Re: Two questions -- About InfraRed Emulsions and ISO

From: Editor - P.O.V. Image Service (editor@p-o-v-image.com)
Date: Sun Jan 30 2000 - 16:00:31 /etc/localtime

With all this talk of why the ISO should be set higher with a red filter on,
it's time for a BASIC PRIMER in LIGHT THEORY!

OK, the IR is as I said is in a ratio to visible light... Just for argument's
sake let's say it's 1/4 of the entire visible light spectrum... When you add
a red filter you are filtering out the only blue and green portions of the
spectrum...

So, now, you have a smaller portion of the visible spectrum, but the IR light
volume is totally unaffected!

If you have a lower total volume of light for the meter to measure (as the
meter measures only visible light) the ratio of IR to visible light is now
higher. If, for example we have halved the visible light, the ir would now
have increased to a ratio of 1/2... Since we are now metering through a red
filter, we must increase the ISO!

If this is tough to understand, allow me to analogize...

Imagine a vase filled with water and half full of pebbles... If the pebbles
are 1/4 the total volume of water and we now pour off half the apparent
water.... the ratio of pebbles to water increases... is that clear?

That's why we increase the ISO on the CAMERA (not on an external meter) when
we meter through a red filter for IR exposure.....

It's also why we us a higher ISO for scenes shot under tungsten light with IR
film.. A higher ratio of IR to visible light...

Keep in mind that if you shoot B&W IR film without a red filter or an IR
filter, that it will not show much apparent "wood effect"... (As an side, you
generally shoot the false color IR E6 emulsion through a yellow or orange
filter)...

Keith 

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