Hello,
The Sigma APO Teleconverter 1.4x EX DG for Pentax has sadly been discontinued and was the best option for Sigma lenses. The only 1.4X option is from Vivitar.
Vivitar's 1.4x Teleconverter for Pentax adds another valuable tool to your camera bag, at a very affordable price. This compact and lightweight 1.4x teleconverter multiplies the focal length of any compatible lens by 1.4--a 100mm lens with this teleconverter attached, for instance, would give you the view of a 140mm lens. The multi-coating on the converter reduces flare and ghosting, common problems with digital cameras.
And, the minimum focusing distance is unchanged. This teleconverter comes with a lifetime warranty.
I might suggest getting Sigma's 2X while available. The
Sigma 2x EX DG APO Tele Converter (
also known as a tele-extender) will multiply the focal length of the existing lens by 2x, and will cut back the amount of light by 2 f/stops. 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.
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