Kood Step-Down Ring 58mm - 48mm

I purchased this 58mm to 48mm step down ring for my Canon 50mm 1.4 and 28mm 1.8 prime lenses in order to fit an astronomy Light Pollution 2" filter (blocking mostly Sodium Vapour and not so much Metal-Halide - based on the filter spectral response charts) sky glow (not so much flat pollution but more gradients which are a pain to remove post processing). These filters are 2" hence the step down adapter ring.

I put on the step down ring and expected it to show bad vignetting, however, I looked...