Best Lens Extender besides Canon

Started Apr 26, 2012 | Discussions
Bellock
New MemberPosts: 17
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Best Lens Extender besides Canon
Apr 26, 2012

What is the best lens extender besides Canon. I have the Canon 5d Mark III and the telephoto lens 100-400 4.5-5.6 L and the autofocus does not work due to the 2 stop loss caused by the Canon extender

Canon EOS 5D Mark III
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GtoJon
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Re: Best Lens Extender besides Canon
In reply to Bellock, Apr 26, 2012

I use a Tamron SP AF 1.4x all the time & have seen no difference between my Canon EF 1.4x II converter except that I can use the Tamron on any lens I am sure the Kenko & the Tamrons are made by the same company.

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rth23
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Re: Best Lens Extender besides Canon
In reply to Bellock, Apr 27, 2012

Bellock wrote:

What is the best lens extender besides Canon. I have the Canon 5d Mark III and the telephoto lens 100-400 4.5-5.6 L and the autofocus does not work due to the 2 stop loss caused by the Canon extender

Image quality-wise I like my Kenko very much.

At the time when I was buying it, it offered some more AF functionality than the Canon ones, not sure if this is still true, though.

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Steve Balcombe
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Re: Best Lens Extender besides Canon
In reply to Bellock, Apr 27, 2012

Bellock wrote:

What is the best lens extender besides Canon. I have the Canon 5d Mark III and the telephoto lens 100-400 4.5-5.6 L and the autofocus does not work due to the 2 stop loss caused by the Canon extender

The Kenko Pro 300 DGX. Tests rate it at least as sharp as the Canon in the centre, though the Canon is probably ahead in the corners.

However your question suggests a bit of confusion so to clear that up:

All teleconverters (Canon calls them Extenders) reduce the light by the same amount - 1 stop for a 1.4x, 2 stops for a 2x. This is unavoidable if you think about it, since by magnifying the image you are spreading the same amount of light over a larger area - hence it has to be darker.

However the brightness of the image is not the issue - AF can work in quite low light after all. The problem is that the cone of light reaching the AF sensor needs to be a certain width for it to function correctly, and for most bodies that requires an f/5.6 aperture. Even if you could trick the camera into autofocusing with the TC fitted (and there are several ways to do that), performance would be unreliable.

Because of the unpredictable performance, Canon actually takes the step of disabling AF if the camera detects a lens (or lens-TC combo) with an aperture smaller than f/5.6. Notice that your 100-400 has more connecting pins than other lenses - these pins are used to detect and identify the Extender, and therefore the camera can work out the combined aperture. To get AF to work, we have to defeat this mechanism.

The first way is simply to tape over the extra pins - see http://www.michaelfurtman.com/taping_the_pins.htm for one of many web pages showing you how to do this. Your AF function will be restored - but as noted above, performance may be unreliable.

The second way is to buy a "non-reporting" teleconverter which doesn't have the extra pins. This means the cheaper TCs which are (while not wanting to start a war about this) optically inferior so it's probably not the best solution.

The third way is to buy the Kenko I mentioned above. Unlike the earlier DG models, which behave like the Canon Extenders, the DGX models intercept the signal between the lens and camera and trick the camera into believing it has an f/5.6 combo attached. This gives you the best of all worlds - excellent optics, working AF, and no messy tape. What's more, the electronics in the DGX is clever enough to report the correct combined focal length and aperture even when using a lens such as the 70-300L which is not compatible with Canon Extenders.

The one think the Kenko DGX can't do is defeat the laws of optics - with a 1.4x TC you still have an f/8 combo and your AF operation will be outside the specification for the camera. How well it actually works in practice depends on both the body and the lens so you will just have to give it a try.

One last comment. You mentioned a 2 stop loss, which suggests you have a 2x Extender. In that case you have an f/11 combo and will be even further outside the specification of the AF sensor. If this works at all, expect performance to be very poor.

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