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Old Feb 22, 2008, 3:28 PM   #1
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Okay, I just bought the Kenko TC, just wonder how well it would work with a sigma 135-400 f4.5-5.6 lens. Do you think I would be able to AF with this set up. I read somewhere that the Sigma is soft at 5.6. Tester suggested to stop down to f8-f11 to sharpening up the image. Now at that range will the AF work at all. thanks
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Old Feb 24, 2008, 6:49 AM   #2
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garman wrote:
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Now at that range will the AF work at all. thanks
It will depend mostly on lighting and subject contrast:
The camera is now operating outside its designed limit so in bright daylight it should work, but on overcast day and under trees canopy IMO the camera will hunt a lot...

-> You will also notice the lens sharpness will increase with lighting and subject contrast (as that's how the MTF is derived) so on a bright sunny day you will also get sharper pictures with the same lens setting as in less contrasty environment
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Old Feb 24, 2008, 10:30 AM   #3
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Thanks for info. I'm still waiting for my kenko to arrive. In time I think I will go with the bigma lens. I'll mostly use this combo during my kids baseball games. Didn't realize the improved sharpness by adding a converter. - garman
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Old Feb 29, 2008, 4:18 AM   #4
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You can't improve shaprness by adding a converter, just the sentence above is little confusing. 1.4x degrades picture marginaly, 2.0x degrades it noticable, but if you use 'em on daylight with lots of sunshine, it is possible to stop down aperture and get very decent and sharp photos.

Actually, the stopping down the lens improves the sharpness, not the teleconverter.

With Bigma and tcon you will meet AF difficilties unless you own 1D or at least 40D, since Bigma is F4-6.3 if I recall well.

This means that not only you 1.4x or double the range of the Bigma, but you influence on the aperture, so Bigma wide open with 1.4x tcon is 5.6-8.8, or with 2.0x 8-13, which makes it little impossible to shorten the shutter speed for sports even with ISO3200.

You may try 1.4x, but I'm not sure whether you will be satisfied with overall AF performance, IQ and shutter speeds (Bigma is not really razorsharp at 500mm - bird watchers use it, mostly on daylight when it is possible to stop down to f8 or f10, but then again it's an issue to shoot without a monopod or tripod at 500mm and acheive faster shutter speeds of 1/500 at ISO400).
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Old Feb 29, 2008, 6:02 PM   #5
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As for the bigma and 1.4x I plan to use it during sunny days. It 's really for baseball, which is mainly played during the day, that's the bonus. So If I can stop down to f8-11 and use 400-800 ISO. Will I be able to catch action shots and get reasonable sharpness. you mentioned that 500mm that the lens is soft. Though I have seen some images from the bigma at 500mm and they look good to my eyes. I guess adding the 1.4x converter jeopardizes image quality. Thanks guys for your help.
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Old Apr 10, 2009, 6:09 PM   #6
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So I finally picked up the bigma. Nice lens even at 500. My issue is the AF with a TC mainly the 1.4x. The bigma will not focus on my 40D. I have read in previous post that the kenko pro 300 will AF with the bigma. I also tried taping the 3 pins on the TC. I get a f0.0 on the camera and still not AF. Any help would be appreciated. Thanks
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Old Apr 13, 2009, 5:25 PM   #7
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So, doesyour Bigma work just fine WITHOUT the TC?

If so, you have nothing wrong with the Bigma itself. However,if the Bigma will not AF when only it is mounted, then get it replaced.

You are exceeding the f/stop requirements for AF to properly work. Read your manuals. Sigma's own charts show the Bigma being MANUAL FOCUS onlywith either of their two TCs mounted to it; they do not show it being able to AF — this would apply to any TC used with the Bigma due to the reduced contrast/f-stop and the Canon body's focusing requirements.

However, with my Bigma,the lens-matched 1.4x by Sigma works just fine with AF inBRIGHT daylight situations on both a 30D and 5DMkII; not on a 20D for some reason.

And, surprisingly,both the Sigma 1.4x and 2x TCs work just fine with my Canon 70-200L f/2.8 IS and allow AF at all times, with virtually no degredation of image quality.
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Old Apr 15, 2009, 1:01 AM   #8
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Yeah I've known about the MF when the lens hits higher value of f5.6. Sigma seems to trick the body into focusing at f6.3 on the long end. So I only presumed that @f/9 and in bright light that the Bigma would focus. I've read in forum that the kenko MC 1.5X TC should work with thte Sigma.
I posted this question on another thread. Some have answered that it does focus with a kenko 1.4X, but seems to only work with the non-DG models of the Bigma. Mind you this is only a preliminary result. I'm waiting for others with a DG model to chime in. Thanks for taking the time to answer. - garman
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Old Apr 20, 2009, 5:45 PM   #9
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There are compromises with a TC, so either don't use it with the bigma, or manually focus!!!

Even if you get it(or another TC)to focus with the bigma, it will most likely be very slow focusing, so it may not be much use in that configuration for baseball anyway.

Use manual focus, and try prefocusing on a spot.
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