Why does a Rosco CT orange + ND filter show 33% transmission for a 1-stop loss?
Asked 11/13/2015
7 views
2 answers
0
I learned that each stop halves or doubles the amount of light, so a 1-stop loss should mean 50% transmission and a 2-stop loss should mean 25% transmission. On Rosco’s filter chart, though, some filters seem different: for example, #207 is listed as “CT Orange + .3 Neutral Density” with 33% transmission and “reduces light 1 stop,” while #208 is “CT Orange + .6 Neutral Density” with 16% transmission and “reduces light 2 stops.” Why don’t those transmission percentages match the usual stop values? What am I missing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
13
If you look at filter #209 you'll see "Reduces light 1 stop (Transmission = 51%)". That is what you expect to see. Filter #210 reduces light 2 stops and has transmission of 24%. Again, what you expect. So what is the matter with filters 207 and 208? They are COLOR filters not neutral. They a) reduce light, and b) reduce it differently in different wavelength. See the datasheet of #207
So it makes 1 stop down (50%) and blocks blue wavelengthes which adds another 17%. Altogether you have ca. 67% loss.
Originally by user44894. Source · Licensed CC BY-SA 4.0
user44894
10y ago
0
Generated from our catalog & community — verify before relying on it.
Your stop math is correct for neutral density alone: about 50% transmission is 1 stop, 25% is 2 stops, 12.5% is 3 stops.
The Rosco examples you cited are not pure ND filters. They are color-conversion filters plus ND:
- #207 = CT Orange + 0.3 ND
- #208 = CT Orange + 0.6 ND
The “reduces light 1 stop” or “2 stops” refers to the ND portion. The lower overall transmission percentage includes both:
- the neutral density reduction, and
- additional absorption from the orange color filter, which blocks some wavelengths more than others.
That’s why the total transmission can be 33% or 16% even though the ND component is only 1 or 2 stops.
You can confirm this by looking at Rosco’s plain ND filters:
- 0.3 ND ≈ 51% transmission = 1 stop
- 0.6 ND ≈ 24% transmission = 2 stops
- 0.9 ND ≈ 13% transmission = 3 stops
So the mistake is treating a colored conversion filter as if it were neutral density only. The stop rating refers to the ND value; the transmission percentage reflects the full filter’s spectral behavior.
Recommended products
UniqueBot
AI10y ago
Your Answer
Related Questions
Why do some circular polarizers lose less light than others?
Why do circular polarizers often lose more than 1 stop of light?
What does a 2-stop ND filter do, and how is it different from changing ISO, aperture, or shutter speed?
Can two polarizers be used as a variable ND filter, and what are the drawbacks vs a true ND filter?
Can I make a DIY ND400-style filter for a 77mm lens?