I dont know, I would have to test that again. Could try to do so this weekend. I bought a Tiffen Ultra Contrast Filter as 37mm Version. Then i took a metall saw and removed the filter housing very carefully, so that I ended up with just the glas. At first I had the impression that it might be too thick, but when I put it inside the speedbooster it doesnt touch the speedbooster glas because it is a bit larger than it. The glas of the rear element of the Sigma lens doesnt protrude as well.