Reason to do it: Crud visible in 5D viewfinder that won't otherwise clear with a blower on bottom side of focusing screen

Tools required: philips screwdriver (jewelers type), flat bladed screwdriver (jewelers type), angled forceps that have taped edges to prevent damage to parts. A small stainless steel pick (use screwdriver if you have no pick) 1 can of "dust off" or rocket blower.

Setup: POWER OFF, remove lens, and place camera flat on table lens mount up, lens cover removed.

STEP 1
Spring the small latch in top center (upward, gently) this will release the focusing screen. Don't worry - the cage will spring open and fall towards the mirror but it cannot contact the mirror.

Image 1 - Focusing Screen Release Latch

STEP 2
CAREFULLY remove the focusing screen with the forceps - try to grab it by the longer tab located on the right side of the plastic screen. Its OK to grab the screen elsewhere, just make sure that you do it lightly and with taped forceps! WARNING - remove screen and do not let it touch anything inside the camera! Set the screen aside on a dirt free surface and NOTE HOW IT CAME OUT!!! Notice that the large tab goes to the right and that convex surface (curved outward) goes down toward the mirror.

Image 2 - removing focusing screen
                 

STEP 3
Remove the two small philips screws (arrows) that hold the shim and plate retainer in place. WARNING - do not drop the screws inside the camera and don't lose them either!! Set screws aside in safe place. Using the forceps, carefully remove the metal bracket (part that has the foam bumpers on it) set bracket aside noting how it was installed.

Image 3 - Screw location and bracket

Step 4
While carefully lifting the left hand tab (#3) use the pick or small flat bladed screwdriver to spring the stainless steel shims (#2) from the plate carrier. There may be up to 4 shims, simply let them flop into the cage for the focusing screen.

Image 4 - location of tabs, shims and plate for steps 4 and 5

Step 5
This time, we will lift the right hand tab (#4) and this will allow the release of the Superimposed Indicator Plate, its a piece of optical coated plastic that also has the small cutouts for the AF points (#1) With the tab lifted, carefully pry it out from its edge. You mayhave to lift the other tab (#3) as well. Once free, carefully remove it with the forceps, again -try to use the tabs on the edge, rather than grabbing it on the surfaces. A note about this part - it is very easily scratched - DONT TOUCH IT with cloth or lens pens!! TAKE NOTE how it came out.. it will need to go back in the SAME. Carefully set it aside on a piece of microfiber cloth or other very soft/ lint free surface.

Image 5 - Superimposed Indicator Plate with AF points
                                      

Image 6 - Parts removed from Camera and set aside
     

Please note - DO NOT MOVE THE CAMERA - remember the shims are just sitting in the tray that holds the focusing screen.

Step 6- Using the dust off or rocket blower LIGHTLY, impinge the bottom of the pentaprism and area around the prism, this should blow away any dust or crud that has accumulated there. Don't worry- if any gets into the camera we will take care of that later. DO NOT use a large blast from the can !!

Step 7- firmly grasp the Superimposed Indicator plate from the edges and give both sides a hard blast from the dust off or rocket. Be very careful not to drop the plate - it is very, very easily scratched (don't ask me how I know that). Do not wipe it with lens cloth or be tempted to run a lens pen over it - you will scratch it! Air is all that it needs- ditto for the prism - DON'T TOUCH IT!

Step 8- Carefully place the Plate back in place and using the forceps, snap it into place (you might have to lift both tabs slightly to facilitate this. With plate in place - use the pick or flat bladed screwdriver to push the shims back in place - again - you might have to lift the left hand (#3) tab a bit to get the shims back in but they go in easily. Make sure everything is aligned and you will be OK.

Step 9- Repeat the blowing process on the focusing screen (again - don't touch it with a cloth or lens pen!) Re-install the focusing screen in its place and snap the cage shut over it. If for some reason the cage does not close easily.. DONT FORCE IT - most likely something is not fully in place- re-check and reposition the other items until the cage shuts easily!

Step 10- Carefully install the metal bracket with the forceps and re-install the 2 screws.

Step 11 - once everything is in place and back to normal, hold the camera upside down (sensor down) and use the rocket to blow any debris or dust off the mirror. Put the camera in sensor cleaning mode and repeat on the sensor. Turn camera off and one last time - use the blower on the mirror and sensor box to remove any lingering debris. Avoid blowing anywhere near the focusing screen !!

Step 12 - install the lens and enjoy a lot less dust and crud than you had before.
Final comments:
If you scratch the Superimposing Indicator Plate- the can be ordered from Canon - they are $17 + shipping, part # YN2-3549

Avoid the problem again by following a few simple precautions - 1> A little dust in the viewfinder is normal 2> Unless your images have dust in them DONT ever get the blower near your mirror or sensor box. ONLY use a rocket (i.e. hand type) and avoid direct blasts anywhere near the edges of the focusing screen.

Credits: reposted from Mark Kilian.

Links

Testing the Brightscreen Focus Screen in the 5D