One high quality teleconverter from Sigma, for Nikon F mount. Used but in excellent condition, from a friend of mine who took very good care of their kit, with soft carry case and original caps for both ends.

Perfect match with Sigma's own bright, high end F2.8 and F4 lenses, this doubles the focal length - great for wildlife, and a lot lighter to carry than a second longer length lens!

Do double check for compatibility! Sigma publish a list of compatibility with their own lenses at that link, but you'll need to check forums etc for user reports of it's use with Nikon or other lenses!

Like any teleconverter, it's not a magic bullet - there's a reason long lenses cost a bomb! This will halve the light getting through to your sensor - e.g. turning an F2.8 into an F5.6, an F4 into an F8. This means you need longer exposures with more chance for camera shake, and if it gets too dark, you'll lose auto-focus as well. Also, this will magnify and exacerbate optical issues with the lens, and is an extra "unknown factor" in the optical path, which hasn't been designed for - do some research!

In summary - if you want good results from a teleconverter, you need a good, bright lens to begin with. Don't expect to stick this on a plastic 300mm and get worthwhile performance at 600! In many cases, you're better off cropping :)

That being said, it's an awesome tool to have in your arsenal, and opens the door to pictures that would not have been as achievable otherwise. With a good lens to begin with, like one of the pro 70-200mm F2.8's, turning that into a 140-400 F5.6 is an excellent option to have.