This still annoys the hell out of me. I briefly mentioned the viewfinder illumination issue in my initial 5D Mark III impressions post. Today I shot with an 1D X (more on the camera in a later post), which has the exact same problem. OK, this is not surprising as the two cameras share (almost) the same auto-focus system.
To quickly sum it up: when anything (AF point or grid line) is illuminated in the 5D3/1DX viewfinder, then the entire screen glows in red. This is especially distracting if you have the grid lines turned on (which will produce a whole-screen red flash every time the AF system locks). The following picture shows what you see in the finder in complete darkness while selecting the focus point (body cap on). You’ll get the same amount of red light for AF confirmation.
A red veil over the finder image is clearly visible in usual conditions I shoot in, such as during golden and blue hours (but surprisingly the same amount of illumination is not enough to show anything in bright sunlight making the situation even more frustrating).
I like grid lines but hate to see them flashing, so I decided to swap out the ground glass in the 1DX with my Ec-D (gridded) screen from the trusty old 1D2. I thought that will cure the problem by avoiding grid line illumination. It wasn’t true success, as the red veil still remains when AF points are lit. Not s smack-you-in-the-face red flash, but still there and still distracting.
Just for comparison’s sake, here is a picture showing how the old-style Canon AF point illumination worked. Just the selected area was lit (actually there was a faint glow in other/unselected AF points, but my iPhone was unable to capture that).
I still prefer the old-style illumination and consider the new system a huge step backward in usability, which plagues otherwise wonderful tools. Hope that Canon will find a better solution in their next generation cameras.