El Sid
(Going potty)
23/07/2008 15:16
Re: Canon 100mm macro and extention tubes

According to the formula given here 100mm of extension would reduce effective aperture by 2 stops (2.8 x (100 + 100)/100). Thus doubling the focal length of the lens by means of extension tubes has exactly the same effect as using a 2x converter.

There's no getting away from Newton.........