Barlow Lenses

Barlow lenses multiply the magnification of the eyepiece in use by a specific factor. So a 2x Barlow will double the magnification, with an eyepiece normally giving 36x then giving 72x. Purchasing a Barlow lens can effectively double your eyepiece collection as each can now produce two powers of magnification. For this reason it is not a good idea, to initially, purchase an eyepiece that has the equivalent double of another eyepiece in your collection. For example a 5mm eyepiece will have the same magnification as a 10mm with a 2x Barlow. 

Get the best Barlow lens that you can, and for visual use keep to 2x Barlows rather than those offering greater amplification. Modern units such as you see here have good optics & high light transmission so that image degradation should be unnoticeable. But remember that increasing the magnification of your telescope close to the theoretical upper limit will reduce the clarity anyway. 

The multi-element Barlows matching the premium grade eyepieces are the best to get. But you will pay nearly double the amount for the eyepieces and the Barlows so these are a pretty serious investment. But like a lot of astro accessories they don't exactly wear out so they will last quite literally a lifetime if looked after.