Image 1 shows a 100 percent crop from an original raw frame of the multiple star system Alcor and Mizar in the handle of the Big Dipper. It was opened in Canon's Digital Photo Professional and converted to a 16 bit TIFF file. That file was opened in Photoshop where it was enlarged to 100 percent and cropped. Image Data
Image 2 shows a 500 percent crop from the original image shot with the Image Stabilization in the lens turned off. Note that these stars are at nearly the center of the frame and the aberrations shown are not coma because all of the stars in the field have the aberration in the same direction. Image 2 was visually focused on Alcor and Mizar with the Right Angle Finder C at 2.5x magnification.
Image 3 shows a 500 percent crop from the original image that was shot with the Image Stabilization in the lens turned on, although I do not know if IS is turned on when the exposure is started with the remote release TC-80N3, or if it stays on during a long time exposure of 30 seconds controlled with the timer with the camera set to bulb. Image 3 was autofocused on the bright star Vega. The focus method does not seem to affect the aberration problem. As demonstrated in the previous tests of the 300mm f/2.8 IS lens, this problem is not because the lens is not focused correctly. Stellar Aberration Response from Canon Examples of the stellar aberrations seen here in the Canon 300mm f/2.8 L USM IS and Canon 400mm f/2.8 L USM IS lenses were sent to the Canon lens development group in Japan. According to a reply received by Rudy Winston, one of the top technical support people at Canon USA, the Canon lens development group feels that the lenses are performing within specifications, and that the images show results they'd expect in shooting this type of subject with those lenses. They don't feel there's anything wrong with these lenses that the service department would be able to fix. Winston went on to say that "with the introduction of the EOS 20Da, there's a clear recognition that astrophotography is increasingly important to Canon." And that "it's safe to assume they're aware that the next generation of super-tele lenses need to be beefed up to be able to perform well in this very challenging field." It appears that the high resolution and linear data-recording capabilities of these digital cameras are now able to detect lens aberrations that were previously hidden when the lenses were used with film. In my opinion, these lenses are not usable for high-quality long-exposure deep-sky astrophotography when used wide open unless the photographer is willing to clip a significant amount of highlight detail in the stars. Further testing may show whether the performance improves when stopped down, but it is incredible waste of expensive glass to have to stop down lenses like these, and defeats the purpose of using such high-speed lenses. It is also possible that pre-image stabilization technology lenses may perform better for this task.
It is more than a little disappointing that the latest Canon super-telephoto lenses, which are the flagships of their lens line, perform so poorly when used wide-open for astrophotography with today's DSLR technology. Image Data
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