Is the Cokin 173 filter just a back CPL?
No. The effect is similar to mounting your polarizer back, but much more pronounced in the Varicolor filter.
This is what my Hoya HD CPL looks like at polarization angles of 0 ° and 90 °, when it is oriented for proper mounting (male threads towards the camera):
- Note that this is NOT a variable ND filter. In the image on the right, the filter darkens the light because the light source is polarized (it comes from an LCD screen, which uses polarization to control the light output).
Hoya circular polarizer, normal orientation, at 0 ° (left) and 90 ° polarization (right)
When you turn the polarizer backwards, this is how it looks, again at polarization angles of 0 ° and 90 °.
- Note that the automatic white balance of my iPhone tried to adjust to the color change in both images. The left bluish mold of the polarizer is actually more pronounced than shown. You can see the yellow LCD monitor outside the CPL. The image on the left should have a cooler color temperature. Similarly, the yellow tone of the right image should be slightly heated (the monitor is a little more blue than normal outside of CPL view).
Hoya circular polarizer, reverse mounting orientation, 0 ° (left) and 90 ° (right) polarization
I don't have the Cokin Varicolor, but I have the Singh-Ray Gold-N-Blue polarizer (same effect). Here is the Gold-N-Blue filter at 0 ° and 90 °, oriented for correct mounting:
Singh-Ray Gold-N-Blue polarizer, normal orientation, at 0 ° (left) and 90 ° (right) polarization
Here is the Gold-N-Blue oriented for reverse mounting:
Singh-Ray Gold-N-Blue polarizer, reverse mounting orientation, at 0 ° (left) and 90 ° (right) polarization
I couldn't tell the difference between the Singh-Ray Gold-N-Blue filter mounted on front and back. But the degree of effect between Gold-N-Blue and a reverse mounted CPL is significant.
- Also keep in mind that in real life, the effect of Gold-N-Blue (and Varicolor, I suppose) is not so pronounced to turn the world into Denver Broncos colors, or to make it look like a Michel Bay color film . Blue and gold / orange are extremely saturated here because the color-separated light of the LCD is already polarized (by definition being an LCD monitor).