Cokin Filter Factors
On a break from work during a sunny fall day, I tested for the correct filter factors to be used with my Cokin Yellow-Green (YG), Orange (O), and Red (R) filters. I used Ilford FP4+ rollfilm developed in John Wimberley's WD2D+ developer.
I used the following testing protocol. I placed a grey card at 45 degrees to the sun. I metered the grey card using a Pentax Digital Spot meter and exposed a frame at the indicated exposure. Then I mounted the filter being tested and made several bracketed exposures at 1/2 stop intervals (I changed only the aperture and kept the same shutter speed for each frame). I developed the film normally and compared the frames, looking for the frame where the density of the grey card with the filter mounted matched the density of the unfiltered grey card.
I found that the published filter factors for these Cokin filters do not hold true in my testing. Here's what I found, and for comparison I've included both the published Cokin filter factors and the Wratten filter factors from Ansel Adams' "The Negative":
| Filter | Measured Factor | Published Factor | Wratten Factor |
| YG | 1.5 stops | 2/3 stop | 2 stops |
| O | 3 stops | 1 1/3 stop | 2 stops |
| R | 4.5 stops | 3 stops | 3 stops |
Well, I guess this goes to show there's no substitue for doing your own testing. I now know why so many of my negatives that were exposed using an Orange or Red Cokin filter were underexposed. The difference between the published filter factor (which came from here: http://www.geocities.com/cokinfiltersystem/exposure_.htm) could be caused by many things, not the least of which is inaccuracies in my testing. For example, the published filter factors could be for a film that has greater blue sensitivity than FP4+. I'll be using my new measured filter factors from now on, and experience should tell whether my numbers are accurate. In the meantime, I hope this information is useful to you!