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02-05-2007, 04:43 PM   #1
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What teleconverter for which lens?

Okay folks! I've never used a teleconverter before, so am pleading an advanced case of Gross Ignorance on the subject! What teleconverters are available for use with Pentax lenses, which ones do you recommend and why, and what experience do any of you have with them?
My (limited) understanding is that they reduce the available light by X stops, with X being an amount determined by the focal length of the teleconverter (i.e. a 2X would reduce your lens by 2 stops.) Enlighten me, oh great Pentax gurus!
Rob W

02-05-2007, 07:53 PM   #2
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Teleconverters fit into two broad categories. 1. special purpose. These are usually designed to only work with a few matched lenses. Often the teleconverter lens elements actually reach into the camera lens. This means they can't even mount on certain lenses. 2. general purpose converters. These are more generic in use and will physically fit any lens. They are not made to match particular lenses.
Teleconverters also come in different strengths. Usually 1.4-1.5X, 2X, and rarely a 3X. You are right that the X is also how many stops of light you loose. Most people go for 2x (it may be same price or less compared to 1.5X). This is not always the best choice. AF won't work very often with 2X unless the lens is very fast to begin with. Also you multiply everything by 2X, including the lens aberrations. A 1.5X is usually better quality because it "stresses" the main lens less. I have had good luck with the Kenko 1.5X converter. Others recommend the Tamron 1.4X. Pentax makes both 1.4 & 2x converters that are further divided into Short & Long. This gives 4 Pentax converters. The right converter must be used with the right lens. Pentax doesn't make an AF converter (except the rare 1.7X which turns non AF lens into AF). No KA mount converter works with the K10D in SR mode. That will probably be addressed in the future. Hope this helps.
thanks
barondla
02-05-2007, 08:28 PM   #3
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A little correction

A 1.4x teleconverter will cause a loss of 1 stop of light while the 2x teleconverter will cause a loss of 2 stops of light. Remember that the f-number represents the ratio of the focal length over the aperture diameter but the amount of light the aperture allows is a function of the area, so that's why f-numbers follow this sequence, based on full stops:
f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11

If you increase the focal length by 1.4 but you don't change the aperture diameter, you end up with a loss of 1 full stop of light.

It's late and I'm about to go to bed so that's enough thinking for today...

Last edited by Jun Park; 02-05-2007 at 08:31 PM. Reason: added a little more detail
02-05-2007, 09:04 PM   #4
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QuoteOriginally posted by macdaddy Quote
Okay folks! I've never used a teleconverter before, so am pleading an advanced case of Gross Ignorance on the subject! What teleconverters are available for use with Pentax lenses, which ones do you recommend and why, and what experience do any of you have with them?

Rob,

I have only limited experience. I have a Tamron 1.4x converter. I can tell you that it seems to work pretty well. I does extend the "reach" of my telephoto zooms by about 40%. I say "about 40%" rather than "exactly 40%" because I'm a bit uncertain about the magnification factor. Apparently there's a 1.5x converter on the market that is identical to the Tamron 1.4x - just marked with a different magnification. You'd think that would be the kind of thing that could be measured accurately.

Sigma makes converters, too, but if you look carefully at their web site, they are rather fussy about what lenses should be used with their converters.

As far as I can tell, a converter is basically a money-saving device. I am pretty sure that a real 400mm lens would be better than, say, a 300mm lens of roughly comparable quality used with an extender. But the Sigma 135-400 costs well over $500; the Tamron 1.4x converter costs under $100.

Will

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