HI first time I have done this so if  I have done something wrong tell me please.
Right I have just bought a second hand Hoya PL CIR filter its like new.
When I compared this to my other (smaller) filters which are PL -I quickly noticed that the ability to reduce glare was vastly limited in the new filter . Putting the 2 together -yes I did get near total blackout so it must be polorising filter. But whats going on? How can 2 same make filters behave differently?
The new filter to me seems quite useless as it does not produce the dark sky for instance I seek.
Do you think it could be a dud filter -unlikely!
I am really mystified and would appreciate comments.
Cheers Hod

Did you actually try the filter on the camera? Circular polarizers are sort of "one-way"; you have to look through them from the camera's perspective for them to have much effect.

Also, remember how to use a polarizer; 90 degrees to the light source is where you'll see the greatest effect.