Canon Extenders - Teleconverters EF 1.4x & 2x II
Remember: Not all Canon Lenses will work with Canon Extenders
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Canon Extender EF 1.4x II
Basically this 1.4x teleconverter (or as Canon aptly name them, an extender) is optically invisible, certainly if you have a quality prime lens in front of it so as not to magnify any optical shortcomings in your lens.

This lightweight but well made item incorporates Canon's weather and dust sealing that is found on their L lenses. This rubber gasket is located on the camera side of the extender, as it would be redundant on the lens side, as your Canon L lens will already have a seal of it's own.

In use, with a quality prime lens, the only downside to using the Canon 1.4x is a loss of one F-stop of light and minor slowing of AF (presuming you have a lens of f4 or less in the first place)

A 100% crop of the head of a Collared Dove. Taken with Canon EF 600mm f4 L I.S. USM with Canon Extender EF 1.4X and EOS 1Ds MkII
No sharpening. A Blue Tit with same combination. Click on images.
Canon EF 1.4x extender
The Canon Teleconverter Trick

Canon non professional dslr cameras such as the Eos 10d  Eos 20d Eos 30d will cut-off auto focus when they detect a f5.6 lens in addition to a teleconverter.

It's quite simple to get around this by fooling the camera into thinking there is no teleconverter attached and continuing to auto focus by a simple trick.

Just place a thin strip of insulating tape over the three pins on the lefthand side (lens side of the extender). No damage will occur to the camera or teleconverter. Auto focus performance will not be quite as good as f5.6 but when you are using a lens such as a Sigma 500mm f4.5 with a 1.4x teleconverter, the amount of light reduction is minor and auto focus will still be very good.
When you try this on a f5.6 lens such as the Canon 100-400mm L  I.S. USM in combination with a 1.4x teleconvereter, auto focus will generally be poor.
• Construction 5 Elements in 4 groups
• Diameter 67.8mm
• Length 27.2mm
• Weight 200 g
Case Supplied LP811
Canon Extender EF 2x II
Where-as the latest version of the Canon Extender 1.4x received only minor enhancements over it's earlier guise, the new version of the Canon Extender 2x was substantially overhauled in optical performance. Apparently it was designed almost entirely to work at its best with the very long Canon prime lenses. Again, as with the 1.4x extender, the EF 2x II is weathershielded with a rubber gasket and is of the same high standard of build quality

It would be a lie to say that adding, even this excellent teleconverter, is free of any problems other than the inevitable loss of 2 f-stops of light. But I will say that it is not as bad as many doomsayers would have you believe and that, if used wisely, it can produce results capable of professional use.

The problems with using a 2x teleconverter are certainly related to the optical degradation brought about by the extra glass elements involved in its construction and the demands it puts on the image delivered by the lens in front of it.... but these are not the only reasons for the commonly held belief that 2x teleconverter are crap.

For one thing, it is often the compression of vast amounts of air between the subject and the lens that can cause a serious loss of quality. The air between you and the subject is not free of pollutants, and these can be naturally occurring or manmade. Compressing this airborne debris when using huge magnifications can have serious effects on the clarity/contrast of the resulting image.

And when are 2x teleconverters commonly used? They are often used when there is a huge distance between subject and lens, usually too much and often the image still needs cropping... It's a recipe for disaster.Cut the distance down, use a 2x at the right time and for the right reason and the quality of results can surprise.

The second, and more obvious problem that cannot be blamed on the teleconverter itself, is the increase in demand for stability, even with the luxury of I.S.

As with all teleconverters, stopping down will improve the results by a large degree and , if you can, try manual focus to overcome some focus accuracy problems that may arise when using 2x teleconverters (although this is more of an issue when stacking teleconverters).

Another issue with 2x teleconverters, even this excellently designed one, is that your AF speed will drop significantly, even if starting out with an f2.8 lens and a pro body.

See which Canon digital cameras the experts pick for you based on the types of photos you shoot, your budget, and size and features preferences.
•  Construction 7 Elements in 5 groups
•  Diameter: 71.1mm
•  Length: 56.5mm
•  Weight 265g
Case Supplied LP811
This image links to a 500kb 100% crop taken with an EF 300mm f2.8 L I.S. USM lens with Canon EF 2x II Extender attached to the Canon EOS 1Ds MkII
No sharpening applied.