Originally posted by aerodave I feel like some constant-aperture zooms are cheating the user out of some potential aperture at the wide end. The DA 17-70mm and the DA* 16-50mm both have aperture blades visible when the lens is wide open at the wide end. As you zoom with the aperture held open, they disappear at some point in the middle of the lens's zoom range.
I think you are getting confused with the difference between what you can see and what the relationship of the elements is.
When zooming, there are lots of things moving, and it may be that while the aperture blades are more visible they do not interfere with the light path,. Watch next time as you zoom and see what happens internally.
Also note that at some point,. the aperture is restricted by the diameter of the front element, especially at the longer lengths, to make this faster, would require bigger diameter elements.
But, you are also partially right. your front element is what 50-55 mm in diameter on the 17-70? (not the filter ring but the element) at 17mm F2.8 only needs to be about 6 mm in diameter