70-300 IS (Insufficient Sharpness) - Canon website post
Service Notice: EF 70-300 f/4-5.6 IS USM Lens May Result in Insufficient Resolution
EF 70-300mm f/4-5.6 IS USM
To Customers Who Use the EF 70-300 f/4-5.6 IS USM Telephoto Zoom Lens:
Thank you very much for your patronage of our products.
We have recently become aware of a phenomenon in which some images captured by the EF 70-300 f/4-5.6 IS USM lens appear in insufficient resolution. This phenomenon may be seen at the edge of the frame at the 300mm setting when the camera is held vertically. We have been studying the symptoms of this phenomenon in order to determine its precise cause and to develop an appropriate remedy. We will inform you on this Web site as soon as this remedy has been decided, which, at this point should be by the end of June 2006.
We are very sorry for the inconvenience this phenomenon represents for customers who use this lens and appreciate your patience as we work to resolve it.
Contact Information for inquiries
1-800-828-4040 (toll free)
8AM to Midnight - Monday to Friday
10AM to 8PM - Saturdays