Originally Posted by
rpcrowe Additionally, some camera/lens combinations can produce better IQ when shot wide open than can other combinations. I often shoot wide open with some tele zooms (such as the Sony 70-350mm) because of the relatively small (f/6.3) maximum aperture at the longest telephoto settings.
Someone who knows more than me, please correct me if I'm wrong, but: the first sentence is clearly true, but I think the second reflects a misconception.
What is relevant to distortion isn't the f-stop number; it's how far out from the center the lens is gathering light. When you shoot wide open, you are going as far from the center as the lens will go. The reason the f-stop is gets larger as you [zoom in] isn't that the aperture is getting smaller; it's that the focal length is getting longer, and the f-stop is the ratio of the focal length to the aperture.