Most consumer zoom lens will have a variable relative aperture at different focal lengths. It is expensive and difficult to maintain a constant aperture at both ends of the focal range and maintain other lens qualities. I don't recall any camcorder zoom lens with the same aperture or f stop at both ends of the focal range.
Please, keep attention: absolute iris aperture and f-stop are not the same. HV20 phisically closes the iris when the focal length grows. Obviously the F-stop (entrance pupil divided by the focal length) grows with the focal length. For example, at the minimum focal length, when the iris is totally opened, you have F/1.8; when it is totally closed, you have F/8 (this is not the truth, because the iris could close until F/22, but only when not in Av). So, at the maximum focal length, the F-stop should be between F/2.8 and F/12 (or F/35 with automatic exposure), but HV20 simply closes the iris following the F-stop measured at 6.1mm.
Originally Posted by Rich
Canon had justified the forced iris closure with image quality, but I didn't see any difference in quality between the 2 frames: see the flower in focus, or try by yourself if you have an HV20.
However, this is a trick in order to have more control on the cam...