Before the rush

Before the rush
by evan-pak

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It is a 2x teleconverter. For all practical purposes, 2x teleconverters are not very useful on slow zoom lenses because it simply costs too much light - two full stops.

The Canon 55-200 f/4-5.6 with a 2x teleconverter will become a 110-400 f/8-11. Keeping the shutter speeds reasonable on such a long and slow lens will not be easy. Autofocus will struggle, and it will not be easy to see in the viewfinder what you are doing.

So, in conclusion: Even if it may work in a purely technical sense you probably won't want to do it.

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Autofocus will not be slow. It will be totally nonfunctional. – Michael Clark Jun 22 '14 at 20:27
Depends on whether the converter tells the camera that it is there or not. If no, the camera will try to AF... in good light and with a high-contrast subject it may even succeed. If yes, the camera will not even try. – Staale S Jun 23 '14 at 0:24
It will try, but in all likelihood unless you are in some VERY bright light all it will do is hunt. The additional glass in teleconverters tend to reduce contrast as well as the effective aperture. – Michael Clark Jun 23 '14 at 0:28
I own this teleconverter and my APS-C cameras (7D, 50D) struggle to AF in daylight when it is attached to an f/4 lens. They hunt and hunt but rarely ever lock focus unless I am using an f/2.8 lens. The FF 5DII, with a wider baseline for the focus array, does a little better, but only in VERY bright light. To use a 2X teleconverter with an APS-C camera you need an f/2.8 or faster lens if you expect to have usable AF. – Michael Clark Jun 23 '14 at 0:38
The AF in most Canon cameras just plain stop working at all once the camera knows that it is shooting a lens with worse than f/5.6 max aperture... and they don't do this to be mean, but because it just plain doesn't work very well with such a small aperture. – Staale S Jun 23 '14 at 11:02

The Kenko Pro 300 1.4x TC says it will not work with an EF-S lens like the 55-250. Everything I've read says the 2x is the same. So, regardless of AF issues, I'd say this is a no go.

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You know, the red circle doesn't align with the white box when I try it, as you'd expect. – khedron Aug 11 '14 at 14:50

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