RX100: Any Lensmate Filter System Reviews?

Actually the limitation is not Phase Detect AF: any setup that use a Mirror, translucent or not, will require a Circular polarizer, because a mirror polarizes the image and you can't have two linear polarizers in the image path unless they are aligned.

So words written at a time where all AF cameras were using a mirror are now obsolete.

Similarly there are now Phase Detect AF systems that use special pixels on the sensor and don't require a mirror so the advice about Phase Detect AF is also obsolete. Any advice is only useful if the reason for the advice is also given, otherwise it becomes a magical belief, that people apply when it does not make sense.

I have seen advice that said linear polarizers are more effective but it did not say why so I don't know if it was true and if it is still true.

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In theory, theory and practice are the same but in practice they are not - Yogi Berra
Buddha was not a buddhist, Jesus was not a christian, Mohammed was not a muslim