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During the last few years I seldom used a TC ( see: http://kennewcombe.com/Teletest.html) as I felt the image degradation and light loss seldom overcame the image resolution gain do to the magnification. The TC I most used had been the 2x on my 300 f2.8L IS. I was not happy with the loss of contrast wide open at f5.6 and the need to shoot at f8 was a handicap. I found myself using the 500mm at f 5.6 more often than not rather than the 300 + 2X. However the 300 is lighter even with the TC by about 2 Lbs so when I needed to hand hold for an extended period I would use the 300 + 2X. Canon stated that the new TCs were optimized to work with the new (not yet released, as of Feb 2011) prime lens, such as version II of the 300 f2.8L IS and 500 f4L IS. It was not clear from the literature if the new TCs would improve image quality on the older big primes such as my 500 f4 IS and 300 f2.8 IS. Extender III TCs have been redesigned and have more elements, florine coating and a new computer chip. The 2X Extender III has nine elements and is heavier than the Extender II.As the Extender III TCs became available, in limited numbers we started to see test reports that indicated that the Extender III TCs did indeed improve image quality on the big primes. There were no claims of huge improvements but A:B comparisons did suggest they may be worthwhile. I decided to purchase a 2x Extender III and test it with my 500mm f4L IS and 300mm f2.8L IS. The test was performed indoors by shooting a $20 dollar bill using a Canon 7D, tripod and flash. Even slight focus errors can throw the test results way off so I mounted my lens on the Arca ball clamp, focused on the target with manual focus and placed tape on the lens focus ring so it would not move and then shot several images of the test target with different focus distances by sliding the lens foot through the clamp 2 mm at a time about the point of focus. The idea was to select the sharpest (best focused) image by looking at them under high magnification on the computer screen and selecting the sharpest image.All images were shot in RAW and converted to 8 bit TIF with no sharpening. I then compared the images side by side on the computer screen with the Extender II and Extender III TCs. Part of the target near the center and the corner was compared at a screen magnification of 250% where pixels are just becoming visible. Lastly the side by side images were screen captures and converted to JPG for the web.
Conclusion: Images from the 2X Extender III TC in both center and corners were generally higher in terms of contrast on both the 500 f4L IS and 300mm f2.8L IS. The most improvement was seen at the fastest f-stop and in corners. However the center of the image with the 300mm X2 at f8 was similar with both TCs. The overall gain was worthwhile but certainly not overwhelming. Looking at a high contrast edge in the corners shows a reduction in chromatic aberation (color fringing) with the Extender III TCs with both prime lens. This probably accounts for the increase in contrast. Resolution seems the same to my eye. I did not notice a change in autofocus performance with the 300mm x2 combination. This test was performed with a crop sensor camera (7D), a full frame sensor may show further improvements in the corners.Comparison images 300mm f2.8L IS X2 f5.6
center 300mm f2.8L IS X2 f8 corner
500mm f4L IS X2 f8
center 500mm f4L IS X2 f11 corner
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