usher99@xxxxxxx wrote:
>Hi,
>One would always expect some image degradation with a 2X TC esp on the edges
>due to the highly curved nature of the optics.
>
I'm not sure this is an issue.
>The TC also magnifies all the aberrations in the attached lens.
>
This is the big issue. The lens designers have to make many, many
compromises and balancing acts to get the best combo of sharpness and
contrast across the image circle size for the target film/sensor, while
at the same time getting reasonable control linear distortion, CA, etc.
As with the new lens designs for small sensors, if they have a smaller
target size, they can make a sharper lens. That's a good thing, since
the image will be enlarged more for any given display size.
When you enlarge the center of field, you also enlarge all the failings
there that exist to make the outer areas that are now ignored decent
quality.
>The 1.4 X TC are said to degrade the image less, as one would expect. I only
>have a 4 element 2X TC and the 7 element Tamron SP 2X BBAR for OM. Some
>combinations work quite satisfactorily if one is careful to keep the obeject
>of interest in the sharpest portion of the field and stop down enough. I
>recall a very nice macro shot posted by Moose with a flower and wasp a few
>months back with a TC. I couldn't relocate it with a quick search.
>
>
Probably moved when I did a little reorganization
<http://www.moosemystic.net/Gallery/tech/Tam902.5/>. It doesn't tell
anything much about performance outside the center. But that's the
nature of most shots I would be taking with this combo anyway. I know it
is very sharp and flat edge to edge without the 2x, but the fl is too
long for my copy stand with the 2x.
>Curiously while looking at a Tamron adaptall review web site, I found a
>reference that Tamron states some of their lenses degrade less with their TC
>than equivalent extension despite no optical elements in the latter.
>Hmmm, that was news to me.
>
>
Well, that could make sense, especially, as here, where the lens and
telextender are designed to match each other. Assume the lens is
optimized for 1:10, has good performance over the infinity to 1:2 range
and gets progressively poorer down to 1:1. With extension, the
performance diminishes. With the 2x, the lens itself is acting like it
is at 1:2 at full extension of the helicoid. The telextender then
magnifies that image to make it 1:1 on the film. So the WAY it gets to
1:1 is optically quite different for the two "extenders". So which is
better at any particular magnification is going to depend on some
complex factors.
Although I have both the 2x and the matched extension tube for 1:1,
fortunately I don't have to run any tests - for two reasons. First, the
lens is on loan to a friend doing copy work with it and a 50/3.5. And I
have no idea when it's coming back. :-) Second, I much prefer the 2x
both because it focuses directly from infinity to 1:1 without taking off
and putting on the tube and because the longer stand-off distance is
better for nature shots.
Moose
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