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CNC Machinist learning with medium format camera build

jaedog

New member
Hi everyone! This is my first post on this forum and any forum for anything ever at all, so I hope I'm going about this properly. I'm starting my very first camera build project as gift for my significant other this holiday season, and as I'm learning more and more through the research I can conduct on my own through the internet I'm finding myself with more and more questions that cant be answered by cut and dry videos and articles so I thought I would take the more interactive route and try to make a forum post! I am building a medium format camera based off of the Goodman Zone which uses a Mamiya RB67 Pro S film back and a Mamiya 65mm lens. My plan is to build an aluminum box body that connects these components using my capabilities in CNC machining. The question I have right now is how to determine the proper distance to space the lens from the film, and as far as I know this is the only critical geometry of the body besides of course the mounting for the lens and back. I was excited to learn this number had a name and is referred to as the Focal Flange Distance, but I still don't know how to determine what that distance is. Any advice is appreciated, I've got almost 8 years of experience building all sorts of things but I'm realizing quickly that a camera is a totally different creature and I am in way over my head. Will definitely be sharing media and documentation of the build if it all pans out.

Thanks if you've read to this point and I hope you're all well!
 

Audii-Dudii

Active member
The question I have right now is how to determine the proper distance to space the lens from the film, and as far as I know this is the only critical geometry of the body besides of course the mounting for the lens and back. I was excited to learn this number had a name and is referred to as the Focal Flange Distance, but I still don't know how to determine what that distance is.
The FFD is measured from the front-facing mouniting surface of the lens bayonet to the front-facing surface of the film or sensor. A quick Google search turns up two measurements for the RB67 series of lenses: 111 mm and 112 mm. (Assuming, of course, that you plan to use an RB67 lens instead of a 65 mm lens from another Mamiya camera series.)

Based on my experience tinkering with custom cameras, I suggest you design your lens mount around an FFD that is very slightly short of whatever distance you determine to be the correct one and once the camera is finished enough to be operational, shim it to the optimal FFD based on your observation about where the actual lens and mount setup focuses at infinity relative to the distance indicated on the focus ring. This is because the optimal FFD for any given lens may vary slightly from copy to copy and it's generally better to avoid adjusting the lens instead of the lens mount.

This is particularly critical if the lens you use contains any floating elements, as the indicated and actual focus distances much match for the lens to perform optimally; if not, then in most cases, it doesn't matter much, if at all, provided the focusing helical of the lens you're using has enough travel to focus the lens across the distance range you desire.
 

jaedog

New member
Hi!

Thank you for such quick and detailed responses! I have a link for the lens I purchased off of Ebay here, using the table Valkyrie shared it looks like the FFD/Register I need for my combination of lens and film back is 43.5mm right? Is it a problem that the lens I've purchased is designed for the Mamiya Universal Press and not the RB67 line of cameras?

Thanks again and I can't wait to get into the design of the camera body. I will definitely share updates with photos as the project progresses.
 

f8orbust

Active member
If you look at an RB67 Pro S body you can see that the flange distance is waaaaay more than 43.5mm.

In this PDF from Mamiya (link), on page 2 (section 1) it states that:

Furthermore, the flange back (distance of the lens mount to film plane) has been reduced by the same amount (from 111 mm on the RB to 104mm on the RZ)

So, I'd work with 111mm. It's pretty big, but then the Pro S is basically a 'box'.

Since film isn't really 'flat' it's far more forgiving than a digital sensor if the flange distance is off a teeny bit - so making it shimmable (say +/- 1mm or so) is a good idea to futureproof the design in case it's used with a digital back in the future.
 

TimoK

Active member
Nope. The flange focal distance is depending on the lens, not the film back. So, the FFD for Mamiya Press lenses is less than 111mm but not 43,5 mm. Mamiya Press body is thinner than RB67. That 43,5 mm in the link's table is for Mamiya 35mm slr.
 
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f8orbust

Active member
I guess the question is, when the OP says a ' Mamiya 65mm lens' - which Mamiya 65mm lens is he referring to? If it's not one designed to fit the RB (which, thinking about it, I guess it isn't or you'd just use an RB), then which one is it? I just assumed it was.
 

jaedog

New member
I was able to actually get my hands on the 3D file for the Goodman Zone body and I measured the distance marked in red to be 58.4mm. I also found this post from another forum
confirming a number around 53-58? Does that sound about right? I'm not sure exactly which one they mean to be the FFD (53 or 58) but It's encouraging that it's closer to the measured value!

