So I recently picked up this lens, and wanted to test it with my old extender, because now I'm broke from buying the lens so I can't get a new tele extender (or my wife will kill me and hide the bones well).
I completed what I feel is a pretty fair test. I took two images, with the same settings, both as wide open as possible, with the same flash settings, and just adjusted ISO to make up for the one stop loss with the extender. I shot in raw, imported to lightroom, then exported as a 1280 JPG, with no other processing done (removed the default processing it tries to apply, and ensured on export it wasn't doing any sharpening, just resizing).
There's a little difference when looking at the images next to each other @ 1280pixels wide. More than sharpness though, I notice the contrast difference.
Then I took a 1:1 crop, still not doing any editing of the images. Unfortunately the 1:1 crops are still over 1280 pixels, the originals can be found on flickr.
There is a slight fall off in sharpness with the extender attached, but what strikes me more than the sharpness drop off is the difference in contrast. I think the image is plenty sharp even at 1:1 except in areas where contrast was lacking, and then the detail was lost. I think at the end of the day it gives me comfort that the extender is certainly usable, but I will likely do my best to exclude the extender and crop from the image. There are certainly cases where I can see using the extender still, but the loss of contrast to me is the thing I am finding most difficult to get over.
