Mamiya RB67 Light Seals

[Barney]

I had noticed a bit of light streaking in the photos and on checking the beck noticed a bit of play in the film back door: thought to myself if its moving that much it cannot be light tight. On inspection the seals looked original and had started to break down in one or two places. I took out the focus screen and the mirror seals were crumbling on the left and almost completely disintegrated on the right.

Time to change them.

New mirror seals




New rear door seals





Not having done this before I am in somewhat of a quandary, Should I use an old expired film to check it or rather just go the whole hog and shoot a roll of good stuff?

 

[Nikon photographer]

Shoot a new roll, if something happens to the film once its processed, you won't know if it's the film or the camera, just try a roll of B&W, as it's cheaper.

 

[droj]

Well I have to say that your workmanship looks excellent - so I'd pile it all in there & just go for it. Trust yourself!

(But I don't understand about the mirror seals - nothing there can affect the focus register, can it?)

 

[Barney]

Well I have to say that your workmanship looks excellent - so I'd pile it all in there & just go for it. Trust yourself!

Thanks for your kind words Droj, its not a difficult procedure at all really.

New film it is! LOL

 

[Barney]

(But I don't understand about the mirror seals - nothing there can affect the focus register, can it?)

I suppose in a correctly functioning lens the leaf shutter is closed when "cocked" therefore the time between removing the dark slide and actually depressing the shutter release there is a potential for light access, before the shutter is released, via the viewfinder. I doubt if the light coming in through the viewfinder would cause streaks, more likely a general fogging or loss of contrast. If there was any massive delay between removing the darkslide and releasing the shutter then undoubtedly light access would be more of a concern resulting in severe fogging or even a blank shot in the roll due to overexposure.

 

[Kevin Allan]

I think that what droj may have been thinking, was that if the mirror seals were too thick, when the focus screen is fitted it may no longer be at the same distance from the lens as the distance from the film plane to the lens, hence what appears to be in focus on the screen may not focus on the plane.

But if you are replacing seals that were already there, and using the same thickness of foam, there should not be a problem.

 

[David_Mug]

When the camera is "cocked" light is prevented from reaching the film by the baffle plate behind the mirror. If you remove the film back and cock the camera you will see it descend, followed by the mirror and seal the mirror box chamber. It has a deep flange around the edge which goes in to an equally deep groove around its seat thus making a light tight closure without need of a foam seal. Light leaking around the viewfinder or mirror cannot get to the film whilst the baffle plate is in position.

 

[Barney]

Good Morning Guys

Thanks for educating me on the workings of the RB67. Having slept on it I think I understand that which you are trying to impart. Kevin, the focus screen sits some 8-10mm above the foam seals when the mirror is in the "up" position.

I have cropped in a bit for greater clarity, the focus screen sits on top of the two stainless (at each side) pins whereas the foam seals locate at the bottom of the pressed housing. The steel is quite thin there and presses into the foam to create the "seal".





David I was not aware of the baffle and now realise that the logic in my previous post has it completely wrong, any light "pollution" from the viewfinder entering the chamber has an effect AFTER the shutter has been depressed and before either 1) the darkslide has been refitted or 2) the shutter release re set to drop the mirror and engage the baffle. Any delay there, such as repositioning for a slightly different view of the scene has the potential to cause degradation of the image. If one considers, when shooting, in the "mirror up" position then the logic used previously is entirely correct with the potential for problems before AND after the shutter button is released. They seem fairly important those seals and in my camera they were completely deteriorated to the extent of ineffective.

Thank you both gents!