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dkm Feb 4, 2009 9:19 AM

I am thinking about purchasing a t-con for use on a 70-300mm and 18-250mm Tamron lens primarily for bird photography. I have a Pentax K200D camera. I have read some real horror stories regarding t-cons. I don't want to spend the price of a brand new long-range lens for a T-con. Are there any decent T-cons available for say up to $150?

Looking forward to your suggestions.

JimC Feb 4, 2009 9:56 AM

As a general rule, you don't want to use a TC on lenses as dim as those. You'll lose one stop of light with a 1.4X TC (only half the light of the original lens gets through). With a 2X TC, you lose two stops of light. That means you'll need shutter speed 4 times as long for proper exposure for a given lighting and ISO speed using a 2X TC.

That also means that the Autofocus Sensors in your cameras can't see as well to focus. Depending on the TC, many cameras will refuse to Autofocus at all if a lens is much dimmer than around f/6.3 (and your lenses would be dimmer than that on their long with a TC). Some TCs don't report corrected aperture information to a camera. So, the camera may still try to Autofocus beyond it's design capabilities. But, in less than optimum lighting, you may find the Autofocus hunting and struggling to lock focus, even if you use a TC that will actually let the camera try to Autofocus with a dimmer lens.

Just to make matters worse, you do tend to get a bit of image degradation with most TCs. So, you may need to stop down the aperture a bit for sharpest results (slowing down shutter speeds even more). ;-)

I'll move this thread down to our Pentax Lenses Forum so that Pentax shooters can let you know what worked best for them. They'd be more familiar with the capabilities of a given camera's AF system and what TCs will allow the camera to try and focus with lenses like yours.

dkm Feb 4, 2009 11:25 AM

Thank you so much Jim. Your explanation makes sense, even to me!

snostorm Feb 4, 2009 7:14 PM

dkm wrote:

I am thinking about purchasing a t-con for use on a 70-300mm and 18-250mm Tamron lens primarily for bird photography.* I have a Pentax K200D camera.* I have read some real horror stories regarding t-cons.* I don't want to spend the price of a brand new long-range lens for a T-con.* Are there any decent T-cons available for say up to $150?

Looking forward to your suggestions.
Hi dkm,

A 1.4x TC will lose a stop of light from the max aperture of your lenses, so it'll put some stress on your AF system. This isn't the fault of the TCs, just a matter of trying to use something at close to the limits that it was designed for.

Cheap TCs are to be avoided. The best auto focusing 1.4x TCs for the K mount have been the Tamron F 1.4x AF TC, the Kenko 1.5x Teleplus AF TC, and the Sigma EX 1.4x APO TC. I've heard that the Tamron and Kenkos have been discontinued, but haven't confirmed this by any means. These originally sold for about $70, but the most recent new listings were a bit over $100. The Sigmas retail at about $200. Pentax has announced a new SDM 1.4x TC, but there's no set delivery date -- and I expect it to be expensive.

As Jim noted, AF performance may not live up to expectations with the lenses that you have because of lens speed. Another consideration is that a TC will magnify any faults of the lens it's mounted on, so the best lenses will show little image degradation, while lenses that show significant faults will give results that are significantly worse. I mostly use TCs with high performance prime teles and a few premium quality zooms.


dkm Feb 4, 2009 7:55 PM

Thanks Scott. Since reading your comments and Jim's, I have pretty much decided to stay away from trying to use a T-con with those lenses. I am thinking along the lines of purchasing a Sigma 150-500mm lens and if I feel the need for a Tcon with that lens, Sigma has one they recommend.

A lot more than I intended to spend, but why purchase a good camera and then not be able to capture the quality of pictures for which it was intended.

And, I am not yet sold on Sigma.

Bob Nichol Feb 4, 2009 8:33 PM

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I got the Sigma APO 150-500mm F5-6.3 DG OS HSM in Nikon mount just after Christmas and the weather finally was pleasant enough to go on a photo shoot this weekend. I quite satisfied with the results to date. There are better lenses if you're willing to shell out $5000+!

It is heavy, 1.9kg, but well built. It's fast to focus as well even in indoor light. It beats a t-con any day of the week.

dkm Feb 5, 2009 2:03 AM

Wow Bob!! What a fantastic picture! Now those are the results I am striving for.

Thanks so much. That is a big help.

bigdawg Feb 5, 2009 4:58 AM

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I have the Sigma 50-500 APO DG EX "Bigma" f/4-6.3 lens and use it with the Sigma APO DG EX 1.4X TC. Works very well but you need bright sun to to Auto focus above f-16 without it hunting. Here is the Bigma with the APO teleconverter mounted.


bigdawg Feb 5, 2009 5:01 AM

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And here is my Meyer-Gorlitz 300mm f/4 Orestegor lens with the TC attached.


bigdawg Feb 5, 2009 5:03 AM

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And a few photos with the Sigma 50-500 and the TC.


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