Filter and 17-40 no focus?

[themtkid]

I just bought a hoya polerized filter. Now im new to filters but i just put this filter on my 17-40 and it wouldnt focus am i doing something wrong? Also what is the white mark on the thing you twist around?

[JackProton]

You lose 2 stops of light with the filter. If there's not much light to begin with, your camera will have difficulty finding enough contrast for auto-focus to function properly.

[Tony-S]

Make sure you have a circular polarizer and not a linear polarizer. A LP will screw up the autofocus.

[Tsmith]

Shouldn't affect focus in anyway unless the cameras focus sensor can't detect a concentrated point to lock onto.

The white mark just gives you the point of 90º from the sun for proper polarization. I never use the filter by the mark. Just look through the viewfinder and adjust till the desired polarization is achieved. Often times you'll find backing off a tad bit will render the best results in the sky detail.

[SkipD]

I just bought a hoya polerized filter. Now im new to filters but i just put this filter on my 17-40 and it wouldnt focus am i doing something wrong? Also what is the white mark on the thing you twist around?Precisely which polarizing filter (model/part number) did you buy?

As said above, if you purchased a "linear" polarizer (as opposed to a "circular" polarizer), that can have a detrimental effect on auto focus systems.

[themtkid]

I have circular polarized filter if that helps

[JWright]

I have circular polarized filter if that helps

It's not outside the realm of possibility the filter is mis-marked. These things happen...

[JackProton]

It's not outside the realm of possibility the filter is mis-marked. These things happen...

Possible but not very likely. Its also possible, but not very likely, that the filter material was mounted backwards (it matters!).

To test this, you can use a pair of polarized sunglasses. Put on the polarized sunglasses and look through the CPL filter as you rotate the filter. If you're looking through the front of the filter, the filter should go dark then light as you rotate it. Looking through the back of the filter (where the lens would be), however, you shouldn't see much difference as you rotate the filter.

If you don't have polarized sunglasses handy, you can use another CPL filter with the back of the filter pressed to your eye. When you rotate one filter in front of the other, they should both go dark only when the fronts of the filters are facing each other.


I'm guessing the poster was just trying out the filter in insufficient light for auto-focus to lock properly.

[SkipD]

The easiest way to be absolutely sure that a polarizer is circular vs. linear is to hold the filter up between you and a mirror. You want to be holding the filter so that, in the mirror, it covers the eye that you'd be using to look through the filter. Thus, you are looking through the filter twice to see your eye in the mirror. Then flip-flop the filter so you are looking at the other side of it.

If you have a linear polarizer, you will be able to see your eye in the mirror with the filter held both ways.

If it's a circular polarizer, you will see it blacked out in one direction and in the other direction you'll be able to see your eye in the mirror.

[foxbat]

I suspect the polarizer is just fine and there was either not enough light or the OP bumped the AF/MF switch when screwing the polarizer on...