Looking for a poor man's super telephoto, I did some testing with a Canon 2X Extender mounted on to my 400/5.6 lens. Although the teleconverter (a.k.a. extender) causes some noticeable loss of sharpness, I wanted to see if it would ever be worth considering for occasional long range nature shots.
First of all, I found focusing to be a significant challenge and was wishing that my camera had a microprism focus screen to make it easier or, at least, a focusing ring that was not so touchy. I also found that at 800 mm FL, vibration is a significant issue. I have a heavy duty tripod with a Wimberly Sidekick gimbal mount, but it was a windy day and with the tripod not being anchored, there was some noticeable vibration. I found that locking up the mirror before each shot made a big improvement in internal camera vibration -- without it, the images were just too blurry. Also, the viewfinder image on the XTi is just too small for my senior citizen eyes. The first thing that I did was remove the eyepiece extender which shrinks the image even smaller.
Anyway, while experimenting with test images, a cardinal landed in the top of a pecan tree about 100 feet away so I gave it the acid test. While the results are not stellar (I could not get the image tack sharp and the contrast was a bit low), I actually was somewhat surprised that the image actually did show some detail in the feathers. The image below was cropped and then rescaled to about 63% of original size. Since the shooting data is not intact, here are the details: ISO100, f/11, 1/160 second, FL 800 mm, and Digital Rebel XTi body. The image was saved in RAW mode and it was processed in ACR/PS CS3, including Neat Image plug-in. After resizing, the image lost a bit of sharpness, so I applied a small amount of USM to the red and blue channels that resulted in a bit of an outline around the cardinal. If I do this again, I'll leave off the final sharpening.
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