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

[benoitvanhecke]

Hi everyone

So I just got back my first scans, and it seems whenever I used a lower f-stop like 3.8 or 5.6, my focus lies farther in the frame than what I focused on. Or it's just blurry altogether. No problem with smaller apertures like f/11 or f/16.

Here are the worst ones:
https://ibb.co/3yx50Wd
https://ibb.co/jWYTBht
https://ibb.co/r0F8h2f
https://ibb.co/H7sqxQT
https://ibb.co/Tk6mKdP
https://ibb.co/48hgTsB

Back seems to be okay, viewfinder/glass and everything as well and I'm preeetty sure I'm focusing perfectly. Not sure about the mirror. Or perhaps I'm not looking at the right things? Anyone know what might cause this difference of DOF in VF and negative? Lens is a Sekor C 90mm.

Here are some pictures of the mirror down:
https://ibb.co/7Y995cs
https://ibb.co/kcChxgQ
https://ibb.co/qjwZ5tr

Thanks so much in advance.

 

[Neil Grant]

Hi everyone

So I just got back my first scans, and it seems whenever I used a lower f-stop like 3.8 or 5.6, my focus lies farther in the frame than what I focused on. Or it's just blurry altogether. No problem with smaller apertures like f/11 or f/16.

Here are the worst ones:
https://ibb.co/3yx50Wd
https://ibb.co/jWYTBht
https://ibb.co/r0F8h2f
https://ibb.co/H7sqxQT
https://ibb.co/Tk6mKdP
https://ibb.co/48hgTsB

Back seems to be okay, viewfinder/glass and everything as well and I'm preeetty sure I'm focusing perfectly. Not sure about the mirror. Or perhaps I'm not looking at the right things? Anyone know what might cause this difference of DOF in VF and negative? Lens is a Sekor C 90mm.

Here are some pictures of the mirror down:
https://ibb.co/7Y995cs
https://ibb.co/kcChxgQ
https://ibb.co/qjwZ5tr

Thanks so much in advance.

not sure that, in general, RB's pick up focus errors like other mf cameras. I believe the mirror has a 'hard stop' unlike others that have a cushion of foam that can deteriorate over time. You dont show us the focusing screen - is it a standard one or 3rd party? We must presume that the camera is basically sound (ie the bellows isn't knocked out of alignment) and that the revolving adaptor/back and focus screen is seated properly. Is your eyesight OK for the viewfinder? Get a friend to try focusing it - do you get the same result? Can you try focusing at the film plane by removing the back and sustituting a piece of frosted glass?? The vf focus and fp focus must agree.

 

[benoitvanhecke]

not sure that, in general, RB's pick up focus errors like other mf cameras. I believe the mirror has a 'hard stop' unlike others that have a cushion of foam that can deteriorate over time. You dont show us the focusing screen - is it a standard one or 3rd party? We must presume that the camera is basically sound (ie the bellows isn't knocked out of alignment) and that the revolving adaptor/back and focus screen is seated properly. Is your eyesight OK for the viewfinder? Get a friend to try focusing it - do you get the same result? Can you try focusing at the film plane by removing the back and sustituting a piece of frosted glass?? The vf focus and fp focus must agree.

Hi Neil, thanks for trying to help me!

Please find pictures of the focusing screen here:
https://ibb.co/0y08M4x
https://ibb.co/Jcd9mPj
https://ibb.co/mG0ffpv

Eyesight is fine and a friend photographer tried as well, had the same issue as me.
Not really sure what you mean with your last line regarding the frosted glass, also I don't have any. :/

 

[b2545]

Hi Benoit, greetings from a fellow Belgian RB67 user!

From the images of your focusing screen, I get the impression that the glass is mounted with the wrong side up. I can check by comparing with my own camera at home. I believe such a situation could very well cause the issues you observe.

Bien à toi,
Morel.

 

[Ian C]

1. The first thing to check on an RB67 or RZ67 is the proper seating of the focusing screen where it mounts on the top of the camera. If the screen is improperly seated, then what the viewfinder shows and what the lens projects to the film will be slightly different. This is the most likely cause of the problem.

2. Another thing to check is the correct functioning of the pressure plate in the film back. If it isn’t holding the film in the correct position, then the film might be somewhat displaced from the image plane. That would result in a fuzzy image recorded on the film.

3. If the mirror mechanism is badly worn, it might cause you to set the focus slightly off. While this might be possible, it’s not likely.

