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From what I've read and watched, a CPL can cut reflections/glare and increase saturation (mainly the blues). Also, these effects can be adjusted by simply rotating the filter. What is not being discussed often is the part that it's also somewhat of a ND filter in that it usually adds one or two stops.

My question is - can that 2 stops also be adjusted or completely turned off? Could a CPL end up as a normal glass without any effect (i.e. no increase in saturation, not cutting of reflection, and no additional stops), kind of like the UV filter where it's mostly used simply as a protection?

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No, the filter cannot be turned off.

Actually, what is perceived as "turning off" (when rotating), is in fact only changing the direction of the affected reflections (which becomes obvious when trying to simultaneously deal with reflections from horizontal and vertical surfaces). The filter does the same all the time.

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    \$\begingroup\$ if your scene contains both directions of polarization (e.g. horizontal and vertical glass panes) you cannot choose to not affect at least one of them. at best you might place the filter at 45° which would reduce both horizontal and vertical reflections but only partially. \$\endgroup\$
    – szulat
    Nov 11, 2018 at 23:38
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  • Polarizers work by blocking light that is not oriented in a particular direction. (Or allowing light through that is oriented in that direction.) Turning the filter just changes the orientation of light that is blocked (or allowed through). Although the orientation changes, the amount of unpolarized light that is blocked (or allowed through) remains the same.

    Look through a polarizer while spinning it round and round. Notice that it never turns "off" (in terms of blocking some amount of light passing through).

  • It's a bit different if you're looking at polarized light. In this case, the filter effect can be turned "off" by aligning it with the direction in which the light is polarized, though the amount of light passing through is still reduced because of the design of the filter. Turning the filter 90-degrees from the "off" direction will result in no light passing through. You can see this effect by looking at an LCD monitor through a polarizer while turning it. This is how 3D glasses present different images to each eye.

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  • \$\begingroup\$ I do try to rotate the CPL and one thing I also noticed is that the shutter speed becomes slower as the CPL comes to full effect. This is one of the reason why I assumed that the ND filter effect can be adjusted. So now, based on the answers, I'm assuming that the added time is because the affected polarized light is being blocked, which results to smaller amount of light entering the lens (thus the need for longer shutter speed). This effect would be on top of the 1 or 2 stops added by default. Is this correct? \$\endgroup\$
    – thedandyman
    Nov 12, 2018 at 23:26
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    \$\begingroup\$ The 1-2 stops of light lost, as reported in product specs, is just an estimate based on unpolarized light. Light reaching the sensor does vary, depending on the extent to which the light is polarized. Before TTL metering, photographers estimated settings and relied on film latitude. In cases where this was not good enough, such as when using slide film, photographers bracketed shots. \$\endgroup\$
    – xiota
    Nov 13, 2018 at 1:14
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How much a polarizer filter affects the light passing through it all depends on the polarization of the light passing through it and the direction it is polarized with reference to the position of the filter.

  • If all of the light shining on the filter is polarized in one direction and the filter is turned to allow light polarized in that direction, virtually all of the light shining on the filter will pass through.
  • If all of the light shining on the filter is polarized in one direction and the filter is turned to allow light polarized 90° from that direction, virtually none of the light shining on the filter will pass through.
  • If the light shining on the filter is polarized in many different directions, then how much and what light passes through is determined by the direction of the filter's polarization. As you turn the filter, some light that was being allowed through will no longer be allowed through, but some other light that was not being allowed through will now be allowed through.
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