Originally Posted by
Doremus Scudder
When you stack colored filters, you just get the effect of the strongest filter. E.g., when you stack red and yellow, the yellow filter passes red orange and green; the red filter cuts out the orange and green, so you just get red. The yellow filter is doing nothing but degrading the image.
The same with yellow and green. Yellow passes red, orange and green, but the green filter only lets the green through, so, again, the yellow filter is doing nothing.
If you stack complementary colors, say green and red, you just get neutral density. If the filters are sharp cut and good quality, you'll get black...
Rule of Thumb: Don't stack colored filters.
A "yellow-green" filter is somewhere in strength (i.e., how much of the blue end of the spectrum gets eliminated) between yellow and green. If that's the filter you need, you just have to get one. You can't make one by stacking the stronger green filter together with the yellow.
Go to the Wikipedia site on Wratten numbers and familiarize yourself with the progression of colored filters from weaker to stronger (yellow - red) and the variations in between. Take a look at the color wheel and the concept of complementary colors as well and it will all become clear.
Best,
Doremus
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