Using a smaller aperture—larger f-number—makes the image appear in better focus due to the increased depth of field about the subject This argues against a pressure plate problem, since stopping down the aperture would give only a slight increase of the depth of focus about the image plane. The image recorded on film would still be somewhat fuzzy, even after stopping down the aperture. The greatest likelihood is a mispositioned focusing screen.

It is useful to do a point-of-focus check to quantify the error (if this is simply a focus error). Photo books used to show example photos of a picket or wrought-iron fence photographed at, say, 30° from the fence line using the largest aperture. In one book this was illustrated with photos showing a hat on one of the fence pickets. The camera was mounted on a tripod and the photographer focused carefully on the picket on which the hat sat. The distance from the lens to the target should be relatively close for good sensitivity, say, 2 – 2.5 meters.

The photo was taken and analyzed. Ideally, the target picket was crisply rendered with rapid falloff in focus at closer pickets and a more gradual falloff at farther pickets. This is a good test to see whether the viewfinder and lens focus are synchronized or not. If they are not, then you can see where the actual focus lies and take steps in correcting it.

Another thing to consider is the manner in which you use the camera when you do the above test. The camera should be on a sturdy tripod and it’s a good idea to use the auxiliary shutter release on the side of the lens. After cocking the shutter you press the main release on the front of the camera to fire the mirror only, wait a few seconds for any mirror vibration to fully dampen, and then fire the shutter for a no-vibration shutter release. This will give you the crispest possible image for analysis.

 

[benoitvanhecke]

From the images of your focusing screen, I get the impression that the glass is mounted with the wrong side up.

Hey Morel, merci à toi! What makes you think that, because I was under the impression it was mounted with the correct side up? On the other side, the dotted lines are foggy. Flipping it over, it cannot be seated properly in the top. So the glass itself is wrongly mounted in the fitting frame according to you? Not at home either right now, I'll be able to send more pics of the screen tonight, these were screenshots of a video I made yesterday.

 

[benoitvanhecke]

1. The first thing to check on an RB67 or RZ67 is the proper seating of the focusing screen where it mounts on the top of the camera. If the screen is improperly seated, then what the viewfinder shows and what the lens projects to the film will be slightly different. This is the most likely cause of the problem.

2. Another thing to check is the correct functioning of the pressure plate in the film back. If it isn’t holding the film in the correct position, then the film might be somewhat displaced from the image plane. That would result in a fuzzy image recorded on the film.

3. If the mirror mechanism is badly worn, it might cause you to set the focus slightly off. While this might be possible, it’s not likely.

Thanks for your input, Ian!

1. Can you tell by the pictures I provided if it seems to be seated improperly?
2. Thought of that as well, though I just put in a new roll of film yesterday so I can't check it... Haven't taken a picture yet, can I take the roll out without having to shoot the whole roll?
3. That's reassuring, since sending it abroad to have it fixed is not something I'm eager to do.

 

[b2545]

Yes, perhaps glass mounted wrong in the fitting frame... A priori, no need for more pics. The dotted lines are clear enough on your images. If they are equally clear on my RB, then the problem is likely elsewhere.

Regarding Neil's reply: Frosted Glass -> 'Verre dépoli'. When placed against the Revolving Back adapter, in place of a film magazine, the plane of the frosted side should concide with the normal film plane. In such a setup, focus on the viewfinder an as it appears on the ground glass should match.

 

[Ian C]

Regarding your question at the end of post #7

1. From the photos, it appears that the focus screen is properly mounted. What others have asked relates to the orientation of the screen in its frame. I believe that they were suggesting the possibility that a previous owner might have disassembled the screen and reassembled it into the frame incorrectly. This is likely not the case, but should be verified.

I think that doing the point-of-focus test as outlined in post #5 should be your next step. It will give you a definite answer about whether or not there is a focusing mismatch between the viewfinder and the on-film image. If there is an error, it can be quantified, and the problem addressed.

2. With 120 roll film it’s not practical to remove the film until its fully exposed.

 

[benoitvanhecke]

Regarding Neil's reply: Frosted Glass -> 'Verre dépoli'. When placed against the Revolving Back adapter, in place of a film magazine, the plane of the frosted side should concide with the normal film plane. In such a setup, focus on the viewfinder an as it appears on the ground glass should match.

Morel, do you have the things and knowledge necessary to perform this test? And maybe other tests if need be? I could pay you a quick visit this weekend, if you were open to that... I'm from Ghent.

 

[b2545]

Knowledge, yes, mostly. The ground glass part I can likely improvise. A side-by side comparison with a working RB67 is highly likely to help reveal any issues. Earlyish sunday works for me.

