This is a fast, sharp telephoto lens.  It exhibits less spherical aberration (coma) or color dispersion (color fringing) than other inferior lenses.   It is well suited for astrophotography using a Canon DSLR (eg. 350D, 50D, etc.).  It is not a zoom lens, however that is one of the reasons it is so sharp across the whole field of view.

The lens used in combination with an APS format Canon DSLR is suitable for wide-field targets up to 6 degrees wide.

 

Wide-open, at f/2.8, it exhibits noticeable aberration in the center of field.  However, by stopping down the aperture just a little bit to f3.2 there is a significant improvement.

 

In summary, at f3.2 it's okay, at f3.5 it's good.  At f4 it's very good and f4.5 should satisfy the meanest eye - right out to the edge of field.

 

The automatic focusing feature is accurate enough to get you started.  Set the lens' focus mode to auto, focus it automatically on a very bright star.  Then take a 15 second exposure and check the results on the computer.  Do this several times and retry the automatic focus and so get the best focus (smallest stars) that you can.  When you've got the smallest stars possible (it should only take you 5 or 6 tries) set the focuser switch to manual and then, if you don't bump the focuser in the dark, you're in for a good night of photography.  I still use DSLRFocus to get the best focus possible.

 

The lens comes with a nice long hood as standard (see picture lower-right), which really saves having to make one. Getting it on and off requires only a quick push/pull with a twist.  The hood is perfect for delaying the onset of dew and really helps keeping stray light off the lens.  The only downside is getting the lens cap on and off (to take dark frames) is tricky if you've got fat fingers. 

 

I've used the 200mm lens to take several images (see examples below) and I can say that this lens (and also its little buddy, the Canon EF 50mm f1.8 II) represent good value for money. 

 

Objects that are particularly well suited to the 200mm lens' field of view are:

      The Large Magellenic Cloud (just fits snuggly in)  [click for first light pics 06Jan08]

     also in the frame for future shoots are:

The Small Magellenic Cloud
M42, Flame and Horsehead in the one frame (easy)
M8 Lagoon, Trifid and the surrounding Milky Way star field  wow!
Eta Carina, the Wishing Well Cluster and surrounding Milky Way
M24 The Sagittarius Star Cloud
M46 & M47 pair of clusters
Baades Window
The Table of Scorpius

This is a (1:1) crop from slightly off-centre of an image taken at f3.5
You're supposed to use a telescope for this object...