RB67 Film Backs: Light Leaks and the Dark Slide Light Trap
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
Seems like everyday I get multiple questions regarding light leaks on Mamiya RB67 film backs. I am making this post so that I can direct inquiries regarding light leaks in this direction.
In almost all cases, this is due to the light seals deteriorating. I usually recommend a DIY approach on this with pre-cut kits readily available online (not a difficult job, just a bit time consuming and messy, the kits come with instructions) If after replacing the seals you still have a leak, attention would shift to the dark slide light trap.
The dark slide light trap is not a foam seal like the rest of the back uses. It is a velvet strip on a spring plate. This light trap does not go bad from age like the foam seals but degrades from the action of the dark slide moving over its surface.
The kits that you purchase for these backs do not include the dark slide seal, probably because replacing this seal is more involved and you need to disassemble the back a bit to access the seal. So, you will have to make one on your own. The old seal needs to be removed from the spring plate and a new one glued on. I use velvet from a fabric store to make the seal.
Photo two shows a film back undergoing dark slide seal testing. As shown, there is a light leak when my light is directed to the dark slide slot (dark slide removed).
Photo three shows the same test with a new seal installed.
The fourth photo shows the film back with the cover removed exposing the dark slide light seal. The fifth photo shows a bad seal on the left and a new one on the right. You can see how the velvet is worn off the base and this will cause intermittent light leaks.
If you have replaced the seals and you are still having leaks, this is the issue. It will take more time to fix, requires removing 10 screws holding the front plate in place and you will need to construct a light seal.
