[Christopher Walrath]

I had a tele-extender (2X) for my Minolta/MD lens system a couple of years back. I had light fall off issues, up to a full stop with certain apertures and focal length combinations. I don't have the data anymore. However you do lose some light, but as long as you're metering TTL that's negligible.

DOI is another problem. The tele-extender spreads out the light reaching the film surface thus enlarging the circles of confusion and in turn reducing Distinctiveness of Image.