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Canon EF 200mm f/2.8L II Lens Test

This page compares various mounting plates, aperture settings, and focus methods for use with the Canon EF 200mm f/2.8L II USM lens. I'm using a side-by-side mounting plate made of MDF for adapting the camera/lens and guide scope to my mount. Suspecting some differential flexure issues, I decided to test one made of Aluminum as well.

The images below are a mosaic of full-resolution crops of the four corners, plus the center, of shots taken wtih a Canon EF 200mm f/2.8L II USM lens through a Canon Digital Rebel (300D) of the "Leo Trio" region. All shots are guided and are a stack of 10 x 2min exposures, processed in IRIS. The mount is a Celestron AS-GT. Note that these images are (over-)stretched to show the star shapes; making esthetically pleasing shots was not my goal. Here are the details for the four images:

ApertureMounting PlateFocus Method
f/2.83/8" AluminumAutofocused on Jupiter
f/2.85/8" MDFManual focus via DSLRfocus
f/4.05/8" MDFManual focus via DSLRfocus
f/5.65/8" MDFManual focus via DSLRfocus

 

Mouse over the text labels below to cycle through the four images:

[Aluminum f/2.8]     [MDF f/2.8]     [MDF f/4.0]     [MDF f/5.6]
 

Note: the mouseover effect requires JavaScript to be enabled. If you don't have JavaScript enabled, click on the links below to load the images in their own pages:

Conclusions

Comparing the two f/2.8 images, it looks to me as if neither plate has any differential flexure issues. I'll probably use the 3/8" aluminum plate for future tests "just to be sure". Similarly, it appears that autofocusing on Jupiter (at least at f/2.8) provides comparable focusing results to using DSLRfocus on a star.

Second, it appears that this lens has a bit of astigmatism when used "wide open". To prove this assertion, note the "kite-shaped" (as opposed to circular) appearance of the stars in the above images, even the stopped-down ones. Also, not the appearance of these two shots of a star just inside, and just outside, of focus, respectively, at 2x image scale to make the effect more visible:

Inside:Outside:

Notice how the two out-of-focus patterns are elongated at 90 to each other, the tell-tale sign of astigmatism. The bottom line of this lens seems to be that it only provides clean stars edge-to-edge when stopped down to f/5.6.