[Larne]

So if I read this right what you are saying is that because at a long focal length we are not expecting to get super sharp pictures (due to the limited depth of field) the manufacturers are happy to use more of the lens, allowing a larger aperture diameter. At shorter focal lengths where the longer depth of field would be more likely to show up limitations around the edge of the lens they reduce the maximum aperture diameter to compensate.

Makes sense. Thanks for that.

On a related question - on some of my photographys in the corners lines (such as tree branches) have a red line down one side and a green one down the other side. (Is this called Chromatic Abberation?) Would this be reduced by using a smaller aperture and is the kind of thing that the manufacturers are trying to avoid? Or is this caused by something completely different?