Hi There,
I'm a newbie to this forum as well as a new DSLR user so pls bear with me for my shallow knowledge in your world.

I've bought a Sigma 135-400mm lens for wild life shooting on my D70 and I like the working distance that this lens gives me. Now, I like to have a flexible range like the above for macro shooting instead of fixed macro but do not have much money to invest on another expensive lens.

Now, I've read some article that a cheap option would be to use a Auto extension tube. The questions I have is:

1. Which model of extension tube should I buy and will it be compatible with my exiting Sigma tele zoom lens and my NikonD70 (retaining the AF capability)?
2. What is the price range for such extension tube?
3. How close can I get for this extension tude without loosing any picture sharpness or quality. My objective is to take dragonfly that could fill the entire frame or even focus onto the eyeball of these dragonfly like those posted in the galleries.

Your expert advice is greatly appreciated and btw, anyone who has a second extension tube to let go will be consider.