I use a GH4 with a Sigma 24-70 f/2.8 lens and MFT to EF adopter that does not have iris control. I set the aperture of a lens on a Canon 7D and put it on the GH4, and even though GH4 is roughly on the same exposure setting with the 7D (f/22, shutter 1/50, ISO 200 - in Los Angeles daylight), the image gets way overexposed.

The only way I can get the right exposure is to switch to one of the auto exposure settings, which raises the shutter speed and is not ideal for video recording.

Does anyone know what the issue is? Is it possible that somehow the aperture size changes while I switch the lens between 7D and GH4? Thanks in advance!

DSLRs don't actually set the aperture until it takes the shot, so merely selecting an aperture on the 7D doesn't actually set it until the shot is fired and then it also opens back up again immediately after the shot too.

With this in mind, can you actually see the aperture blades in the closed position while transferring the lens to the GH4 or does the lens appear to be open (as I would expect) ?

When setting the aperture on a Canon EOS body to transfer the lens to a MFT body, the DOF Prevue button must be pressed in and held while the lens unlock button is depressed and the lens twisted off the Canon body.

If the DOF Prevue button is not held in while the lens is "unlocked" and twisted at least 1/4 inch, the lens diaphragm will simply open back up and remain wide open.

With the old Canon T1i there was no provision for manual settings in video mode, so to manually set the aperture (at some setting other than wide open) we had to set the aperture in Manual mode then use the "unlock and twist" method above while holding the DOF Prevue button in.

The 7D, T2i, and subsequent models solved the problem with full manual in video mode.