is this normal or do these cavision filters suck?

[cheezweezl]

i got this set of 4x4 filters made by cavision. it has a polar, a black mist, and 3 grads (nd, blu, orange). the thing is they seem to be throwing off my focus. for instance, i zoom in a nd focus on an object. then i drop in any of these filters and it's all the sudden out of focus. i can adjust focus to bring it back in but it's not as sharp as it was and it seems to have a blurry vignette. it is very severe at the long end of the lens. it doesn't seem to affect wide shots but i assume the problem minimizes as dof increases. wtf is this? just crappy glass? they seem to be decent quality. i have used cheapo cokin plastic filters on an hvx and not had this trouble.

any ideas?

 

[Noel Evans]

Its the filters and they are not very good. Had this happen on a 2 camera shoot in Hiroshima with a crew from US. They had a beautiful canon lens, and for a moment we were adjusting back focus - until we popped the filter and saw its havoc.

 

[cheezweezl]

wow. they have really f'd this up. i have shot thru cheap screw on filters before and no problem. i have also used cheap cokin resin filters. it's funny because i don't see any distortion when looking thru these with my eye but obviously there is some....

well i guess these will be flying back to b&h.