Here's a quick and easy way to pretty well eliminate minor vignetting
from your photos with Photoshop Elements. This has just been my first time
using layers, so if I can do it, anyone can.

First take a picture of an evenly illuminated smooth white surface with whatever
equipment your having vignetting problems with. Make sure your white balance is
set right - use manual if your camera has that option. You'll wind up with a shot
like this:
http://www.fototime.com/{DCEB21A4-289D-417D-8441-9C1225824B7A}/picture.JPG

Open this file in Photoshop.
Go to - Select > All
Then - Edit > Copy
Next you tear off the layers palette and place on desktop.

Now you open the picture file you want to fix.
Then - Edit > Paste
Image > Adjustments > Invert
Now go to the layers palette and set blending mode to Soft Light.
Next go to - Layer > Duplicate layer

That's pretty much it. Depending on the photo, the first adjustment
layer might have eliminated the vignetting. If it needs more you can
fine tune it by adjusting the opacity of the second adjustment layer.
When you like what you see flatten it and your done.
It's real easy and fast to do a series of files this way.
There's some extra noise left from the dark areas and it's not as sharp,
but for many shots it's not even noticeable. I'm sure with a bit of extra
work this can be improved too. I'd like to hear any ideas on that - I'm
just a Photoshop beginner.

This will help a lot of the stacked teleconverter shots be able to use their
full resolution.
Let me know if I screwed up the directions!

Here's some examples of how it works.
All these shots were taken with my E100 and a B300 stacked on a TL 46.

First shot is of the vignetting shot canceling itself with two layers at 100%
opacity
http://www.fototime.com/{5FA5F7AC-1CAE-41BB-9476-AE01CE3C782A}/picture.JPG

Here's two examples of other shots.

The bird shot is an extreme example with the center being lit with a shaft of
light and a dark background.
http://www.fototime.com/{23C09072-F345-46A1-8285-3B6B5D41CAB3}/picture.JPG
http://www.fototime.com/{92D14A29-45BC-4943-8F7C-B54AEAB36B0F}/picture.JPG

http://www.fototime.com/{49EE2232-338A-421A-943E-C233BED042B2}/picture.JPG
http://www.fototime.com/{79D80463-C46E-4935-82F4-9AA521045267}/picture.JPG