A couple years ago I purchased a set of macro filters. After doing some research, I realized that filters work exactly the same way as everyday reading glasses. They work by changing the working distance and magnification of the lens. The filters has a number printed on the side (+1, +2) that indicate how much the working distance will change. The new working distance can be calculated by 1/D (D being the number on the filter). Df (f being the focal length in meters) gives the new magnification.