CPL filter and AF issue

[emanresu]

I recently purchased a Circular polarizer filter and tested it today with my XSi.  But several times, the automatic focus kept going back and forth and would not let me press the shutter.  Is this normal?  Or did I do something wrong (confession: I left my Hoya Multicoating UV filter on and added the CPL on top of it)?  Thanks.

[Bob Atkins]

Shouldn't be a problem unless the light was really low and the CPL dropped it below the AF threshold. Double filters aren't a good idea, but I doubt that was the cause.

I suppose a really bad, cheap CPL filter might distort the image enough to bother the AF, but it would have to be pretty bad. I have Hoya and Tiffen CPL filters that were not expensive and which work just fine with no noticeable effect on AF.

[emanresu]

Thanks Bob.  My CPL filter is sunpak.  From Amazon, it receives good reviews albeit the number of reviewers being low, and the only complaint is about the filter's fitting rather than its optical quality.  I will try to remove the UV before mounting CPL in the future (it is just a bit hassle to juggle with two filters, increasing the chance of dropping one, getting dust on them etc.)

http://www.amazon.com/Sunpak-CF-7059-Circular-Polarized-Filter/dp/B00009W3U9/ref=sr_1_2?ie=UTF8&s=electronics&qid=1260735497&sr=8-2

[Bob Atkins]

That's a pretty cheap circular polarizer and CPL filters are fairly complex to make. Maybe it's OK but in general I'd advise sticking with the better brands, especially for polarizers. Tiffen and Hoya are usually OK.

I'd worry that anyone reviewing one probably hasn't compared it with a Hoya, Tiffen, B&W or Heliopan and so may not be aware of any issues it might have. I'm not saying there's necessariliy anything wrong with it, just that I would not recommend it

For example: Tiffen 58mm CPL and Hoya 58mm CPL

I actually have both Hoya (58mm) and Tiffen (77mm) circular polarizers and both of them are pretty good. I tested them for image degradation and neither results in any significant sharpness loss or AF problems. Of the two brands, if the filters were the same price, I'd probably incline towards the Hoya. Hoya have been in the glass business for a long time and are one of the most respected industrial, commercial and specialty glass makers.

[emanresu]

Thank you for the recommendations, Bob, and I understand perfectly why you don't recommend the sunpak brand.  Living on a student budget, I figured that for the UV filter I would get a good one because it would be mounted most of the time.  But as for the CPL, since I am new to photography, I decided to get this cheap one just to get a feeling of when and how to use a CPL filter and later go for a better one when I am more familiar with it.

The AF issue may be caused by the glass after all, and I guess I will have to use MF when this happens. :-S