Other people have already commented on this one. When adding a filter, the focus point shift by 1/3 the thickness of the filter. This is negligible for most glass considering how thin it actually is. But with digital, the focus could shift if the filter is causing reflections. This is what digital lenses and filters actually prevent through added coatings and in some cases a different shaped opening at the rear of the lens. Case in point, the Canon 24-70 original had a round shaped opening, newer ones are rectangular to handle digital better. Canon did this about a year or so back. The only way to truly determine if your test is 100% correct, is to mount the lens on a tripod. Focus then switch off AF. Take a shot with and then without filter and compare. AF could be causing this. If not, and the results hold true, try it with another brand filter and see if it still holds true. You may have a bad filter. Just my 2 cents.
Peter K * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
