As in the previous test, I photographed a stuffed valley quail to understand how a rented Canon EF 300mm f/2.8L USM lens (www.borrowlenses.com) compared to my copies of the Canon EF 400mm f/4 DO IS USM and Canon EF 100-400mm f/4.5-5.6L IS USM lens.
Disclaimer : I am only testing one copy of each lens type and I am not sure that the copy tested is representative of the population.

**No sharpening or other alterations were applied to any of the cropped images other than to convert the images from RAW to JPEG using PS CS3. The images were cropped to 700x700 pixels and then cut and pasted side-by-side for easier viewing.**

The lenses were microadjusted on the Canon 50D camera immediately prior to testing. All the pictures were taken with the lens mounted on a tripod and with a cable release to minimize vibrations. The pictures at each f-stop were taken in duplicates and the best one (if different) chosen as the best example. The pictures were taken using available light on a sunny day, with center AF, in single shooting and aperture priority modes.

Click on "original" on the "other sizes" line below any image to see the largest image.

This is what I learned:

1) The Canon 300mm f/2.8 lens tested had very good image quality even when fully opened at f/2.8 and the image quality did not change (improve) significantly as the aperture decreased (fig. 2).
2) The center image quality produced by the 300mm f/2.8 and the 100-400mm lenses (set at a 300mm focal length) at the same f-stop are very similar. Perhaps, with the 300mm f/2.8 being slightly sharper to my eyes (fig. 4-6).
3) The center image quality produced by the 300mm f/2.8 with a Canon 1.4x EF Extender II (Teleconverter) is very good (fig. 8) and it improves further as the lens is stopped down. The quality almost matches that of the 400mm DO lens when tested at the same f-stop, specially at smaller apertures (fig. 9-12).