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IR Filter/Hot Mirror


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#1 Carl Fischer

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Posted 22 November 2009 - 12:10 AM

When using a IR filter with the red is it just used for day EXT? It just takes care of UV pollution from what I've read. Does it change any color or have a large exposure compensation? Also do you use it in day INT.?
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#2 Adrian Sierkowski

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Posted 22 November 2009 - 10:23 AM

an IR filter has nothing to do with UV. It has to do with IR (infrared light) you use it when on the red and using a lot of ND, which primarily happens outside. It doesn't effect visible light (or it shouldn't) but keeps the infrared light from interfering with the recorded image when using a lot of ND on the lens.
While this primarily happens outside you could use it inside if you're using a lot of ND on the camera inside (though can't think of why you would?). Rule of thumb; when you get into double digit ND you should consider an IR.
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#3 David Mullen ASC

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Posted 22 November 2009 - 12:46 PM

IR and UV are at opposite ends of the visible light spectrum, just outside of it, but sensors and film stocks can still be sensitive to it. Infrared is just below red and ultraviolet is just over violet.

When you stack a lot of ND filters over the lens because of extreme levels of sunlight outside, a lot of visible light is blocked but infrared light is still passing through, so the proportion of visible to infrared light shifts in favor of more infrared. You then get infrared pollution in your blacks and certain colors.

UV is blocked by glass, different amounts depending on the glass, so the problem becomes worse when you aren't shooting with any filters outdoors, and UV tends to show up as exaggerated atmospheric haze. It also tends to be heavier on clear days at high altitudes.
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