The Sigma TC-2001 2× Teleconverter extends the focal length of your lens. This teleconverter is made for Nikon F-mount lenses and cameras, and lets the lens communicate with the camera to allow an autofocus of up to f/8. There are two Special Low Dispersion lenses in the optical design of 10 elements in 5 groups that reduces chromatic aberrations. The light loss is limited to two stops for maintaining the image quality and ensuring accurate focus and exposure adjustments. You may use the teleconverter in challenging weather conditions, thanks to its dust and splashproof construction.
This is the magnifying factor of a lens. Macro lenses will often be described by their “magnification factor”. A lens with a 1:1 magnification factor produces a projected image on the sensor which is the same as the subject.
The individual pieces of glass that form the overall optical construction. Most lenses have between four and eleven elements to bend the light rays to ensure a perfectly formed image appears on the light sensitive surface, such as a film or CCD. The elements are arranged in groups and may be seen in specification sheets as, for example, 6 elements in 4 groups. That configuration may have two single elements and the other four grouped in pairs.