Kenko 1.4x Extender Tradeoff

After having used the Kenko 1.4x extender this afternoon, I started wondering if that had been a good idea. Am I better using the tele-extender to increase resolution or am I better simply enlarging the image? This evening, I setup a test chart, placed at 30' from the camera, and used the Canon EF 100-400mm f/4.5-5.6L IS with and without the extender. After importing the images into Photoshop, I enlarged the images taken without the extender by 140%. This gave both sets of images the same pixel resolution.

The results are shown below (you may want to click on the images to view a full-sized version). Not unexpected, the image quality is degraded by introducing the extender, whose optics are not as good as the lens. For the coarse details, the image quality is better with the enlarged, 400mm image, both in terms of sharpness and contrast. For fine-grained detail, however, the story is not so simple. The contrast is still better at 400mm but, a some point, the details are simply too fine to resolve at 400mm.

Not sure exactly how this applies to a bird image. I am thinking that for the Brown Booby images, where pixelation was noticable, I was better off using the extender. For the Osprey shots, I am not sure.

With Kenko, 560mm f/8, original resolution

Without Kenko, 400mm f/8, image size increased in Photoshop by 1.4x to match pixel resolution

Without Kenko, 400mm f/5.6, image size increased in Photoshop by 1.4x to match pixel resolution