New Member
Jan 20, 2010
i have read that aauto focus 2x teleconvertor does not work for lens above 50mm.

Autofocus can work reliably only down to about F/5.6 or F/6.7 at best. In best lighting condition and if there is something contrasty to lock on, it can work down to F/8, maybe with some hunting.

With a 2X TC, the effective max. aperture is 2X of the max. aperture of the lens. For example, the Tamron 18-250mm F/3.5-6.3 when combined with the TC becomes a 36-500mm F/7-12.6, too slow for autofocus.

so any one here has successfully used an auto focus 1.4x teleconvertor instead for 250mm lens?

Dream Merchant

Staff member
Jan 11, 2007
Depends on what's the AF limits of a particular body.

Let's say it's f/5.6 - fast and assured AF
f5.7 - f/7.9 - AF slows down and hunts
past f/ 8 - AF not useable

You can use ANY focal length and brand/system-specific tele-converter as long as the multiplied maximum f-stop is within the operating limits of a camera's AF system.

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