> Several posters are getting in real deep with the XL-1 focus mechanism.
> There seems to be some suggestion that lenses with an internal focus
> mechanism cannot track whilst zooming - this is piffle!
> My broadcast Canon 8 x 6 and many of its modern cousins and rivals often
> use internal focus so that the lens front element is static and can
> mount a filter holder.
> Perry Mitchell
Perry:
All internal focus mechanisms are not the same. A professional zoom lens
with internal focus still uses separate elements for focus control, just
like a traditional zoom lens. The focus ring moves one or more elements just
behind the front one. Those elements are entirely independent of the zoom
section of the lens.
The zoom lenses with the focus problem being discussed have NO elements,
front mounted or otherwise, for exclusive focus control. They accomplish
focus by changing the distance between two groups of zoom elements. They
accomplish macro focus the same way.
Your broadcast lens has macro focus too. Most of these do so by moving the
prime lens element group at the extreme rear of lens, like adjusting back
focus.
You are fortunate to be using professional lenses, Perry, you're missing out
on the 'fun' of tyring to use a lens that zooms in and out of focus. :-)
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