All models feature the exclusive Ceralin Plus coatings on all air to glass surfaces, and HD models have flourite lenses for apochromatic correction, awesome resolution and unmatched color rendition. EXCEPTIONALLY close focus, with fine adjustments. Rugged, built to take it. Waterproof and nitrogen purged. The HD models utilize fluorite lens and provides extraordinary detail and virtually perfect color resolution in very low light or high magnification. All Optolyth spotters come with a lifetime warranty.