This is a test of the Canon EF-S 18-55mm f/3.5-5.6 IS Autofocus Zoom Lens on a star field. This is a very inexpensive lens that comes in a kit with the Canon Digital Rebel cameras and costs about $140 if purchased separately. Gemini is near the center of the frame, and the Hyades is in the lower right corner. This is the full frame image at 18mm of focal length, the lens' widest setting. These images are taken from the in-camera JPEG, with in-camera sharpening turned off. The camera was set to a custom 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/3.5 at 18mm of focal length, and stopped down one stop to f/5.0. Performance of this lens is very good when used wide open at f/3.5 at 18mm focal length. There is a bit of chromatic aberration seen as blue fringing around the brightest blue stars, and a bit of coma and astigmatism in the corners, but neither is objectionable. Chromatic aberration improves considerably when the lens is stopped down one stop to f/5.0, but the coma and astigmatism only improves a little bit. Stars also get a bit 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/3.5. Almost all lenses have some vignetting when used wide open, so this is nothing out of the ordinary with this lens. Vignetting improves as the lens is stopped down, but it is certainly not objectionable wide open.
In the comparison above at 500 percent enlargement, we can see the chromatic aberration near the center of the lens wide open at f/3.5 at 18mm of focal length, and stopped down one stop to f/5.0. This color fringing improves as the lens is stopped down, but it is only visible around the brightest blue stars in the frame. I shot other similar tests with the lens at 35mm of focal length, and zoomed all the way in at 55mm of focal length, with similar results, except the lens performance improved wide open at the longer focal lengths, although the f/ratio did get slower as the lens is zoomed in. Conclusion This lens' price - performance ratio is outstanding. It is certainly usable wide open at its fastest f/ratio and widest zoom setting for astrophotography with very good results. It is also very good at its other focal lengths when used wide open. The lens is even usable wide open at f/3.5 at its widest angle of 18mm for constellation shots on a fixed tripod at ISO 1600 with about 30 to 45 second exposures. For longer focal lengths, because the lens gets slower as it is zoomed in, star trailing be become more noticeable sooner on a fixed tripod. For use on a polar-aligned equatorial mount, a fast aperture is not as much of a consideration for most wide-field shots, since a longer exposure can be used, and tracking is not that critical at the focal length range covered by this lens. At its widest setting, the Canon 18-55 f/3.5-5.6 EF-S IS zoom lens is only 2/3 of a stop slower than Canon's top-of-the line 16-35mm f/2.8 EF L USM IS zoom, and at a comparable aperture of f/3.5, it's performance is remarkably similar. The 18-55mm costs $140, and the 16 - 35mm costs about 10 times as much at more than $1,400. Here you can see a star-field test of the 16-35mm f/2.8 lens. Of course, the 16-35mm is made to cover a full frame sensor and the 18-55 is only made to cover the smaller APS-sized sensors, so it can't be used on a full-frame camera. But if you can afford a full-frame camera, you can afford the more expensive lens. The other difference is that the 16-35mm stays at f/2.8 throughout the zoom range, but, all things considered, the Canon 18-55 f/3.5-5.6 EF-S IS zoom lens is a remarkable bargain, especially considering it has Image Stabilization built-in for daytime photography. Exposure Data
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