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Image 1 - Comet Machholz passes by the Pleiades. Shot with a Canon EF 70-200mm F/2.8 L USM IS zoom lens at 200mm at f/2.8 with a Canon 1D Mark II digital camera.

Hold the mouse cursor over the image to see the sections of the image that were enlarged below.

Image 1 shows the full frame from an original processed raw frame. It was opened in Canon's Digital Photo Professional v 1.5.03 with the following settings:

• Brightness Adjust: 0
• White Balance Adjust: Click White on the sky background
• Dynamic Range Adjust: 0 (-9.0), 4095 (3.8)
• Tone Curve Property: Linear
• Color Adjustment: Shot Settings (none)
• Convert and Save: 16 bit TIFF
• Color Profile embedded: Adobe

The Image was opened in Photoshop CS and the mid-tone gamma was adjusted to 3.18. No other adjustments were made to the image.

For display on this web page, the images were re-sized using bi-cubic resampling, sharpened with an unsharp mask of 100 percent, a radius of 0.5 pixels and a threshold of 0. The files were then converted to sRGB color space and saved with a JPEG quality of 8.

The original high-resolution file was opened in Photoshop where it was enlarged to 100 percent and cropped for the following examples of the lens' performance on a star field.

Image 2 - Center of frame, 100 percent enlargement.

Even in the center of the frame of Image 2 we can see that the stars exhibit a blue halo from chromatic aberration in the lens when used wide open at f/2.8 at 200mm focal length.

Image 3 - Center of frame, 500 percent enlargement.

Also note that the stars are not circular in Image 3, which they should be, and as seen in the previous examples shot with the 300mm f/2.8 and 400mm f/2.8 L series USM IS lenses, the brightest core of the star is not centered, but offset to one corner, with a large diffuse halo around it, that is not chromatic aberration.

These particular stars were chosen very near the exact center of the frame. because they display a varied range of brightness.

Image 4 - Top right corner of frame, 100 percent enlargement.

Stars in the corners of the frame of Image 4 exhibit lateral chromatic aberrations, as well as astigmatism.

Image 5 - Top right corner of frame, 500 percent enlargement.

The lateral chromatic aberration can be see here in Image 5. It is caused when all three color planes in the image are not brought to the same focus.

Image 6 - Lower left corner of frame, 100 percent enlargement.

Here in Image 6, in addition to astigmatism and lateral chromatic aberration, a diffraction effect can be seen, which is curious since the aperture diaphragm was not stopped down, which usually causes these effects.

Image 7 - Lower left corner of frame, 500 percent enlargement.

Diffraction effects can be easily seen around the brightest star in the frame in Image 7.

Image 8 - The Pleiades, enlarged 100 percent.

Misshapen stars in the Pleiades are seen in Image 8.

Conclusions

The disappointing performance of this lens when used wide open is not that surprising. It is a zoom lens after all and contains many elements in many groups, plus it has the image stabilization element. Stopping it down improves the performance, but stars never look like they do in an apochromatic refractor. This is not unexpected as an apochromatic refractor is designed to do one thing well, image stars at infinity. A zoom lens is designed to provide a wide field at different focal lengths and at different subject distances. It is not really fair to expect a zoom lens to perform as well on the stars as an apochromatic refractor.