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05-21-2008, 06:46 PM   #1
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TAMRON 1.4 TC converter has trouble auto focusing.

My camera is a pentax k100d super. I just bought a TAMRON 1.4 TC converter and it has trouble auto focusing. The lens I am using is a TAMRON AF di II 18-200mm 1:3.5-6.3 and AF Di 28-300 1:3.5-6.3. What can I do to make it auto focus.

05-21-2008, 07:15 PM   #2
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QuoteOriginally posted by jon pafford Quote
My camera is a pentax k100d super. I just bought a TAMRON 1.4 TC converter and it has trouble auto focusing. The lens I am using is a TAMRON AF di II 18-200mm 1:3.5-6.3 and AF Di 28-300 1:3.5-6.3. What can I do to make it auto focus.
You will need very bright light for this lens to autofocus with the teleconverter on it. AF is usually rated as operable at f/5.6 and faster. With the converter, your lenses hit 6.3X1.4 or almost f/9 (8.92) at the long end.

One way to help your lens focus is to use the center focus spot only (the camera doesn't have to decide among the 11 points) and lock your focus on a sharp, contrasty edge tone. A table leg with a different colored background, for example.
05-21-2008, 07:31 PM   #3
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QuoteOriginally posted by jon pafford Quote
My camera is a pentax k100d super. I just bought a TAMRON 1.4 TC converter and it has trouble auto focusing. The lens I am using is a TAMRON AF di II 18-200mm 1:3.5-6.3 and AF Di 28-300 1:3.5-6.3. What can I do to make it auto focus.
Teleconverters normally are designed to work with lens with apertures f2.8 and above lens.

VR
Darryl
05-21-2008, 07:54 PM   #4
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Tamron TC's that I've used specify an aperture of F/4.5 or larger. The 2X units require F/4.0 to guarantee AF will work. I confirmed this - just checked my unit's pamphlet/manual - it's a Pz-AF MC4 unit, Pentax mount.

Even the Tamron 1.4x "Pro" in Canon mount doesn't work well with anything other than the Mark II or Mark III bodies (thanks Codiac for the reminder/explanation of that problem). It's practically useless with the Sigma 500/4.5 - AF constantly trying to fine tune the AF lock, so I missed some very good shots... Back it went to Adorama...

Note that even with my FA* 300/2.8, the Tamron 1.4x TC still has issues in lower light or lower contrast images. CR has it on the money - some contrast is needed too... and your lenses have smaller apertures at the long end... that's tough on the AF to work. No TC is really a catch-all solution - something is a compromise - every time.

Regards,
Marc


Last edited by Marc Langille; 05-21-2008 at 08:09 PM. Reason: clarification
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