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Old Jan 10, 2011, 9:29 AM   #1
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Hello all:
Looking for a teleconverter for my sony A300. I have a kit 18-70, a Sigma 70-300 APO DG, and a 50mm f1.7 i recently purchased. I would like to purchase a teleconverter to extend the reach of the 50mm and 70-300 mm. I realize I will loose a few stops of light but think it would be a good addition to my present collection. Can someone give me some good brands to look for? I have been on Ebay and found several brands I have never heard of. Also what size tele converter would be best? Thanks in advance.
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Old Jan 10, 2011, 10:01 AM   #2
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Most inexpensive teleconverters are made by Kenko. In addition to Sony's high end teleconverters, Sigma and Kenko also make their own high end teleconverters. (Tamron also sells a line of high end teleconverters that are made by Kenko.)

Adding optical elements to any system will degrade image quality. The high end teleconverters have the least impact, however. In general, even the inexpensive 1.4X and 1.5X teleconverters are reasonably good. Inexpensive 2X teleconverters tend not to be so good. They are available with either 4 optical elements or 7, and the ones with 7 are supposed to be better.

In addition to adding to the focal length of a lens, teleconverters also increase the f-number. You should know that even a 1.4X teleconverter will increase the f-number of your Sigma 70-300 APO DG to f/8 for much of its zoom range, and dSLRs in general will not AF when the f-number reaches f/8 (with the noteable exception of the Sony/Minolta 500mm f/8 Reflex lens), so using a teleconverter on that lens may actually make it less useful to you.

Some teleconverters make their presence known to the camera and some don't. They make their presence know by increasing the reported focal length and f-number that the camera stores in the EXIF data of the images it captures. If the teleconverter doesn't make its presence known, the camera may actually attempt to AF when the true f-number is f/8 or smaller, but the lower light will cause the AF to struggle. There's no way to know whether the teleconverter makes its presence know without trying it.

I've had good luck with a 1.4X and a 1.5X teleconverter. The 2X teleconverter I once owned earned the nickname of "The Purple Fringe Machine".
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Old Jan 10, 2011, 10:04 AM   #3
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For more info, see A-Mount Teleconverters Guide.
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Old Jan 10, 2011, 10:42 AM   #4
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Great information Tcav. Thank you
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