>From: Andrew Gullen <andrew.gullen@xxxxxxxxxxxx>
>
>In the manual for my 65-116 tube, in the "care for use" list, it says not to
>add any other sort of extension...
>
>Does anyone know why this isn't a good idea?
I can think of a couple reasons: 1) moment of inertia, and 2) auto-aperture.
1) By lengthening the optical path with additional extensions, you are exerting
more stress on the tele-auto tube. It's pretty sturdy, but I can see why they'd
not recommend increasing the load on it. Particularly if you have a monster
like the 90/2 sitting on the end of a big lever, like a full stack of
third-party auto tubes!
2) The fancy mechanicals that couple the auto-aperture lever in the tele-auto
may soak up enough energy that an additional mechanism may make the total
system slow, inaccurate, or sloppy. If you hook up every auto-aperture lens
coupling device you have, at some point, there will be too much mechanical
resistance for the camera to properly operate the lens aperture!
That said, I've used the auto-tube with the 1.4X and 2X tele-extenders, and it
seemed to work well.
Bottom line, I don't think it will make ANY optical difference, and with care
(like, don't go swinging the 90/2 around at high-G) and within reason (like,
only ONE additional extension), I don't think it makes any practical impact on
wear or performance.
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