Hi Steve, all I can say is that in my days as a pro-tog I occasionally bought s/h fast Zeiss lenses with a coating mark on the front element as a means of affording ownership. Even in extreme lighting conditions, I do not recall any of the thousands of slides betraying a problem except for one occasion with a 15mm where I got some unusual flare when shooting with strong directional light entering at near right angle to the part of the element with the coating mark. Nowadays, this would have been a breeze to correct in software if producing prints. As most of my work was gymnastics photography, usual lighting conditions were subdued indoor and of course, no flash! Lenses were always wide open - no problems. Even when shooting at events with TV lights, no problems. Mind you, I was often shooting with GAF500, so spotting any problem through the golf-ball grain was difficult! Those were the days...
I'm sure that the likelihood of problems arising is entirely contingent upon the extent of coating marks and the extremes of light travelling through the affected part of the lens. I reckon from experience, in over 95% of cases, no one would ever spot a problem in the resultant image. If you sell lenses, the drop in perceived value is a bummer. If like me you buy lenses primarily to use, they can be a real bargain!
Cheers,
Graham
