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Old Jan 14, 2005, 9:08 PM   #1
gpp
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does this converter work with the is lenses and do i understand the auto focus does work also ?im using a eos rebel and do i love this canera!
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Old Jan 18, 2005, 7:01 PM   #2
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My tamron 2x works with my 75-300is, but it will not function thru all of the lenses zoom length(up to about 200m depending on light conditions).
It is also a very slow lense(in shutter speed)

I should have opted for the 1.4x then the lense would focus in alomost all of it's range.

Also contact your local camera shop they should be able to give you a better explaination on all of the specs. They would aslo let you connect the lense and convertor you want to make sure everything works. Then you can see exactly the auto-focus capabilites with the equipment your looking at.
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Old Jan 18, 2005, 8:52 PM   #3
NHL
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It depends on the camera, for example with the DRebel, the AF will be lost around f/5.6 -> with a 1.4x one lose 1-stop and a 2x one lose 2-stop so when mounting theses devices on a variable aperture zoom, the AF works when the resulting aperture is larger than f/5.6, but will cease to work when the apertureis smaller thanf/5.6... Remember the aperture is the ratio ofa lensdiameter opening over its focal lenght? -> the focal lenght changes when you zoom in while thelens diameter stays fixed hence the f/ratio changes

"This also explains why AF systems won't work with some teleconverters or
with lenses of slow maximum aperture. It isn't just that the light hitting
the sensors is dim -- otherwise, it should still work on a very bright day
or with very bright subjects. The problem is that if the lens' aperture is
too small, the AF sensors' view of the right and left edges is cut off."
http://www.a1.nl/phomepag/markerink/af_expla.htm


The IS however is independent and works off accelerometers built-into the lens and is independent from the AF system which is in the camera! :idea:

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Old Jan 19, 2005, 12:14 PM   #4
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Well since you have the tamron 1.4x TC which doesn't report correct aperture to the camera body, the AF will work fine if max aperture on your lens is f5.6. The AF might be slow but it works.
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Old Jan 19, 2005, 12:16 PM   #5
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On 10D, 300D, 20D etc. AF suppose to work upto f5.6. On pro bodies it is f8. If you use canon 1.4x TC, it will report aperture of f8 (assuming max aperture of f5.6 on your lens) so on 300D, it won't AF. If you tape the pins, camera would not know about it and AF will work.
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