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Message: <blockquote data-quote="Tony_79" data-source="post: 1863359" data-attributes="member: 86917">David, I have very similar experience. My 100-400 is sharp enough at wide open. It sharpens only very minimally when stopped down to f/7.1-f/8 (and it requires 100% crop to see that difference). Down to 50-70% crop and there is no difference.Once I removed the front lens assembly, there was dust inside and i just wanted to clean it (yes, i know those dust particles are only cosmetic trouble, does not affect image quality).However there was somekind of mechanism that will let you calibrate front lens distance in realtion to the secondary lens assemblies. I have no idea if it has something to do with image quality/lens calibration... It was sealed and tightened to one position. I left it original position.</blockquote>