What does the symbol on my lens mean

After the focal and aperture ranges, the other measurement you'll see on most dSLR lenses is preceded by ? and describes the diameter of the screw mount on the front of lens barrel. Check this number each time you head out to buy a filter or hood as you can't guarantee that it will be the same for each lens in your collection, even if they are all designed to be used on the same camera.Check the diameter of your lens when heading out to buy a new filter.