CPL filter on canon ef 70 - 300

[mneame]

Just bought myself some filters - at last. one of which is a circular polarising (sp) filter. fine on my kit ef-s 18 - 55mm lens but makes the 70 - 300mm out of focus.

any ideas why and if i need a different brand of filter for this lens, can anyone recommend which one. uv / skylight filters are fine.

btw am using them on a 30d.

cheers,

matt.

[_dobbo_]

Hi Matt,

I get the same problem with my 80-400 Nikkor - basically at anything lower than about f/8 the autofocus stops working. As a polariser robs you of a couple of stops of light it can mean the AF can't work.

Edit to add - I believe this is actually due to the way AF works, the system requires areas of contrast and at around f/8 there is not enough light getting to the AF sensor for it to work.

Edited by _dobbo_ on Monday 11th December 15:27

[mneame]

i'll give it a go with manual focus and see what happens or will it just not play ball at all?

[_dobbo_]

Manual should be fine, as will the CP filter on any lens that's faster than f/5.6