13th March 2010, 05:45 PM
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?
13th March 2010, 09:13 PM
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.
Last edited by Dream Merchant; 13th March 2010 at 09:15 PM.