When choosing a filter you’ll want to choose one to match the diameter of your lens. Often times the filter size you need to buy for the lens will be listed in the product specs. This number is almost always different than the focal length.

Which Filter do I buy?

When you go to purchase a filter you’ll find a lot of different prices. Tiffen is a good brand and since most times you’re just looking for protection, the cheapest will usually do just fine.