infrared filters question

Started 1 month ago | Discussions thread
OP Montanawildlives Contributing Member • Posts: 540
Re: infrared filters question

AlbertTheLazy wrote:

Montanawildlives wrote:

I have a 590nm converted Fuji xt1. I wanted to go for a more "pure" infrared experience, so I was going to buy a lens filter--maybe 720nm, or even 850nm. The reason I did the camera conversion rather than just using filters in the first place was because I wanted to be able to hand hold (which I can, no problem, with the current 590nm conversion). So my question is: if I attach a 720nm or 850nm IR filter to my lens on this camera, how much will my exposure be affected? I think not much, because so much more light from 590 getting through since the sensor-IR filter has been removed/replaced. So reducing the light in the range of 590 to 720, for example, might cost me a half stop or stop but not....10 stops, for example. Am I correct?

Not necessarily. It entirely depends on the passband of the filter you already have in the camera.

Whoever provided the '590nm filter that you have permanently mounted in your camera should be able to supply you with a transmission graph.

You can think of your choice of infra-red filters as being directly analogous to a tricolour set for visible light

These will have transmission curves something like this:

Stacking the blue and green together will pass very little light -- probably similar to the result of using a 590nm filter on an unconverted camera. If you stack the red and blue filters together you should pass no light at all.

For the purpose of this discussion you could consider the blue to be equivalent to a 590 IR, green to a 720 IR and red to an 850 IR.

If, on the other hand, your existing filter has a passband shaped like this

then your idea will work pretty well.

That transmission curve is roughly the equivalent of UV cut filter for visible light.

Trouble is, without knowing the characteristics of your existing filter you can't predict the result.

Thanks for the information! In fact my conversion was done by lifepixel so it is precisely that super color filter from the graph you provided. So from that graph, if I go on the x-axis to 850 and follow that line up I can see that maybe 88% of the light will make it through, giving me a fairly small change to my exposure. Similarly if I got a 950 nanometer filter it might pass about 82% of the light. Definitely not looking for anything exact just wondering if these are Ballpark figures in which case yes I think my plan will work.

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