This lens was purchased for its image quality and fast autofocus speed and as a backup to my 500mm f4L. For working in close at bird feeders and for flight of raptors and Owls that allow a close approach I have long felt it would be a good choice.

Over the years I have gotten in the habit of testing each new lens that I purchase to see if it is up to snuff.  I only purchase from reputable dealers and I make it clear that I will return it if the lens is not sharp.  For years I have used a USAF 1951 chart to give me line pairs per mm numbers but recently I have found it somewhat easier and more intuitive to simply compare the new lens with a known good lens. For this test I have printed a test chart on 8.5" x 11" paper and hung it on a wall illuminated with one 150 watt tungsten flood lamp at a distance of four feet. All lens were used with a Canon 40D camera at ISO 400 on a sturdy tripod with mirror lock up and 2 second timer to reduce vibration. The lens I use for  reference was a 50 mm f2.5 macro. At f4 - f 5.6 this lens is about as good as a lens gets. Various tests by different web sites including photodo.com have shown this lens to be in the top 1% in optical quality. The new lens to be tested was a 300mm f2.8L IS canon lens reputed to be of very good quality. The chart below shows the area to be compared in the blowups hi lighted in red. A 100mm f2.8 macro lens was also tested.

Conclusion: The 300mm f2.8 L IS at f4 is probably the sharpest telephoto lens I have tested. At f2.8 it loses a bit of contrast but is still very good.  Contrast stays high until the X2 teleconverter is used but even then it is good. The camera resolution (10 megapixels) was the limiting factor in the tests. Edges and corners were also viewed and were quite sharp but only the center is shown for comparison. With stacked teleconverters (x1.4 and x2) the contrast is low and resolution finally is reduced some what but the image quality is still sufficient in a pinch.  This lens is slightly sharper that the 500 mm f4 but not by much, it is 2.7 Lbs lighter though and this is a major factor in handhold ability for many people. In addition the tripod collar can be easily removed to save 3\4 Lb. The Autofocus speed is very fast without teleconverters and I was able to obtain a head on flight shot that I would not have been able to get with a slower lens. With the x1.4 Teleconverter AF speed is slightly less and it is slower again with the X2 but still usable for flight with slower birds flying across the film plane.
 
 

 
 
 

Actual Pixels

 
 
 
 

400% enlargement of actual pixels