Canon 300mm f/2.8 with 2X teleconverter Home ยป content

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The Canon 300mm L f/2.8 EF IS USM is alleged to be sharpest lens ever made, and the best lens for use with a teleconverter. Can it really be used as a poor man's 600mm lens? I am testing with a Canon 5D, a camera with a full-size image sensor, and only 12 MP. So, my pixels are bigger than perhaps any 35mm camera ever made, and intuitively this would be the best camera with which to test a teleconverter. Mike Baird has a photo on this site that I think shows definitely that with the 5D a perfect 600mm picture is possible; 300mm + 2X photo. Look at the detail in the bottom half of the photos. My example is not bad either, but does not feel quite as sharp.

Now the remaining question is if one can get the same results with a camera like a 50D, with a 15 MP crop-sensor. Or the 5D Mark II and 20+ MP sensor.

Now one might ask if the if the 5D and 2x teleconverter are so great, can I add a 1.4x converter as well, giving 840mm? An obviously better solution to get more zoom would be to get a camera with a more pixel-dense image sensor, the reason being that the teleconverters add and amplify optical imperfections in the lens, while the image sensor should be a pure capture of the image. But right now the 5D Mark II is just too expensive for me. So I've done just a little experimentation with this, and again think that you should only try this with a full-size sensor and perfect light conditions. But I took this photo with imperfect conditions and am keeping it! Note that with two teleconverters Canon does not compute the correct mm information in the EXIF header; it shows 600mm. One last note, amazingly, autofocus works with two teleconverters; with my old 300mm f/4 lens it did not with just the 2x converter.

Here are Amazon links to some of the items discussed. Amazon might not be the absolute cheapest site on the net (but it may be), but they will have a very good price, and you are guaranteed a problem-free shopping experience.
Canon EF 1.4X II Extender
Canon EF 2X II Extender
Canon EF 3000mm f/2.8L IS USM Telephoto
Canon 5D
Canon 5D Mark II

2x teleconverter with Canon 5D Mark II and Canon 5D Mark III

With the 5D Mark II I hardly used the 2x teleconverter, finding the 1.4x very easy to use. With the 1.4x I would guess 80% of my shots came out perfectly focused, while maybe 20% with 2x. This is of course because it is much harder to focus at 600mm because of less light, longer shutter times, and increased sensitivity to vibrations.
However with the Mark III I am having much more luck with the 2x, because of the focus system is greatly improved, and because I can shoot at higher ISO settings and a faster shutter time with the same results.
My next questions will be about the new III teleconverters and if they are worth the increased price. The main reason for the update is faster automatic focusing, something very important for bird photography. From the reviews I've read, there is little difference in image quality, and they are quite a bit more expensive, and require a III lens as well.