Using 25A filter with IR modded 40D


Senior Member
Oct 26, 2003
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Hi guys

Recently just got my EOS 40D modded for IR by non other than fatigue. When comparing to without using any filter on the lens & camera on AV mode :

Tried CP with it (though internet say no effect, but what the heck, have it use it lor):
Color shifted
No cutting of reflection in windows
Needs to compensate 1/3stop less.

But when I tried 25A:
Color didnt shift
Needs to reduce 2stops exposure compensation

Any idea y so much difference in the metering?

one eye jack

Senior Member
Jun 11, 2011
For 25A (RED) compensation is about there = 2 stops.For CPL because it passes all light spectrum (for IR so..)

Filters blocks and passes certain light spectrum and the energy wavelengths differ.There is a calculation method called filter factor which is not the same as F stops but translate to it.
Below are links that explain what it is and the calculation of it.Google black and white filters to know more.