An optical system is only as good as its weakest link. Putting a filter in front of a lens will only ever cause a small amount of degradation in image quality, but this shouldn't be noticeable provided the filter is made from high quality optical glass. I was very sceptical about the image quality this polarising filter could offer at its low price (£18.99 at time of order) considering a similar item from Canon costs around £200. Having tested it thoroughly, I have to say that even from my perspective as a retired professional photographer with over 40 years of experience, I am incredibly impressed! It looked great from the start as it came in a lovely silver foil box with a plastic storage case. The manufacturers have wisely included a high quality lens cap and cleaning cloth. There was also a wrist strap included in the pack which was a bit random but welcome nonetheless. When I examined the filter in detail, I was honestly amazed at its build quality. The metal outer ring is very slim, yet it is made in two parts, allowing the polarizing filter to rotate smoothly and freely once attached to the lens. It fits perfectly with my Canon 16-35mm f4 L-series lens. My first impressions were so unexpectedly positive that I felt I had to confirm them with an objective test. You can see the results in the attached photos. These were not created to demonstrate the polarizing effect (which I had already verified works as expected), but simply to find out what effect the filter might have on image quality. The first thing you notice about the full frame is that due to the narrow filter rings, even at the widest focal length of 16mm, there is no visibility in the corners of the image or darkness in the corners of the image. There are also two images taken in the same way as the wide angle view. I cropped them to show the level of detail in the center of the frame. One shows the result with the filter attached and the other without. I really can't see any noticeable difference between the two and will therefore feel comfortable using this filter without worrying about it significantly affecting image quality. Overall (and I must admit rather to my surprise) I can't say anything bad about this filter and am therefore very happy to give it the well-deserved full five star rating