Five plus or so years ago the similar teleconverter made by Olympus that
Bill is refering to was bringing up to $200 on yabe. At the time there
weren't many telephoto optons for affordable digital cameras. The 1.7x
version that Olympus made was one of the best ways to improve the range of
the cameras. Since then affordable digital cameras have improved and lenses
availabilie for them has dramatically improved.
I believe several people on the list had tested the Olympus 1.7x version on
their OMs and found that most lenses having a focal length less than about
70mm didn't provide a worthwhile combination. Longer moderately fast lenses
would vignette. I believe there were some lenses for which it worked fairly
well. (I believe the Olympus version was called B300 because in converted
the IS3 lens into a 300mm lens)
A 140mm x 3 = 420mm f6 lens requires at least a 420/6 = 70mm front element
diameter to avoid vignetting. So maybe this Tokina lens would be useful on
your zoom.
As Bill mentioned putting this type of lens on front of your lens may not
impact the most open aperture. Putting a teleconverter between your lens and
the camera spreads out the light changing the effective aperture by the
multiplication factor of the teleconverter.
I've never seen a Tokina version before. There were some cheap 3rd party
lenses of this type ( "Crystal" or "Digital" brand name? ... that were
considered worthless)
-jeff
----Original Message Follows----
From: "Bill Pearce" <bs.pearce@xxxxxxx>
This is based on my experience with one on a IS3.
It should not affect f stop, as it is in front of the lens. Some T stop
difference, of course, but not a teleconverter.
It is likely only good at the maximum FL of the lens, and likely vingnettes
at wider FL.
AF should be OK.
It is probable acceptable, but not as good as a lens of equivelent FL, but a
whole lot cheaper.
Bill Pearce, yes, he has no elk!
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