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Finally have had a little time the past few days to do some experimenting and to try to get some results together.
Rather than post ten's if not 100's of images from all the TC's of the centers and corners etc, I think I can somewhat summarize some results.
Basically tele conversion TC's fall into 2 camps, those that vignette severely and those that don't (though they may still have very soft edges)
Interesting thing is is that many of those that have a strong vignette are actually very sharp. They simply weren't designed for wider focal lengths though. Rather they were made for using the long end of zooms and only at those setting will they not vignette.
What this means for the X100 user is that we can basically rule out all the Canon, Panasonic, Ricoh etc TC's at 1.5x and higher powers, they just aren't going to really work in a practical manner.
I'm posting an image, this one from the Olympus 1.9x c-210 (and actually the best performer of the higher powered bunch) to show what I'm taking about.
As you can see, you get a crazy amount of vignetting, and stopping down can cause slightly more, though its more defined and sharp so no real net difference.
You are throwing about probably 75% of the frame once you crop out the dark circle, and the exposure can be a bit off as well since the camera is seeing so much dark.
Good news is that the middle of the frame is actually pretty sharp. As you can see from the 100% crop with the TC, there is good per pixel sharpness there.
I also uprez'd the non-TC image to match the pixel dimensions and its no contest, TC is much better.
Thing is though, after you crop out all the dark parts, your left with about a 2000 pixel wide image, or about 3 megapixels. Looks okay for the web, or a very small print I suppose, but thats about it.
Basically you've got about a 150mm equiv lens on a 3megapixel camera when using these type of TC's after cropping. If you wanted to take a picture of something really far away I guess its better than nothing, and yeah, it beats cropping the native image, but what practical applications are there ?
One interesting side note is that in the video mode, where you can apply an automatic crop using just the center of the sensor as is, is that your then using just the non vignetting part of the image so your getting something like a 5x crop effect. Thats kind of a cool, but again, not very practical for video work given the lack of stabilization.
So here are some results, and the next part is where things get a bit more interesting with the lower power, yet non-vignetting TC's that can give you nearly total coverage, but with a more modest 1.4x or so increase, roughly giving a 50mm equiv.
http://img191.imageshack.us/img191/1708/x100tc.jpg
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