The Vivitar Series 1 Teleconverter for Pentax (7 Elements) with a Pentax mount effectively doubles the focal length of your lens, maintaining its resolution and contrast. The optical design featuring 7 elements yields superior field flatness and edge sharpness compared to converters with fewer elements. The lens elements have an anti-reflective coating which reduces flare and aberrations.
This teleconverter will multiply the focal length of your existing lens by 2x, and will cut back the amount of light by 2 f/stops while maintaining full autofocus coupling. For example, using a 2x teleconverter with a 300mm lens will effectively increase the focal length to 600mm. A lens set to f/2.8 will now be f/5.6.
Teleconverters are not recommended for lenses wider than 50mm, e.g. 28-105mm. However, full AF operation is only possible with lenses with open aperture of f/2.8 or brighter. Please be aware that AF will work properly only if there is enough light and contrast on the subject to activate the camera's AF sensors. Manual focusing is recommended when using lenses with smaller open f-stop values than those given above.