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Mamiya RB67 Focus Problem

Wilson Barr , Dec 31, 2009; 09:32 p.m.

Hey guys. I recently purchased a Mamiya RB67 with a 90mm Sekor C lens. I took about 8 rolls of a mix of landscape and street photography. All of the photos that were focused to infinity (primarily landscapes) came back great. They were sharp and clear. However, nearly all of the portraits and close-range (5-10ft) photos came back with the subject not in focus. Instead, often, the background was in focus, even though I DEFINITELY focused on the near subject. If it were just a couple of pictures I would assume I had incorrectly focused or bumped the knob before taking the picture, but almost all of them came back with these results. A few were overexposed slightly, but otherwise I can't figure out what the problem is. Any ideas? I posted an example photo below.

Responses


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Karl Fermedfor , Dec 31, 2009; 10:05 p.m.

Not to insult your intelligence or anything, but there's nothing obvious, like having the back of the bellows impeded from moving in or out by a loose eyepiece cover, right?

That wouldn't exactly explain your situation, though, as that would effect shots from 10+ feet (3+ meters) or farther from the lens as your prime focal point.

Also, as this isn't a 50mm lens, with a floating front element, that wouldn't explain it either.

Wilson Barr , Dec 31, 2009; 10:13 p.m.

Nope, nothing is blocking the bellows from moving in or out. I was looking in the forum and I seem to have a similar problem to this guy: http://photo.net/medium-format-photography-forum/00UpuB

Could it be that I need to calibrate the groundglass? If so, how would I go about doing this? The wya they explained it on the other thread was kind of confusing.

Jim Momary , Jan 01, 2010; 08:14 a.m.

Well, to recalibrate in a nutshell -
1. Take the film back off. Place your frosted glass (scotch tape on a piece of glass, such as a glass slide, etc.) on the film rails exactly where the film would lie (in the same plane). Afix it there with tape or whatever.
2. Take WLF or prism off the camera body.
3. Put the shutter in "Bulb" (always open) or use a locking cable release. Cock and fire the shutter.
4, Focus the lens for a sharp image on your make-believe film.
5. Now, without bumping or jarring the focus (lock it in) you will find a point where if you raise or lower the finder ground glass the image it displays will become sharp. It's normally adjusted with screws (like jack stands) or with shims.
Make sure to use some sort of magnifier to tell when things are sharp, don't rely on just your eyes.
In summary, you must focus the camera to give the best focus on the film plane first. Then, and only then, adjust the ground glass to match.
I'm not sure if it's best to focus to infinity or close up for best results. Apparently close up may be deemed here. Others will chime in.
If it already look finem, then you have other issues.
Jim

paul RON , Jan 01, 2010; 10:38 a.m.

Don't make any adjustments yet. Does this happen with all of your lenses? If it';s only one lens, the lens needs adjustment, not the camera.
Jut to clarify... don't take off the film back, you only remove the insert and place the GG on the rails n rollers the film would ride on. Tape the GG in so you don't have to worry about it shifting.
First check to see your screen is mounted properly and not upside down. Frosting is down. Are you using a split image screen or micro dot type? Textured side is always down.
Make some reference marks on the 4 big screws to align with a small scratch on the spring below each. THis way you ensure you are doing all of em the same and can always go back just as accurately.

With the GG on the film plane and your camera on a tripod, focus the film plane to a well lit target with lots of details like a news paper and use a loupe to get the sharpest image possable.

Now check your screen through whatever you use to normally focus, prism WLF with the diopter in place...etc. Is your screen in focus? If not then remove the prism or WLF and check again to see if the screen is in focus?

If the screen is in focus, you will need a diopter to correct your problem or maybe new reading glasses. If it doesn't make any difference and it is out of focus... lift the screen to see if that makes any difference? If not we will have to lower the screen using the screws a 1/4 turn at a time to each n every one or if rasing corrected it, raise the screws 1/4 turn each.

David McNiven , Jan 01, 2010; 10:42 a.m.

Do not adjust the screen position without first making certain that you can restore it to exactly where it is now. The problem may be elsewhere - trial & error may compound the difficulty of repair. The screen position is fixed securely by the 4 screws you can see in the corners below the screen. The screen itself, not the frame, rests on those screws and they do not move without help.
The mirror also is critical to sharp focus at other than infinity. More than once I have examined cameras in which the mirror has been reversed (turned over) to present a better appearance for sale. This on an RB is a simple job, just 2 screws. If you look carefully at the edge of the mirror you will be able to see whether the silvering is at the front where it should be. Mirror carrier taking position on an RB is unlikely to move without interference.
It is worth checking that the lens capsules are fully screwed into the shutter and that your shutter is not running slow - your subjects (apostrophe, PLEASE!) boots look sharper to me than her face.

paul RON , Jan 01, 2010; 09:44 p.m.

Whatever you do, don't mess with the mirror. That is not your problem. Too many mirrors get broken by people messing with em.

David McNiven , Jan 02, 2010; 05:08 p.m.

Paul, my contribution was not a response to yours, my fingers are much too slow these days to post in 4 minutes.
I suggested that Wilson might "look carefully" at the mirror and for the reason stated. Which unless checked remains a possibility.
Flipped mirrors are not that uncommon and do affect the optical distance to the screen, increasing it.
"That is not your problem" may or may not turn out to be correct but it is a judgement for which I see no evidence as yet.

paul RON , Jan 02, 2010; 06:51 p.m.

David, Sorry. I wasn't worried about what you said, it was all correct n accurate from a repairer POV, it is a possability. I have never seen it done but might be something to look at in those fdeceatful flea-bay cameras deals.

I am more concerned about the inexperianced DIYer starting to mess with a mirror and end up cracking it because they look bent and out of alignment to someone that isn't familiar with the tilt they have and the overwhelming urge to straighten it, beleiving that's the cause of their focus problems.

The most common problem I see in my shop is these old RBs were sold with their original screens. But they may have been readjusted sometime in their life to take a bright screen. The reseller not realizing the difference removed the bright screen n is selling it with the reinstalled original, with out refocusing again. Also if the origianl screen was a fresnel type and a GG is reinstalled, it must be recalibrated again. Best way to check is the GG on the film plane, then make judgements based on the findings.

I'd say let him try the least destructive easiest n most obvious first, then deal with the least common problems later. Perhaps at that point he may have sent it to a shop anyway.

Wilson Barr , Jan 03, 2010; 04:43 a.m.

i just checked my screen and it had been foggy side up, clear side down. The frame that holds the screen only fits in one way, so i couldn't flip it over and try it - I would have to unscrew the glass and flip it over and screw it back in. foggy side should definitely be down? Could this be the reason my focus distance is off?


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