Manolo, this is going to be REALLY HARD to believe! Some people cannot remove the Viso-Flex from their cameras! And people ACTUALLY pay money for these un-removable viso-flexes. Fortunately, you can use an accessory RF in the accessory-shoe and then set the distance to the lens using focus scale.
One of my "Stuck Viso-flex's" lets me at least lock the mirror up and use with an accessory viewfinder. It came with the 21mm lens.