blue skies SOOC, no channel swap and minimal processing. The look approximates some Kodak Aerochrome film look, at least if not pushed in saturation.
The more permissive the filter, the lighter the skies will be if going BW. If you want super dark tone in the skis for BW, 850nm or 720nm is the way to go.
Many examples of various filters, either from full spectrum or filter-specific cameras here (several pages, usually flagged for filter used):
Kolari and LifePixel have sample images of IR filters. 720 gives a sepia tone to skies. I stopped using filters and went to converted cameras. The entire switcheroo became too tedious. These are done using Nikons but IR is the same everywhere.
I highly recommend you to to the Kolari Vision website. They have extensive comparison photos of the different effects of the different filters, both with and without channel swaps.
For 850, no need for a channel swap as it generates a full black and white image.
Can those lover nm filters also be used for IR photography? I mean, are they passing spectrum above the nm they block + IR, or only pass through red or whatewer color they are supposed to pass?
Judging by the curves, it should be fine - as transmission is cut below nm but it does not block IR which is further above.
I wanted to obtain b+w 091 for start (which is not called IR), however, got confused if it is a right thing to do.
Also, is red-blue channel swap in post required for all IR filters?
I used the Hoya 720nm filter and a less expensive Chinese 720nm filter since 77mm could be expensive, used to be $300 for the Hoya IIRC. It blocks most of the visible spectrum. I doubt 091 works for one thing cameras block IR for regular photo iq. That's just a red filter. You'll need tripod.
I've never heard of the channel swap, so I'd say it's unnecessary
I got 58mm Hoya from keh used to save $$$. Adapted it to 77mm still able to use long end of 11-16 uwa zoom, and ff 17-35 on aps
