An optician told me how to check for naff lenses. It was 30 years ago, so cant remember fully. You pick a vertical line and tilt the lenses sideways. If the line distorts they will screw your eyes up. At the time, I used to test all sunglasses. 90% were faulty and harmful. Even prescription ones. I just buy the darkest I can now. I squint for cleaning windows, or close my eyes lmao