This is a test of the Canon EF 16-35mm f/2.8 L USM Autofocus Zoom Lens on a star field. This is Canon's very expensive, top-of the line zoom lens that costs about $1,450. The constellation of Cepheus is near the center of the frame, and Cygnus is in the upper right corner. This is the full frame image at 20mm of focal length. These images are taken from the in-camera JPEG, with in-camera sharpening set to normal. The camera was set to a daylight white balance, and no color correction or other enhancements were applied. All images were manually focused with Live View on a bright star. Note that this zoom lens is not parfocal. This means that it must be re-focused if you change the zoom setting. Below are the corners and center of the frame seen at 100 percent enlargement:
In the comparison above, we can see the performance in the center and corners of the lens wide open at f/2,8 at 20mm of focal length, and stopped down one stop to f/4.0. Performance of this lens is ok when used wide open at f/2.8 at 20mm focal length, considering it is reasonably fast at f/2.8. There is a some chromatic aberration seen as blue fringing around the brightest blue stars, and coma and astigmatism in the corners. Chromatic aberration improves considerably when the lens is stopped down one stop to f/4.0, but some coma and astigmatism remain in the upper and lower right corners. Uneven coma and astigmatism may mean lens elements were not in collimation inside the lens. Stars also get tighter and sharper, as is expected, when the lens is stopped down. In general, almost all lenses improve in performance when stopped down a stop or two from wide open. It is hard to see in the full-frame image at the top of the page, but there is also some vignetting present in the frame shot wide-open at f/2.8. Almost all lenses have some vignetting when used wide open, and more so with fast lenses, so this is nothing out of the ordinary with this lens. Vignetting improves as the lens is stopped down.
In the comparison above at 500 percent enlargement, we can see the chromatic aberration near the center of the lens wide open at f/2.8 at 20mm of focal length, and stopped down one stop to f/4.0. This color fringing improves as the lens is stopped down, but it is not that objectionable when the lens is used wide open as it is only visible around the brightest blue stars in the frame. Conclusion Canon EF 16-35 f/2.8 L USM zoom lens' price - performance ratio is not that great considering it costs more than $1,400. It is usable wide open at its fastest f/ratio and widest zoom setting for astrophotography if necessary. The lens is good for constellation shots on a fixed tripod at ISO 1600 with about 30 to 45 second exposures at f/2.8. For use on a polar-aligned equatorial mount, it can be stopped down to improve its performance as longer exposures can be used at slower f/stops. The 16-35mm is made to cover a full frame sensor and stays at f/2.8 throughout the zoom range. Exposure Data
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