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Old Aug 22, 2007, 6:18 PM   #1
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I have a cheep canon 75-300 lens. Can I use a teleconveter on it. I see some on ebay like the Kenko 300 pro. Will this work? Or is there a beter one for the price that will work?
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Old Aug 22, 2007, 9:49 PM   #2
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Maybe, maybe not.

Most cameras require an aperture of around f/6.3 or wider (smaller f/stop numbers) before they will Autofocus.

You will lose 1 stop of light with a 1.5x TC. So, your 75-300mm f/4-5.6 with a 1.5x TC would be like mounting a 112-450mm f/5.6-8 lens. That means it would not normally Autofocus if you zoomed in much.

A 2x TC would cause you to lose a full 2 stops of light (your 75-300mm f/4-5.6 would behave more like a 150-600mm f/8-11 Lens).

But, some TCs do not have any electronics, and the camera may not be aware that they are in place. As a result, the camera may still attempt to Autofocus, despite the light loss from the TC. You'd have to try one to find out if it could still Autofocus in good light or not.

A TC is best used on a brighter lens (i.e., a lens that can maintain f/2.8 throughout it's zoom range).

Otherwise, the camera may not attempt to Autofocus at all, and even if it does, it may hunt a lot trying to achieve focus lock due to the light loss associated with a TC in less than optimum lighting.

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Old Aug 23, 2007, 7:59 AM   #3
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And then there's the matter of quality. You're talking about taking a lens with already poor optical quality and putting another piece of glass on it. So even if you are manually focusing, I think the results will disappoint.
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Old Aug 24, 2007, 2:59 PM   #4
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i used to have a Sigma 70-300 F/4-5.6 with a Kenko Super 300 2x teleconverter oon a Canon 300D.I found that i will autofocus at 70mm and as the sigma does not loose focus when you zoom it was kinda okay.
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