This gallery contains images for testing these lenses in combination with a telephoto extender:
1- Canon EF 70-200mm f/4L IS USM
2- Canon EF 300mm f/4L IS USM
3- Canon Extender EF 2X II (tele-extender)
The subject is the Golden Pioneer Man high atop the State Capitol building dome in Salem,Oregon USA. Camera used to take the pictures is the Canon 5D Mark II.
All tests were from a Manfroto tripod mount with mirror lockup turned on and shutter triggered with a wireless remote and camera set to a 2 second delay to minimize camera shake. The shots were taken with image stabilization turned off. Two pictures for each focal length and f/stop were taken to minimize mistakes.
The images are taken at 200mm and 300mm without the extender; and 400mm and 600mm with the extender.
The first 4 images are out-of-camera(except for web resizing) for the 4 lens combinations.
The last image was taken a week later during a second set of more extensive tests when it was discovered that the 600mm looked soft compared to the other combinations during the first test. This time the image quality was much better at 600mm and more contrasty lighting conditions are not the reason. All lens combinations were retested.
Apparently during the first test the 600mm combination was not properly manually focused or the image stabilizer was still turned on while taking tripod mounted images.
1-Images at 200mm, 300mm, 400mm and 600mm(using the Extender on the 200mm and 300mm) were tack sharp. I was very, very impressed with the 70-200 image quality with the extender attached.
2-The 600mm achieved with the 300mm and extender is just a fraction softer than the others but almost indistinguishable.
3-There is some degradation of image quality at higher f/stop values with the extender for both lenses, but both seem to perform well in the f/8-f/16 range.
3-All images were better image quality and sharpness than the EF 100-400mm f/4.5-5.6L IS USM lens I tried out at a camera store.
4-Camera shake is the biggest obstacle to overcome while using these lens combinations.
Based on these results I recommend the Canon Extender EF 2X as an inexpensive alternative for Canon owners to double their lens focal lengths.