The 2x teleconverter allows you to increase the focal length of the lens by two, fixing it between the lens and the T2 adapter. The lenses used in the converter are made of the highest quality optical glass and have anti-reflective coatings.
The converter, when screwed into the lens, doubles the resultant focal length of the telephoto lens, for example, screwed into a 500 mm F / 6.3 lens, converts it into a 1000 mm F / 12.6 lens. The converter has a T2 thread on both sides (M42x0.75), so it is a universal solution, independent of the body manufacturer (the T-2 ring is screwed to the body of the converter).
(below: similar converter 2x screwed to the Samyang 500 mm f / 8 mirror lens)