Film Miscellaneous How-To & Reference
How many stops should you correct for an orange filter? How many stops does it affect the camera metering?
Canon AE-1 with 50mm lens. Tiffen #16 Orange filter (52mm).
Let's say the AE-1 recommends 125th second at f/11.
With the filter on, now it says 125th at f/8.
That's a one-stop difference because of the orange filter. But wait, is that correct? An orange filter is not full-spectrum.
The Film-Speed Dial
The Tiffen Orange 16 actually has 1 2/3 stops of light loss. (The orange filters article has info for others, too.)
The AE-1's meter can already see 1-stop difference. So we need to adjust the ASA/ISO for the remaining 2/3 stop.
With 400 film, move the dial two clicks toward 200. (The AE-1 allows you to set the ASA/ISO in one-third stop increments.) That gives something like ASA 250, which is DIN number 25.
From here, simply use the camera normally with ISO 400 black & white film. With the AE-1 in manual mode, let's suppose you set it to 1/60th. It will then indicate which aperture setting to use. Whatever the light meter now indicates, that should be correct now for the filter.
So, once again: the camera's light meter can see 1-stop difference when you put the filter on. For the other 2/3 stop, you have to adjust the ASA dial.
Will This Work With Other Cameras?
This was for an AE-1, but it should work for any camera that uses a silicon-based light meter.
I don't think it would be the same for a CdS or selenium meter. I think but am not certain that CdS meters will not give accurate results with this method, because they have much more sensitivity to blue-spectrum light. (Orange filters block the majority of blue light.) That might be better, because the CdS meter might see the orange filter as almost a neutral-density. So with CdS you might be able to use an orange filter with no ASA/ISO adjustments. Or, just a milder one.
Meter without the filter on. Then meter with the filter on. Note the difference. Then use the ASA/ISO dial to give the additional number of stops necessary for whatever your filter requires.
So, if you had a Red 25 filter (3 stops), and your meter saw it as two stops darker, you'd need to adjust the ASA/ISO dial to one stop slower.
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