A different way of looking at the focus breathing of the 70-200mm f/2.8 VRII

Started Feb 3, 2013 | Discussions thread
Samuel Dilworth
Samuel Dilworth Regular Member • Posts: 249
Re: And a better way to look at it…

pluton wrote:

Samuel Dilworth wrote:

Whether zooming or focusing, the size of the diaphragm doesn’t change. (The diaphragm is placed in or near the regular lens behind the afocal lens.) Although the physical diaphragm doesn’t change size, the entrance pupil does change size when the lens zooms, because the magnification of the upstream Donders lens changes. But since the Donders lens is afocal and appropriately sized it doesn’t affect the relative aperture (f-number).

Samuel, Unfortunately, I've never closely examined the current Nikon 70-200/2.8 lens, but I can tell you that every other post-1970's Nikon zoom I've looked at has the aperture change size as the lens is zoomed; My 14-24, 17-35, 25-50, 17-55, and others I've seen. The aperture opens---through a mechanical linkage of some sort---as the lens is zoomed to the tele end. You can see it with your own eyes; usually it's easier through the rear of the barrel. Are you sure that Nikon has not employed this well-established "trick" in the lens under discussion?

Are you sure you’re not instead observing a change in the size of the entrance pupil (if viewed from the front) or the exit pupil (if viewed from the rear)? I know some lenses do use cams to link the diaphragm to the zoom position, but I’d be mildly surprised if all the lenses you list do.

In any case, the f/2.8 telephoto zooms use an optical construction that doesn’t require a change in size of the diaphragm to maintain f-number when zoomed. That’s because the zoom effect comes from an afocal lens in front of a conventional lens, as I described in an earlier post.

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