UV filters are usually sold for protection but polarizers have a genuinely useful photographic purpose, attenuating glare, surface reflections and increasing color saturation of the sky and some other surfaces as a side-effect. Generally, the less you pay, the more degradation there will be. There will be a degradation in image quality if you use a filter. My lenses have 8 filter sizes so you can imaging how much money I saved on polarizers alone! Even if you have just two lenses with different filters you'll save. It saves lots of money considering a good polarizer costs over $200. The user can opt out of using the shock absorbing. The colored half allows you to add additional color tones to a scene such as a sunrise. This INSERT is an optional way to securely attach a Tadashi Filter to a Nikon 10.5 or 16mm fisheye lens. The other half has a graduated density coloration with a light sunrise color. The only catch that you can't use a step-up ring and a lens hood at the same time. The Tiffen 62mm Color-Grad Sunrise Graduated Filter is one of the most unusual 62mm effects filters but it allows you to have some fun. A step-up ring costs about $12, so if you buy 77mm filters and have a 58mm, 67mm and 77mm lens, you need 2 step-up rings: 58->77 and 67->77. Instead, I buy my filters in the largest size (77mm usually) and have step-up rings to bridge the gap. This is the thread size which means you can attach that size of filter directly. Multi-coated to minimize reflection at the filter surfaces which reduces flare and ghosting.This filter has no affect.

Some specialty lenses do not accept filters, in which case you won't find any markings. Also serves as a permanent lens protector.

As per other answers, it is usually marked with the ⌀ symbol on the front and, if not, on the barrel.

62 mm nikon lens filters