Teleconverter Help Needed

Dexter247

Dexter247

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Chris
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I have a Sigma 70-300mm f4-5.6 APO DG on a pentax K-r but I would like a little more reach but with out breaking the bank, Could I use a Sigma or kenko 1.4x teleconverter on this lens and still retain autofocus.
If this combination works how much extra reach will I get?
 
You'll have to check with kenko/sigma as to lens compatibility, as some catch the tele. If it works you'll get 1.4 * 300 or on a dx 1.4 * 450.
It will auto focus but only in very good light so you'll prob find it hunting a lot and having to use manual.
 
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Teleconverters work best on telephoto primes, and can be good on high end zooms. They are likely to disappoint on on your lens. I had that lens in the past and got some good pics with it, but it won't thank you for adding a 1.4x.

Phil
 
that might not even work
they're designed for the specialist lenses. not the 70-300 standard telephoto ones
infact it might be that your rear lens element would come back far enough to damage the TC and your lens
as a general rule TC's do not work with 70-300 in my meagre experience.
also you'll lose a stop of light, so AF might not work as mentioned above

I put a 2x tc on a f2.8 lens and it was fine, apart from the drop in quality.
but that's because it made the lens around an f5.6 wide open which was still functioning.
if I'd started with F4 then maybe it wouldn't have
 
Your lens will effectively become a 398mm F8, you will most likely lose autofocus as well as most autofocus points require an aperture of F5.6 or wider in order to work.

You'd do better picking up a secondhand sigma 400mm APO f5.6 and giving that a try
 
I use a 2x teleconverter on my beercan...minolta 70-210 f4 and it works a treat. Autofocuses across the range. No problems.
 
This is my understanding of how things work:
Reporting TCs: A TC-compatible lens (e.g. 400/5.6) detects the presence of a TC through the extra contacts and reports the correct f-stop for the combination to the camera.

Non-reporting TCs, or taped TCs, or TCs with non-TC-compatible lenses: The lens does not detect the TC and just reports its own f-stop. AF may still work, because the camera doesn't know about the true f-stop, and tries to AF even if the combination is outside the specs.

Kenko DGX: Unlike the above TCs, the DGX manipulates the communication between the lens and the body so that the f-stop is always reported correctly for the combination, and so that AF is retained even if the combination is above f/5.6. From your and others observations that f/8 is reported but AF still works, my guess is that wide-open f-stop and used f-stop is reported separately and the camera looks at the wide-open f-stop to determine if it should try to AF.

Personally unless you have fantastic light conditions a 2X TC on either of your lenses won't produce great results. Nikon's slightly different to Canon, Canon limit autofocus to f5.6 for all but there 1D series camera's, with Nikon autofocus still works (I've been led to believe) but with both brands it depends on the compatibility of the lens with the TC as only certain lenses work.

TCs are really designed for the faster prime lenses not zooms because the optics can handle the drop off in image quality and hit on the auto focusing, but you can use a zoom and TC but be aware you'll need decent light conditions to get anything worth while image wise.

Personally I would never even think about attaching a TC to a 70-300mm lens
 
Thanks to you all for taking the time to answer my question, looks like I will have to save my cash for a fast prime lens of around 400mm. Thanks again
 
Just a note, and it may be only me, but I did notice a notice image quality diference when I put the TC on. The images were still fine, but at closer inspection imperfections were more obvious, and by imperfection I mean granulation or something similar.
 
DewanDemmer said:
Just a note, and it may be only me, but I did notice a notice image quality diference when I put the TC on. The images were still fine, but at closer inspection imperfections were more obvious, and by imperfection I mean granulation or something similar.
Click to expand...

That's correct about IQs.
You lose 1 f-stop for a 1.4x TC (f2.8 become f4) and about 50% auto-focus speed with little or no image quality degradation. 2 f-stops for a 2x TC (f2.6 become f5.6) and about 75% auto-focus speed plus a degrade in image quality.
 
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