Sirius Glass
Subscriber
Using box speed, taking a light reading and adjusting for the filter factor is more accurate that arbitrarily setting an effective ISO. Filter factors are supplied by the manufacturer, who actually know what they are doing, not like the random number pulled out of the air, guessing, without any knowledge.
R23 filter factor 2
R25 filter factor 3
R29 filter factor 4
720 aka 72R filter factor 5
filter factor =5 (25 A = 8 filter factor) or 5 stops = 32 filter factor? What I used is effectively to 6 or 7. I am not finding R72 filter factors on Hoya or any websites. I found one unofficial listing of filter factor=20 to 40 .
I did not make this up, but checked to see what worked with other photographers on Photrio and Flickr, and back calculated filter factors as a check. It made sense that the filter factor for R72 should be higher than 25A, I think. I just found it easier to use effective ISO around 6-16 with the filter on. You end up in a very similar place.
Look at my post: R25 [25A] has a factor of 3 [23 = 8 times the exposure time], 720 [72R] has a factor of 5 [25 = 32 times the exposure time]. Mathematics shows that 5 is greater than 3. I have found the filter factor for the 720 listed as 6 but that was a little too high sometimes from my work with it.
https://en.wikipedia.org/wiki/Wratten_number
Last edited: