I'm not clear here that we are all on the same page, so help me out, please.
There are IR filters that you put on the lens. More or less blocks visible light and lets the IR thru. Your need to compose and focus before installing the filter. Exposure times are extended. I've read that D90's work very poorly at that. Not sure about the 40, but did ask a few years ago and told it was a very poor choice. Has to do with the (hi or lo pass) filter built in --- which can be removed, I'm told.
Then there's that internal filter, mentioned above, which needs to be removed for normal IR work. Pretty much rendering the camera useless for normal work.
I know I'm missing something here, so please help me. This is something I've wanted to do for awhile.