Teleconverter (TC) reviewsTeleconverters (also known as TCs, extenders or multipliers) are used to increase the focal length of a lens. So with a 2x TC, your 300mm lens becomes a 600mm lens. However, there is no free lunch! The sharpness of the image will be lower and you will lose 2 stops of aperture, so your sharp 300/4 will become a less sharp 600/8.
Some basic facts:
Canon 1.4x and 2X (version I)
The Canon TCs only work with some EF series lenses. Both TCs have a front element that sticks out from the mounting flange, so the lens they are used on must have a large rear opening and a recessed rear element. Basically that means the telephoto prime lenses (200mm and up) and a few zooms such as the 70-200 and 100-400. They will not fit, for example, the 75-300 or 100-300 zooms.
The Canon TCs report the correct aperture of the lens+TC combination to the camera body. So, for example, if you mount a 300/4 + 1.4x TC, the camera body will report a maximum aperture of f5.6.
The older (I) models of the 1.4x TC and 2x TC cannot be stacked together (to give a 2.8x TC) without including a 12mm extension tube between them. With this tube some lenses (telephoto primes like the 300/2.8) will still give infinity focus. Sharpness drops quite significantly with stacked TCs.
AF speed slows down when a TC is mounted in order to maintain AF accuracy.
Image quality with the 1.4x is excellent, in fact with many lenses (300/4, 500/4.5, 600/4 etc.) it's hard to see any drop in image quality much of the time (though it is there if you measure it carefully). The 2x is still good, but most of the time you can tell there is some drop in sharpness.
Canon 1.4x and 2x (version II)
In 2001 Canon released the mark II versions of their TCs. The new versions have a rubber "O-ring" seal for better weatherproofing (only effective when used with the new IS telephotos and EOS-1v which also have the seals). The 1.4x II is optically identical to the earlier version, but the 2x II is optically different. Both are said to have improved control of flare (though that was never a problem with the original versions as far as my experience goes).
Though both the 1.4x and 2x II have the same protruding front element as the earlier versions which prevents their use on many EF series lenses, they can now be stacked without the use of a 12mm extension tube between them since the 2x II has space at the rear for the front element of the 1.4x II.
I haven't tried the 1,4x II, but I have tested the 2x II on a 500/4.5L, 300/4L and 70-200/4L. On the 500 and 70-200 (at 200) there was no difference between the original 2x and the 2x II. On the 300/4L there was a very slight improvement, just visible in shots of a USAF high resolution test target on ISO 25 Agfa APX high resolution B∓W film, but not noticable in "real world" images. It's possible that a larger difference would be observed between the original 2x and new 2x II if they were used with the new IS telephotos, but it doesn't seem worth "upgrading" for use with the lenses I used in these tests.
Tamron 1.4x and 2x
Note: these comments refer to the black TCs made by Tamron in the late 1990s. Designs and specifications may change without notice
The Tamron TCs will fit on any lens since they do not have a protruding front element. They can also be stacked without an extension tube between them.
Unlike the Canon TCs, they do not report the "true" lens aperture back to the camera body, so with a Tamron 2x on a 300/4 lens, the body will think the maximum aperture of the combination is f4, not f8. This isn't normally a problem since metering through the lens is still correct, but it can confuse the user and may present some problems when using flash in non-TTL modes.
AF with the Tamron TCs isn't as good as with the Canon TCs, but AF will be attempted as long as the prime lens is faster than f5.6. By "attempted" I mean that sometimes AF will lock onto the subject, sometimes it will fail to lock and sometimes it will hunt for focus. With the Canon TCs, the aperture of the lens+TC must be f5.6 or faster for AF (but the AF will be positive, with no hunting).
Both the 1.4x and 2x TCs show some image vignetting with long telephoto primes like the 300/2.8, 500/4.5 and 600/4. Center sharpness is comparable to the Canon TCs, but edge/corner sharpness is lower. Note that on small sensor (APS-C) digital SLRs like the Digital Rebel and EOS 20D, vignetting will not be a problem and edge quality will not be as large an issue. The Tamron TCs are significantly cheaper than the Canon TCs, so they can be good value.
The Kenco 1.5x which I tested was identical to the Tamron 1.4x in terms of both image magnification (1.4x) and image quality! However this was done some years ago and more recent designs may be different.
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