Sony alpha + kenko teleconverter = bad times!

[plasticmorph]

Hi,

I just recieved a Kenko TelePlus MC7 2X AF TeleConverter

When fitted it behaves badly!

with the kitted 18-55 it constantly tries to refocus when on AF and it is IMPOSSIBLE to get it to focus. With my tamron 70-300mm, in bright light it will focus (just) however the focussing ring has become very stiff and the motor is very slow.

Is this normal??

[ap4a]

What max aperture is the lens? A 2x converter will usually reduce it by two stops, which may be enough to kill the autofocus capability of your camera (Canon's consumer SLRs for example won't autofocus with lenses that can't achieve f/5.6 or higher (f8 centre focus point?) - including when the limitation is caused by a TC).

[plasticmorph]

the 18-55 for example, has 5.6 on the side of the lens, however on the aperture settings on the camera it seems to go down to 4.5 when on about 30mm

(amateur)

[ap4a]

Originally Posted by plasticmorph

the 18-55 for example, has 5.6 on the side of the lens, however on the aperture settings on the camera it seems to go down to 4.5 when on about 30mm

Okay. Without a TC your camera can autofocus fine with that lens. When you add a 2x TC that f/5.6 actually becomes f/11 and the f/4.5 becomes f/9. So your camera likely won't be able to autofocus with that set-up, leading to the issue you're having.

[plasticmorph]

ok great, thanks for the help. so it that just because the AF sensors need more light to detect a sharp edge?

I'm happy with that then because I dont mind manual focussing at all, my only concern is how the tamron focus becomes stiff and 'grainy'. It seems that the 18-55 has a motor built in and the tamron relies on a small spinning tooth that projects out of the camera body into the ring on the lens mount. I guess the teleconverter will always add some resistance but this much??

[ap4a]

Yeah, autofocus is done with the lens wide open. I've just checked and confirmed that, for example, my 40D has horizontal and vertical sensors that can AF at f/5.6 and a central precision cross-type sensor that requires f/2.8 or wider. The pro bodies also have sensors that can AF at f/8 or faster, but the consumer and prosumer lines don't (the 7D may be different).

It's likely that the same is true with Sonys.

I can't help with the issue with the Tamron as I have no experience of it.

There are people having similar experiences (though not with the same kit), and a couple of suggestions of possible causes here: http://photo.net/nikon-camera-forum/00X0DQ