If the FFD is specific to the lens, any ideas where I could look to find an accurate number for my specific lens? It doesn't seem to be represented on the table based off of the information from TimoK. The information from the other forum is encouraging but I'd like to find a way to confirm from multiple resources if possible.

I plan to iterate the design of the camera body most likely with 3D printing and in the end I will machine the final aluminum body with the perfect FFD. Is there a good process to test for the lens' FFD once it arrives? That way I could perfect it if the number I determine now is off.
 

MartinN

Active member
Without a ground glass you have to measure the camera to target distances accurately.
And maybe you develop many films :rolleyes:

And as little stopping down the lens as possible.
 
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jaedog

New member
Thanks a ton f8! That info is just what I was looking for. The poster in the link you sent seems to have my exact lens and the discussion between him and Nick looks like it settles the question of FFD for good. I think the next steps for me are to wait for the RB67 back and 65mm lens to come in and then I can dig in to the design of the body. If anyone has CAD resources for the mounting geometry of the lens bayonet system or the RB67 back that would help a lot, otherwise I think I can make do with careful measurements on my dial calipers. I'll be sure to share photos and possibly some nicer looking renders of the 3D model as it progresses!

Thanks again to everyone here this is an awesome community!
 

Shashin

Well-known member
If you want to judge your tolerances, simply calculate the depth of focus (not depth of field) for the lens and format. Simply multiply the permissible circle of confusion by the f-number (maximum aperture) and then double it (you double it because depth of focus is both in front and behind the film plane (if you don't double it, then it is +/-). The permissible circle of confusion is the largest a point can be rendered and still appear sharp in an printed image. One common formula is to divide the film format diagonal by 1,500. You can adjust that value if you want tighter tolerances.

We won't get into film flatness and how that effects things.

Looking forward to seeing the results. It is an exciting project.
 

TimoK

Active member
I have used some times Mamiya Press and Universal with 65mm lens. But it was 40+ years ago! But I did not own those. That time I also had my RB67. ( If my memory serves)
There is Graflock 23 back in both RB and Press G or Universal. So you can use RB rollfilm backs on those.
Some information about Graflock 23 and Mamiya Press , here more . Read the Appenix #3 in the first link!
 
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jaedog

New member
Thanks for the clue to Appendix 3 Timok! When I added 4.85 listed in the article to the 54.11mm I measured in CAD (using a different reference point than one of my earlier posts based off the new information shared by f8) it comes out to be 58.96 which is plenty close enough to the 60mm suggested in f8's link for me to feel confident moving forward with that number. Lens arrived today and the film back yesterday! Hopefully will get an opportunity to get to work on the design and share a photo update later tonight.
 

jaedog

New member
Quick update: Lens and back have both arrived! Spent some time playing with them both and printing the Goodman Zone body just to get an idea of how their tolerances feel on the camera components. Got to work on the model a little bit, its unidentifiable at this point but in the computer image there are the two "rails" ill be using to mount the main camera body to the RB67 film back. I promise it'll make sense in the end, I'm going to have to do a lot of tricky things with the design to make this practical for machining out of aluminum. Here are the photos I have from tonight.
 

Attachments

TimoK

Active member
Can you open those rails to unmount the back? I ask because the next thing you'll need is some kind of ground glass to focus your camera. You have to trade the film back to ground glass, focus the picture and then set the film back back again to expose the film.
In the original Graflock there are sliding steel plates to keep those at the place. Look at pictures Here If You look at the Mercury back I believe You'll understand how it works when sliding those things.
 

jaedog

New member
Thanks again Tim, I wasn't even thinking about that. With that design I was planning to lock the back in once the bolts were tightened but obviously that wont fly. I took a look at the graflok mechanism and I think I can imitate it. I will share another update once the CAD has some more meat on it.
 

Godfrey

Well-known member
Hi!

Thank you for such quick and detailed responses! I have a link for the lens I purchased off of Ebay here, using the table Valkyrie shared it looks like the FFD/Register I need for my combination of lens and film back is 43.5mm right? Is it a problem that the lens I've purchased is designed for the Mamiya Universal Press and not the RB67 line of cameras?

Thanks again and I can't wait to get into the design of the camera body. I will definitely share updates with photos as the project progresses.
I have a Mamiya Press 23 Super and the Mamiya 50mm f/6.3 lens, configured at present with ground glass back.



I just measured the lens mount flange to film plane focal distance for you: This is the distance from the lens mount seating surface to the ground glass. That distance is 61.4mm, which would be exactly the same for all lenses compatible with the Mamiya Press 23 lens mount regardless of which film back you use.

G
 
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