Why is my star filter fighting my AF?

[cw318is]

Evening all, hope someone can help me with a problem I had earlier. I was doing some product shots of some jewellery for a friend of mine, who wanted the full of QVC-esque glint, so I tried my new Cokin Star 6X. It gave the right results, giving a bit of glint in the right places but also made the AF hunt, had to go to manual focus in the end. Focused without the filter, switched to MF and dropped the filter in and its fine and sharp, so no idea whats gone on.

Anyone got any ideas? I'm stumped. Focussing using center point only, One Shot AF. Lens was my Sigma 28-300 Macro at 300, using a very well lit softbox (tried a Cokin ND4 in case it was not enough light for AF but that had no issues and my star filter isn't tinted).

 

[Musicman]

I would conjecture that the fresnel lens that makes up a star filter is interfering with the (phasing of the) light in such a way to make the phase detection AF system used on your your 20D unusable; in the same way that using a linear polariser does.

e2a: If you had a more modern body with Live View and contrast detect AF, you might find it worked if you switch to Live View.

Why don't you just shoot normally and add the stars in PP? Much more control over the finished result

e.g. http://photoshoptutorials.ws/photoshop-tutorials/photo-effects/star-filter.html

 

[lawrie29]

Be fine if you had a Nikon you know, ask Arkady.....

 

[cw318is]

got to the bottom of it today, and I think its the AF on the Sigma. Tried it on a friends Canon 100 on my 20D and its absolutely fine. Then tried my Sigma 28-300 on his 450D and the AF went crazy again.

Maybe its time I invested in a proper macro lens!