I did these tests this evening. I was as careful as possible to keep room lighting the same for each shot.
See the thread about "50mm lenses that don't suck wide-open", which made me get off my butt and do these tests.
Note that the aim for these tests was only to compare lens brightnesses (T-stop) and bokeh, with all lenses wide-open.
The scene is too dark to determine relative sharpnesses.
I tested the Canon 50mm f/1.4 wide-open, both connected and NOT CONNECTED electronically, by rotating lens in body while holding the lens-release button down. Mechanically it is still completely mounted, but the camera cannot communicate with the lens, so does not know what it is, so it cannot "decide" to sneakily increase the ISO.
Canon 5D Mark 3
t=1/15s, ISO400 (nominal)
Focus was on the same point on the lantern, using live view, magnified x10.
Dude was right. The "CONNECTED" Canon 50mm f/1.4 image is noticeably brighter, and I took the picture both ways, several times, and the results were completely repeatable. So it seems that the 5D3 DOES sneakily SEEM TO boost ISO without telling you, if it detects that the 50mm f/1.4 lens is connected. When I review the images on the camera, they all say ISO400.
Dude was also right that the Olympus 50mm f/1.2 is significantly brighter than the Canon 50mm f/1.4 wide-open, EVEN brighter than the "CONNECTED" image. That makes me feel slightly better about having dropped just shy of £400 on this (perfect, mint, unmarked) example of this lens, back in Jan. 2010!
Here's the album on FB:https://www.facebook.com/media/set/?set=a.4505135767033.1073741834.1849695638&type=1&l=7e5db91bd2
Here are the files on Dropbox, including raw files:https://www.dropbox.com/sh/rhtr3k0ru902sbr/yMKl0KWANf?lst
The tests I did were:
Zuiko (Olympus) 50mm f/1.8 - Dimmest, as expected
Zuiko 50mm f/1.4 Silvernose
Zuiko 50mm f/1.4 Non-Silvernose (later model) VERY SLIGHTLY brighter than Silvernose
Zuiko 55mm f/1.2 - Has the "biggest" bokeh / OOF blur. NOTE! This is a 55mm lens, not a 50mm one.
Zuiko 50mm f/1.2 This is the brightest of them all
Canon 50mm f/1.4 NOT CONNECTED, because lens was rotated to disengage electronic contacts
Canon 50mm f/1.4 CONNECTED - Should be same, but IS BRIGHTER