Earlier this week a friend of mine asked me to take some test shots with her 100-400mm IS L lens. She said she has never been happy with the sharpness of the lens. I didn't have a tripod, so I set a beanbag on the hood of my truck and rested the lens on that. I also placed a bag of rice on top of the lens for extra stability. I didn't have a lot of time, so I only took some shots at 400mm because that's where I figured she would use this lens the most. I'm sure I would too.

When I got home a looked at the pics starting from wide open, and working my way through the apertures, I was a bit surprised. Wide open, I didn't think it was too bad, and it continued to get better as I stopped down, until I got to f/8. This was by far the WORST image of the bunch! I was expecting f/8 to be one of the sharpest images of the bunch. f/9, f/10, and f/16 all looked very good. I just couldn't believe how bad f/8 was.

Granted this wasn't the best testing environment (no tripod, cable release, outside on a windy day) but I was expecting much better performance out of an L lens.

I don't own this lens, nor to I plan on buying one, but I do have a rental reserved for a trip coming up in about a month. After seeing these images, I'm starting to wonder if I made a mistake. I will have a few days with the lens before I leave so I can test it out and figure out where the sweet spot is, but the test images make me a bit nervous.

Has anyone else who uses this lens had any problems with soft images? I wonder if she just got a bad copy of the lens, or if it needs to be sent in for adjusting. I scanned through a bunch of the reviews on this site, but didn't see anyone say anything about this lens being soft, except for wide open.