I don't think the 2x will produce an image circle of 88mm, I think it
just magnify the center of the image of the prime lens and gives the
same image circle of normal 35mm lenses.
Concerning image degradation, I also have the same thought as you ten
years ago, I expected only the prime lens has resolution, the 2x will
magnify the image but half the resolution. But based on my experience,
Gary's test report and an test essay I read from Pop photo. I have
totally changed my mine. The tele extender perform much better than I
thought, a good matched 2x will give very little lost on resolution.
Give some trial with your lens you will know.
C.H.Ling
dreammoose wrote:
>
> 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.
>
<snip>
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