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04-12-2009, 04:49 PM   #1
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Teleconverter and Pentax 55-300

Hi all-Happy Easter!

Can a TC be used on the Pentax 55-300 and still get quality IQ??

If yes, what TC would you recommend?



04-12-2009, 07:21 PM   #2
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QuoteOriginally posted by Rmpjr7 Quote
Can a TC be used on the Pentax 55-300 and still get quality IQ??
Happy Easter, Bob. The main issue with those consumer zooms is that the maximum aperture is quite limited.

If you want a teleconverter, I assume it's because you need more reach, and not for macro use. So you will always use the lens at 300mm, where the aperture wide open is a paltry f/5.8. With an 1.4x teleconverter, you will get a maximum aperture of F/8.3, and with a 2x teleconverter, the lens will become a F/11.6 telephoto.

Needless to say that the autofocus will not operate at F/11.6 -- and even at F/8.3 -- and that manual focusing at F/11.6 will be nearly impossible except in bright sunlight. Let me add that you will have to close down the lens at least one or two stops to get good optical quality -- that's true even with the best teleconverter -- and you will see that this kind of combo is what I call a "moon lens," because getting moon shots with the help of a steady tripod is about the only use I can see for it.


04-12-2009, 07:43 PM   #3
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Rmpjr7: Can a TC be used on the Pentax 55-300 and still get quality IQ??
The consensus is no. this zoom is not good with a tele. Even when the obstacles which Abbazz highlights above are overcome, the IQ suffers too much.
04-13-2009, 12:04 AM   #4
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QuoteOriginally posted by Rmpjr7 Quote
Hi all-Happy Easter!

Can a TC be used on the Pentax 55-300 and still get quality IQ??

If yes, what TC would you recommend?


TCs are usually designed for lenses that are 2.8 and above.

You lose one stop with a 1.4x or 1.7x.

Two stops with a 2x.

04-13-2009, 12:18 AM   #5

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QuoteOriginally posted by Rmpjr7 Quote
Can a TC be used on the Pentax 55-300...
I think so. I tried a Tamron 1.4X TC with my Tamron 18-250mm. With the lens at 250mm (max aperture f/6.3), if there is something contrasty for the AF sensor to latch on, the lens will focus. It hunts, but will focus.

QuoteOriginally posted by Rmpjr7 Quote
... and still get quality IQ??
I don't think so. For most zoom lenses, the IQ at the long end is already so-so at best. Now with a TC added, good IQ is really iffy. There's no point to use a TC if the lens's focal length has not max'ed out.

In the case of my Tamron 18-250mm, I find that cropping is better than with TC.
04-13-2009, 09:30 AM   #6
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I must be doing something wrong because I occasionally use a TC on a telephoto lense with mixed results. This photo was taken with a K20, sigma 50-500 at 500mm with a Promaster Spectrum, 1.7 sr turned off and using the truck window as a steady rest. Auto focus worked just fine.

04-14-2009, 02:47 AM   #7
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I have used my Tamron x1.4 TC with my crappy Pentax FA-J 75-300 a few times with half decent result .... not the best I guess ... but neither is the FA-J ... but it gave me the reach I needed at the time.

It was used on a bright day and AF worked surprisingly well at the long end too. The key being a "bright" day. Don't expect AF work on a dull-ish day though.
04-15-2009, 02:52 PM   #8
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The result is much better if you just crop the image. You will loose both sharpness and contrast with a TC.


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