Infrared (R72) filter. Should you be able to see anything ?

Some time ago I bought a roll of ectachrome colour infra red slide film which has been sitting in my deep freeze ever since. I bought a Hoya Infrared filter (R72), a year or so later, and I'm sure at the time I held it up to light and it just looked black and opaque.
Today I was tidying up my odds and sods draw and found the filter, but this time by holding it
up to a south facing window (but not AT the sun) , after a second or so my eyes could see everything, but coloured red. I thought Infra red filters only passed 'invisible' light ?. I'll go and check up what an R72 filter is, though someone here may have an explanation.
I assume it's a bit useless for digital camera without a lot of 'hacking' ?.

Andrew

An infra red filter just passes IR light, but the cut off point is different depending on the filter. The R72 cuts off from 720nm so might pass a small amount of near-IR red. The R95 will look even blacker, only passing IR light beyond 950nm.

The Hoya R72 should let light through from 750nm - and may let a small amount of visible red through. See: https://hoyafilter.com/product/r72_infrared/

You can use IR filters on many digital cameras. However, some have aggressive IR cut filters that will block what's coming through an IR filter. The K7 worked fine with a 720nm IR filter for me, but the K5 + K3 get zero as they have IR-cut filters over the sensor.

Even when the sensor can accept IR, there is still a measure of IR-cut so the exposures tend to be tens-of-seconds (or minutes). Added to that - you can't see anything through the viewfinder, and the focus tends to be slightly off compared to visible light. So, it's fun to experiment, but can be difficult to get good results.

Here's one I did ages ago (tinted in PS) : https://www.pentaxuser.com/photo/user-77/-infra-dead--25354

If you are serious about IR photography you can get the IR-cut filter removed from the sensor; and the focus adjustments made - that then enables more regular exposures and the ability to see what you're shooting! (And doesn't require a front filter). You can even get cameras modified to be sensitive to UV in addition (very interesting if you're into the insect-eye-view of flowers!).

Hope that helps.
Matt