Should focus (MF) be affected by polarizer?

[jo1]

I was looking through my 135mm Nikkor MF lens (f/2.8) with my FM2n

today and I noticed something quite strange: Using the split-image

part of the screen to focus I found (as expected) that a building

about 1 kilometre away required me to focus at infinity. However,

when I attached my Fotar circular polariser I found that in order to

get the split image to line up I had to bring the focus in a bit

(about half way to the next marking at 20 metres). Is this a common

problem? It doesn't make much sense to me that the polariser will

change the focal properties of the lens.

Thanks in advance for your responses.

[rana_dayal]

This happens with cheap polarizers where the glass is not exactly flat so it changes the focal length of the lens. Use a good Carl Zei$$ polarizer and you'll see the difference.

[stephen hazelton]

That would have to be some MAJOR unflatness to shift focus from infinity to 20m or so! It would also be clearly visible by just looking through the polarizer.

[jo1]

Thanks for your answers. The focus isn't changed all the way to 20m and who's to say exactly where 2/3 of the way from infinity to 20m is anyway? Also, there is no obvious magnification when I just look through it. Anyway, you've confirmed what I suspected: that it's cheap, I'll go and find a decent one and replace it.

Thanks