IR Filters on flashes

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Ong

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Just a random question, but I'm tempted to try some IR Flash photography, and was wondering about this.

When you put a IR filter over a flash, I'll assume it works the same as filters over lenses where there will be a reduction in the light.
Just how much light will actually make it through the filter? Does it work in stops so that it reduces the flash's power by x f/stops, or is it a number you divide by (ie, divide whatever GN by x) ?

I've also heard that unexposed E6 works well as a filter for flashes, and I'm just wondering if anyone would know how much the power reduction is for that as well.

Thanks RFF :D
 
Ong said:
Just a random question, but I'm tempted to try some IR Flash photography, and was wondering about this.

When you put a IR filter over a flash, I'll assume it works the same as filters over lenses where there will be a reduction in the light. ...
Click to expand...

What light ??? ... or more accurately "which" light?

The usual "IR filter" that you be placed over a flash for IR photography is a "band pass" or "cutoff" filter. These pass nearly 100% of the light in one frequency band (the IR portion of the spectrum in this case) and cutoff (read: block by either absorbing or reflecting back toward the light source) the rest (all visible light in this case).

For conventional films and for digital sensors that are bundled with an IR blocking filter (most) the filter factor is nearly infinite with a true IR filter. With extremely deep red filters (e.g. Wratten 29) there will be enough red light passed to yield reasonable exposure although without any real IR contribution. With IR films and unfiltered sensors there is a moderate filter factor as these are both sensitive to visible light as much or more than they are sensitive to IR.
 
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