Quote Originally Posted by John Koehrer View Post
No light loss w/diopters is correct. That's the biggest advantage to them.
Light weight & compact size also help.
There is light loss w/tubes or bellows though.
I don't think so. Light is not quite 'lost.' It is spread out to cover a larger area and so correspondingly loses intensity. Just like an enlarger making an 8x10 print will need 4 times more exposure (2 stops) than when making a 4x5 print (1/4 the area of an 8x10). This spreading out happens no matter what method you use to achive the magnification and so there is an equal loss of light intensity.

-Anupam