Some time ago i made this basic magnifying glass lens, i installed a aperture in front of the lens, it's about 27mm from the lens element. Does this affect the way i should calulate F stops? The lens is 195mm in focal length, largest opening is 60mm. So 195 / 60 would be something like f3.2? But it started wondering if the gap between the iris & the element would screw this calculation up, as the lens is i bit larger than the largest iris opening..