I know that the lens has 72mm filter threads, but do you believe that there would be any vignetting present if I used a 72mm-67mm step down ring? I ask because I already have a number of 67mm filters from when I had a Panasonic 100-300, and if I could use them on a 40-150 it would be ideal since 72mm filters are expensive. Looking at the image of the lens below, there is quite a bit of real estate between the edge of the front element and the filter threads (it looks like there is more than 2.50mm between the element and the filter threads per side). To any owners of the lens, would you be able to measure what the diameter of the front element is and let me know? I would greatly appreciate it. Thank you!