Yeah, my 7 element is a "ROKUNAR 2X MC7 AUTO TELECONVERTER". I've seen
several brands, including Vivitar and Tokina on eBay that look identical
in the pictures except for the substutiion of the brand name. I suspect
they all came off the same assembly line. I also have the Tamton SP
dedicated 1.4 and 2x extenders.
These are useful devices, but we should remember what they really do to
understand their limitations. Let's assume I have a 'perfect' 2x
telextender, that adds no abberations of any kind except the one it's
intended to. Lens designers have given all their skills and used the
best materials available to optimize all the image characteristics of a
100mm lens into a circular image with a diameter of about 44mm. The
extender expands that image to a diameter of 88mm, ample to cover the
85mm diagonal of 6x6mm medium format. 3/4 of that expanded image is lost
in the baffling of the extender and the mirror box (And any that isn't
absorbed bounces around to reduce contrast!), which is why two stops are
lost, only a quarter of the light the lens gathers ends up on the
film.The result can never be as good as a reasonably decent lens of
twice the focal length designed to optimize performance in the 44mm
diameter needed for 35mm film.This is one reason a 3x extender that
'really works' doesn't exist, it 'blows up' the limitations center of
the prime lens image too far and then adds it's own inevitable abberations.
For print film users, if film grain (or scanner dpi) is fine enough, it
might be better to use the prime lens alone and later enlarge the image
for a few reasons.
1) All image degradation of an extender, which has to make compromises
to be adaptable to many focal lengths, speeds and makes of prime lenses,
is replaced by the lesser problems of a dedicated enlarger or scanner lens.
2) All that physical light in the enlarger or virtual light in the
software handles the enlargement, rather than the natural light where
the photo is taken, so 2 stops are available for slower film, better
depth of field, faster shutter speed, etc.
3) One less thing to carry around and fuss with when actually taking
pictures. Sometimes the eagle flies in the time it takes to put on the
extender.
4) The 7 element extender is about the size of the 28mm f2.0 and almost
as heavy and larger and heavier than the 24/2.8, 21/3.5 and 18/3.5.
I haven't shot slides in years except to make slides for my friend the
art professor's lectures. I can't remember for sure the last time I used
a telextender...Hmmmm, maybe I need to rethink my 'light kit', to be
lighter or to have more perspective!?
Moose
Jim Couch wrote:
Tokina also has a 7 element 2X convertor that is highly rated by many as
well. I picked one up off of eBay not to long ago, but not yet used it
so I can't comment as to it's optical quality. Keep in mind when
comparing convertors, the lens it is attached to has a big influence on
the overall resolution ahd sharpness. If you are going to do any kind of
fair comparison you will need to use both convertors on the same lens.
I don't know if Gary Reese has done any testing on any of these besides
the Zuiko. It would be ideal to have one person get all the convertors
together and run tests using a singe lens or set of lenses with the same
film and targets to get a fair comparison. I am certainly willing to
loan the tokina out to anyone who would have the time and inclination to
do such a test.
Jim Couch
"W.Xato" wrote:
I have the Zuiko 2X but if there is something better
for less I would like to test one out. Do you have any
shots posted we could see to compare the Viv to the
Zuiko?
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