I have to admit, I was quite disappointed at how poor the stellar images were in the Canon 300mm f/2.8 L USM IS lens.

Canon 300mm f/2.8 Star images enlarged 500 percent.

Note that the stars in the image above were taken from near the center of the field. Although it looks like coma, it is not. These aberrations, a bright stellar point surrounded by an off-centered halo, were present on all stars in the field, in the same orientation.

The image at far left was focused with the Canon autofocus system at f/2.8 on the bright star Capella. The autofocus system did not have any apparent problem focusing on the star.

The middle image had the focus tweaked manually just the smallest amount that I could physically move the focusing ring, past the infinity mark.

The image at far right had the focus tweaked manually just the smallest amount that I could physically move the focusing ring, to the close focus side of the infinity mark.

These different focuses were tested in case the lens did not autofocus exactly at infinity. It can be seen that the best apparent focus is in the first frame where the lens was autofocused. It has the tightest stars. The other two frames show that best focus is not on either side of the autofocus point.

When I initially examined the stellar images from this lens visually with a high-power, high-end eyepiece, and they were awful. When the image was in the best focus I could obtain (smallest star size), they had a giant halo around them that was off-centered. I originally attributed this to some slop in the eyepiece holder that I had rigged up, thinking the eyepiece was not orthogonal to the optical axis. But I found exactly the same aberrations in the stars in the images I made with the 1D MarkII, as can be seen in the images above.

I realize that these camera lenses were designed for daytime terrestrial photography and not astrophotography of point sources across the entire field. Even using ED and Fluorite glasses I would not expect the color correction to be perfect on a point source, especially given the speed of the optical system in these super telephotos.

However, I have considerable experience with these types of lenses for astronomical imaging on film, and haven't seen these kinds of gross aberrations before. Initially I thought the halos might have something to do with the low pass filter in front of the CMOS sensor in the camera, but I don't see them on shots taken with the telescope. And that also would not explain why the halos are off center. That is what I really don't understand.

Quite honestly, I don't know what is going on. I don't know if it's the IS elements possibly being tilted slightly or what.

The fact that these aberrations were not present in the images taken through the apochromatic refractor indicate that the problem is in the lens and not in the camera or sensor.