 

[b2545]

Having examined my own RB67 Pro S, the viewfinder glass is in its correct position in your pictures, Benoit. (Leastwise, mine matches yours and I have never observed the focus issue you describe.)

The problem is then likely elsewhere. IanC's points remain the best course of investigation, in the same order he presents them.

 

[cramej]

The focusing screen is adjustable on the RB. You need to use a ground glass-like surface at the film plane and adjust the screws on the focusing screen until they both agree.

 

[benoitvanhecke]

The problem is then likely elsewhere. IanC's points remain the best course of investigation, in the same order he presents them.

Then I'll have to develop a roll of film to test this? This isn't possible to do together then?
But the ground glass test is? Multiple people have suggested this on different platforms, shall we try that?

 

[b2545]

@cramej: I was unaware of that possibility. Can you indicate precisely which screws you mean? If it helps, I have identified three candidates, circled in the various colors in the images here:

https://ibb.co/sqvpsy3
https://ibb.co/m6PnSdV

 

[b2545]

Then I'll have to develop a roll of film to test this? This isn't possible to do together then?
But the ground glass test is? Multiple people have suggested this on different platforms, shall we try that?

Checking without developing film is the quickest, I would think. That does mean with ground glass at the film plane, providing immediate feedback as to the effect of adjustments.

Without either ground glass or film development tests, comparing side-by-side against a camera of the same model, itself in proper working condition, can also provide important insights.

 

[cramej]

@cramej: I was unaware of that possibility. Can you indicate precisely which screws you mean? If it helps, I have identified three candidates, circled in the various colors in the images here:

https://ibb.co/sqvpsy3
https://ibb.co/m6PnSdV

The one circled in blue is correct.

See here for more details: https://www.photrio.com/forum/posts/?post_id=1403382

By looking at your sample photos, you would need to lower the screen.

I had to do this to my RB. At that time, I did not have a loupe and now that I do, I think I need to check this adjustment on mine again.

 

[b2545]

The one circled in blue is correct.

See here for more details: https://www.photrio.com/forum/posts/?post_id=1403382

By looking at your sample photos, you would need to lower the screen.

I had to do this to my RB. At that time, I did not have a loupe and now that I do, I think I need to check this adjustment on mine again.

Thanks @cramej!

I feared it would be the blue one. I don't have a two-pronged screwdriver.

 

[b2545]

Crudely, the verification procedure being suggested is this (substituting ground glass for the paper in the photos, and using a loupe for accuracy):




If it is indeed the focusing screen seating which needs adjustment, the screws in question are the ones circled in blue here:

There are four of them, one at each corner.

To make ground glass, I have found the following to work well enough: Starting with piece of clear glass (about 6.5 cm by 7.5 cm in this case)... grind it evenly rough on one side with abrasive powder (protect the other with tape).. The best is to grind one piece of glass against another (larger) with abrasive powder (and a few drops of water) in between. Wash thoroughly with plenty of water, and then remove the protective tape. When finished, the glass should go in place of the film in the camera back, rough side towards the lens.

 

[cramej]

Two mechanical pencils with metal tips can work in a pinch. Insert them at an angle in opposing directions and push - very carefully. It's like trying to pick up a grain of rice with a chopstick in each hand.

 

[b2545]

Two mechanical pencils with metal tips can work in a pinch. Insert them at an angle in opposing directions and push - very carefully. It's like trying to pick up a grain of rice with a chopstick in each hand.

Ok, so perhaps two pieces of stiff wire of the appropriate guage, taped firmly to a piece of wood of rectanglar section and thickness matching the hole-to-hole distance would provide more assurance?

 

[Pieter12]

Something like this might work, it looks like the tips can get close enough together. The tool itself is rather large, maybe 8x10cm.
https://smile.amazon.com/Neewer-Pro...rds=lens+tool&qid=1562887921&s=gateway&sr=8-2

 

[benoitvanhecke]

By looking at your sample photos, you would need to lower the screen.

Are you sure? Because it doesn't look like it can be lowered anymore to me, but you have more experience, could you check these photos please?
https://ibb.co/5TwxBWf
https://ibb.co/Cn5WNbp

I don't have a cable release, how can I open the shutter with the mirror up? Tried it with the knob on the left side of the lens, but it doesn't work...

 

[cramej]

I could be wrong on which way the screen needs to go...

Just need the shutter on T. Check focus on the screen for a particular point of focus, trip shutter, check back.

 

[John Koehrer]

The amount of adjustment isn't very large, you're looking at fractions of a turn, maybe 1mm or less.
It's very possible you wouldn't be aware of the screen actually moving.