I read a post on photo.net dealing with TCs and depth of field. The basic reasoning was that with a 1.4x for example a 200mm 2.8 effectively becomes a 280mm 4.0. The ratio between the new focal length and the existing aperture changes the geometry to a 4.0. Therefore the depth of field is like a 280mm lens with 4.0 aperture not 2.8. Makes total sense. But then I began to think about constant aperture zooms like 70-200mm 2.8. I'm stumped. How can this be geometrically possible?