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Old Mar 29, 2008, 8:08 PM   #1
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I was looking at buying a Sigma APO EX DG 2x Teleconverer, but realised in the shop that this and also the 1.4x Teleconverter, are recommended for use with only a select few Sigma lenses. (mainly longer APO zoom and fixed lenses, but not for example the APO 70-300mm DG)
And of the 12 Sigma lenses listed, only three of them will autofocus, while the other 9 will only manually focus.

So I guess that if these Teleconverers are used with other Sigma lenses, or other lenses from Canon, Pentax, Nikon etc, they will either not work properly, and the image quality will be poor. As I understand it, when used with any of the 12 APO Sigma lenses listed, the results are extremely good.

Does anyone know more about this or have experience with these Teleconverers?

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Old Mar 30, 2008, 6:51 AM   #2
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You will lose 1 stop of light with a 1.4x TC. IOW, only half the light gets through.

You will lose 2 stops of light with a 2X TC. IOW, only 1/4 the light gets through.

So, a lens with f/2.8 available effectly becomes a lens with only f/5.6 available using a 2X Teleconverter.

Many cameras will refuse to Autofocus with apertures smaller than around f/6.3.

The lenses you see shown that Sigma's listings show will Autofocus with a 2x TC will have f/2.8 available without the TC. The lenses you see that should Autofocus with a 1.4x TC will have f/4 available without a TC (for example, a Sigma 100-300mm f/4), since they would become equivalent to an f/5.6 lens using a TC.

Basically, a TC is not recommended for dimmer lenses (like the 70-300mm f/4-5.6 you mentioned), because you lose too much light for this type of lens trying to use one.

Also, you will tend to degrade optical quality using a TC. So, it's best to use one with a lens that has better quality to begin with.

Depending on the Teleconverter used (some third party converters don't report the corrected aperture to the camera), some cameras will still try to Autofocus with a TC. But, Autofocus may be much slower with lots of hunting, and the focus system may not be able to lock unless lighting is very good.

Remember, the Autofocus Sensors need to have enough light to "see" to Autofocus, and you're reducing the light the camera sees using a TC.

As for using a Sigma TC on a different brand of lens, I wouldn't suggest it. It may not interface properly with some lens designs.


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Old Mar 30, 2008, 7:49 AM   #3
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I've used TCs with film cameras years ago, so I am familiar with the loss of stops, etc. And that being the case, I did not have to worry about AF, because my lenses back then were manual focus !!!!!! :-)

I would use manual focus if the AF does not work, but in real terms, is this why they have listed MF for the slower lenses, or do they just not work?? Only the APO lenses are listed as useable (AF or MF).

I would have thought that an Apochromatic TC would work better with an Achromatic lens because it produces less additional chromatic aberration.
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Old Mar 30, 2008, 8:02 AM   #4
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Most of the time, the problem is with the camera.

Most will just refuse to Autofocus if they don't see around f/6.3 or brighter apertures being reported by the lens (although some of the pro bodies will still attempt to Autofocus at f/8 ).

Depending on the design of the TC, the correct aperture may or may not be reported to the camera (some TCs have electronics built in that correct for the loss of light when reporting the aperture to the camera, and some just pass the existing connections from the lens through to the camera, so the camera isn't aware that a TC is being used).

I think Sigma TCs probably fall into the first category (report the corrected aperture). Some other TCs don't (so a camera will still attempt to Autofocus, since it doesn't realize that the aperture is too dim).

As for using a TC with a given lens, you have lens mount design issues with some TCs, too (for example, rear elements colliding with the TC). Not all TC's will work with all lenses, even if you don't care about AF.


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Old Mar 30, 2008, 5:54 PM   #5
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Ok, soI'd need to check the rear elements on any lenses I was thinking of using to stop rear elements colliding with the TC?


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Old Mar 30, 2008, 10:54 PM   #6
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You'd have to try one to see if it worked or not.

Personally, I would only buy a Sigma TC if I were planning on using it with one of the Sigma lenses it was designed to work with, in case of any compatiblity issues (physical or electronic).

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Old Apr 1, 2008, 4:20 AM   #7
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Ok, so if that's the case, do you have any other suggestions for some good quality TCs?? (to suit Pentax )
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Old Apr 2, 2008, 6:21 AM   #8
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Here's an article that discusses some of the differences in TCs, and even shows the clearance you have from the TC front element with some. Some of the generic TCs may have 8 or 9mm clearance. But, some of the dedicated Sigma and Kenko TCs are going to have far less clearance.

This article is specific to Minolta. But, I suspect the same type of limitations are going to apply to some of the other manufacturers (you have to worry about things like clearance issues with TCs).

http://www.dyxum.com/lenses/TC/index.asp

Your best bet would probably be to ask Pentax users what TC works OK with a given lens, if it's not a lens a TC would normally be used with.

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Old Apr 2, 2008, 10:43 PM   #9
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Ok!!

Thank you very much!!
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Old May 19, 2008, 12:20 AM   #10
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dnas wrote:
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Ok!!

Thank you very much!!
I have a Pentax K10D and use a Sigma APO DG EX 1.4TC with a couple of my lenses. I have a Vivitar 300mm f/5.6 that has a TX mount and the APO TC works great with it. I also have a 50-500mm sigma APO DG EX lens that I use it with...The Auto Focus works pretty good at wide open but around f/8 it starts to hunt all over the place. I use it now either Manual focus or wide open for Autofocus.

This is with auto focus and wide open using the sigma 50-500 and the APO TC from about 40 feet distance.

Dawg

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