2018 May     Film      Miscellaneous How-To & Reference



How many stops should you correct for an orange filter?  How many stops does it affect the camera metering? 

Let's see.


The Gear


Canon AE-1 with 50mm lens.  Tiffen #16 Orange filter (52mm).


The Metering


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.


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Conclusion


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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