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11-16-2010, 01:24 PM   #16
Pentaxian




Join Date: Feb 2009
Location: Alameda, CA
Posts: 3,194
QuoteOriginally posted by jolepp Quote
One aspect with TCs and AF would be that these might push the effective minimum aperture beyond AF capability: phase detect AF is based on light entering from the borders of the lens focusing at the same point when the lens is in focus and on different points otherwise. When the aperture gets smaller, the difference when not in focus gets smaller and the AF sensors eventually stop working.
The autofocus incompatibility of Tamron 2X TC and some lenses (Pentax 100mm Macro WP F/2.8, or Ltd. DA 70mm F/2.4) is not due to the insufficient light hitting the autofocus sensor. The combo Tamron 2X TC + DA 40mm F/2.8 autofocuses just fine, albeit a bit slow.

My Tamron 18-250mm F/3.5-6.3 autofocuses OK with the Tamron 1.4X TC, even at the long end of the zoom range at effective aperture of 8.8. It is slow and needs something contrasty to lock on, but will autofocus.

There must be something else at work here.

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