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02-19-2016, 06:48 PM   #1
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Recommended tele to use with Kenko 2X?

Hi all,

I recently came across a Kenko TelePlus 2x MC7 P/AF Tele-converter.

I purchased it in hopes of finding an auto-focusing lens on the cheap that I could effectively go birding with a Pentax K-S2.

I've done some research after the fact, and I've read that people suggest to only use a 2X converter with lenses faster than f/2.8 or shorter than 50mm.

1) Is this true?

2) Could you please suggest some lenses that are relatively cheap and common that I could look for and use with my new converter? Auto-focus is much preferred. And hopefully longer than 300mm?

3) I might be able to get a Pentax SMC DA 55-300 f/4-5.8 for about $150 CAD.. Would that work well with my 2X converter?

4) Any links you could recommend that already discuss this sort of topic?

Thanks for your help everyone!

Last edited by FozzFoster; 02-19-2016 at 07:18 PM.
02-19-2016, 08:05 PM - 1 Like   #2
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A 2x TC enlarges the image by 2x (length and width) and thus 4 times the area, and only 1/4 of the image reaches the sensor. Thus you have a dimmer image by 2 f stops--so your f/2.8 lens becomes f/5.6 (as regards light transmitted). And f/4 becomes f/8. Can your camera AF at this effective f stop?

I have never heard it said TC's work best for less than 50mm focal length, and indeed mostly they are used with lenses 50mm or longer--although there is nothing to prevent using it w/ longer or shorter focal length. The optical formula will do better with some lenses than others, and maybe yours does better w/ shorter focal length. Nevertheless usually the better the prime lens it is used with the better the results (and thus a general consumer zoom is not likely to do well--and a fime prime lens is likely better than a professional grade zoom), and usually the best f stop is about 2 to 3 stops closed down.
02-19-2016, 08:20 PM - 1 Like   #3
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I have tried 2x TCs with my 55-300 and they do work provided it is very sunny or lighting is very good. Other than that forget about it. Also, image quality might not be spectacular
02-19-2016, 08:30 PM - 1 Like   #4
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In general it is best to use teleconverters with fast prime lenses, because they cost you about 2 stops of light for the 2x and about 1 stop for a 1.4x. I have tried the 1.4x on the 55-300 and it works in very good light. Anything less and forget using the AF. A 2x on that lens? I doubt you will be very happy. Generally Pentax says their AF will work at f/5.6 so when you add the 2x TC I think that makes the 55-300 a f/11 lens.

Here is a thread with some folks reporting results.

Also, note there are several versions of this TC, that look very similar. Some are autofocus some not, some work with SDM lenses, some do not.

02-19-2016, 09:39 PM - 1 Like   #5
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Won't the DA 55-300 put you at nearly f/12 with a 2X converter? I take quite a few pictures of birds (I'm certainly no expert, though). I mainly use ED telescopes, so I have to manually focus. I can't imagine manually focusing at f/11 or f/12. Maybe if it's a really bright day, but all too often it isn't. Small birds tend not to sit around and wait while you are struggling to focus. Depending what type of birds you are hoping to photograph, sometimes 300mm is enough. Depends how close you can get.

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