Mamiya RB67 Pro S focusing problems (Sekor C 90mm)

Seeing that the RB67 uses a Graflok style back, is it possible to use Graphic film holders or Grafmatic 23 for single shot testing?

choiliefan said:

Seeing that the RB67 uses a Graflok style back, is it possible to use Graphic film holders or Grafmatic 23 for single shot testing?

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You can just cut a piece of film and tape it into the back - cut it long enough to wrap around the film guides.

I do this often when I need just one shot (like, I'll make a print of clouds and shoot it to have a neg to mask into a print with a blank sky). I have a roll of acros that's unrolled in an old 5x7 paper bag that I trim off bits as needed.

And, in my opinion, for things like focus testing, get a pack of 4x5 orth-litho film from freestyle for like ten bucks. You can cut it and handle it and develop it under safe lights, and I just use paper developer - it develops to completion in about a minute in paper-strength liquidol or dektol. It has a very low ISO but it's cheap and fast to work with (and you can start making unsharp masks, which don't require precision registration with 6x7 negs, there's a bonus!) I use it for all sorts of camera testing, esp. since I've been playing with various flipped-lens ideas, I can suss out the focus distance of non-SLR cameras in a few minutes. it's also great for finding light leaks, tape a sheet inside the camera, mark what side is up with a sharpie, take the camera out in the sun for a few minutes.

The film surface when contacting rollers is 0.197 +/- 0.007 inch from the contact face of the rollfilm back.

Similar to methods shown by b2546 and M Carter,
this is how I check the rollfilm backs, (shown on the Speed but could also be done on the RB67)

https://app.box.com/s/bdg1k4tss2tl0zlrs292wlwl5za6yu20

Cut a flat square of stiff transparent plastic ( shirt box etc) to fit in as shown
Put some magic tape on camera side of plastic instead of ground glass.

press it against the rollers and tape in place temporarily.
Now the focus can be checked directly, using a loupe or a magnifying headset.

Tried baking paper, didn't work well, and then putting the focusing screen on the back like here:
https://ibb.co/9vHPMW1
https://ibb.co/k0DnJyV

Tightened the screws a bit but there's still a small difference, I think. Might try your technique next @wombat2go

wombat2go said:

https://app.box.com/s/bdg1k4tss2tl0zlrs292wlwl5za6yu20
Put some magic tape on camera side of plastic instead of ground glass.

Click to expand...

So, on the inside? The side that's in the camera or on the outside?

benoitvanhecke said:

So, on the inside? The side that's in the camera or on the outside?

Click to expand...

The magic tape is on the lens side of the plastic rectangle . The magic tape is contacting the pin rolls of the rollfilm back as shown in the photo
The image is viewed from the rear side, through the clear plastic rectangle.
To see the full image, you can completely cover the rectangle with strips of magic tape. (I was only looking at the corner sharpness in the one in the photo)

I recommend at CLA [Clean, Lubricate and Adjust] by a repairman who can put you camera in a jib to adjust and verify that the camera is square and that the shape meets the manufacturer's specification. That will cure a lot of problems with one stop.

As an alternative to ground glass, I bought a cheap photoframe with glass in it and cut the glass to the width of 120 film (needs to be wider than the image to allow contact with the film rails). You can use a cheap glass cutter for this, score the surface a few times and then snap on a couple of matchsticks or a ruler. Then cover one side with strips of non-overlapping scotch tape. Mount this with the tape towards the lens in your film back. Focus on something relatively close using the waist level finder (with magnifier flipped up) and then check on the glass in the film back that the focus is the same - best to use a loupe, or if none available, a 50mm lens for 35mm cameras works well.. Try it a few times at a few different distances. Another check is to lay out a tape measure on a table, aligned towards the camera and focus at a wide aperture on say the 1m mark. When you get the film developed it should be crisp at the 1m mark. If it's sharper at the 0.95 m mark or the 1.05 m mark, you know you're out. Take a number of frames as focus will vary a little each time but the error will be consistently in one direction.

wombat2go said:

The magic tape is on the lens side of the plastic rectangle . The magic tape is contacting the pin rolls of the rollfilm back as shown in the photo
The image is viewed from the rear side, through the clear plastic rectangle.
To see the full image, you can completely cover the rectangle with strips of magic tape. (I was only looking at the corner sharpness in the one in the photo)

Click to expand...

Darn it, I was testing with the tape on the other side and it was looking pretty well.
Until I turned it around, of course.
So I still have backfocus, and I can't screw the screws in any more than I have with the tools at my disposal. (which are wooden sticks, really... metal pins were bending)
What now? :/

Have you absolutely ruled out a reversed focus screen?

choiliefan said:

Have you absolutely ruled out a reversed focus screen?

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Yes, everything looks to be the right side up.

It’s useful to get an idea of the magnitude of adjustment required.

In the second photo of post #1 we see a group of four men standing in front of some bushes and a building. Based on the approximate horizontal span of an average man facing a camera, it appears that the scene spans about 2.3 meters horizontally in the plane of the men’s faces (assumed intended plane of focus).

From that and the use of a 90 mm lens on the RB67, calculation shows that the photo was shot from a subject distance of about 3.1 meters (subject plane to first nodal point of the lens). These numbers are only an estimate, but give us some idea of the distances involved.

The faces are assumed to be the intended plane of focus. The actual focus appears to be about a meter deeper, or about 4.1 meters. That corresponds to the focus screen positioned too high by about 0.65 mm. If so, that’s a gross error.

The focus error on photo #1 shot from a much closer distance seems less severe. This makes me think that you’re setting the focus somewhat inconsistently from shot to shot. But that doesn’t account for the gross focusing error shown in your photos. Based on the photos you’ve shown it seems likely that the focus screen is positioned too high in the camera.

Continuing to use it as is will give you spoiled photos. Trying to fix it yourself has apparently yielded no cure. The situation calls for analysis by an experienced camera technician.

Thanks for you assessment, @Ian C !
I'll try again tonight to get it right myself.
If I don't succeed again, I will have to get it looked at by a technician, though I haven't found one yet in my area. (Ghent, Belgium)

Hello benoitvanhecke. I think I arrive late to this issue, but I have the same problem, how did you fix it? Or didn’t had solution? Leave some pics of the focusing problem in my mamiya.

Thank you for your help

https://drive.google.com/file/d/15deB4wiDhSIFJjCu1bYSeCLTN_UkDoh9/view?usp=drivesdk