I don't scoff at the idea. A Barlow can produce some flattening of the field, but the idea again is intent. The Barlow may not be able to completely compensate for a severely curved field. This is where *deliberately* designing a field flattener out in front comes in, as it is set up to *match* the field curvature of the positive lens set of the eyepiece, thereby canceling it out. It isn't a "built-in" Barlow used so that cheap eyepiece designs might be used at shorter f/ratios; its intent is for field flattening from the word "go". Clear skies to you.