A fellow photographer, Michael Quan, and I were having a discussion at Glamourcon 2011 in Long Beach this past Saturday and the subject of Singh-Ray Vari-ND filters came up. He stated he was looking to get one and I had mentioned how I have a 77mm Singh-Ray Vari-ND in a standard mount, but that a thin-mount form factor was also available.
While I had no problems with any vignetting on my Canon EF 70-200 f/2.8L IS II with the standard mount form factor, he was curious as to whether or not there were problems with a Canon EF 24-70 f/2.8L.
It just so happened that I bought a used a 24-70L from lensrentals.com this past week, so I decided to test the 77mm Singh-Ray Vari-ND standard mount filter on the 24-70.
I put a piece of red gaffer tape on an off-white wall for a focus point, setup a Paul C. Buff Alien Bee B1600 flash with no modifier (bare bulb) and dialed it in to about 3/4 power. I threw the 24-70 onto my Canon 5D Mark II and dialed in 125s, f/8, ISO 100. These settings in combination with the Alien Bee strobe were meant to blow out the wall so any vignetting would be visible.
Without any filter (or hood) on the 24-70, I took a test shot at each 24mm and 70mm for control / reference shots.
The following photos were taken with the Singh-Ray Vari-ND 77mm standard mount ring on the 24-70 at varying focal lengths (no lens hood).
From my testing with the Canon 24-70 f/2.8L and Singh-Ray Vari-ND 77 standard ring that I have, there is definite vignetting at the wide end of 24mm up to ~35mm. There does not appear to be any vignetting at 50mm through 70mm.
Conclusion: If you want to use a Singh-Ray Vari-ND filter on a Canon EF 24-70 f/2.8L across the entire focal range, you cannot use a standard mount and will need to get a thin-mount version.