Posted by Mike
Well not really ruining the lens, but definitely ruining the optical performance. Glass is not glass. There are different types of glass, so the silica glass that makes a window, does not make a high quality optic. Silica, fluorite, lead and other minerals are used to make glass that has different refractive (how the light bends) and reflective (how the lights reflects) indexes. The way the glass allows the light to transmit, or not be transmitted it part of what makes your lens.
But it is also the super thin coatings that are put on the lens. These coatings manifest themselves in the colored reflections you see when you look into the lens. Better, deeper reflections typically mean a better richer coating which translates into better sharpness, colors and contrast.
So a cheap filters, with little or no coating will decrease the sharpness, contrast and color charistics of your lenses. The Promaster HGX filters are some of the best made. The coatings are designed to transmit the light in the best possible way. You can visibly se the difference between having a cheap filter and having an HGX filter installed on your lens.
We also warrant our HGX filters for life. If they are defective, or are damaged in any way, for any reason; we will replace the filter. HGX filters can be found at this rather long address: http://spartanphotocenter.com/products.html?catalog%5Bsearch%5D%5Btext%5D=HGX&catalog%5Bsort%5D%5Bon%5D=relevance