Mamiya Press Back Tolerances

[JChrome]

Bonjour Amigos y Amigas!

I have a Mamiya Press with two different backs. I got a sneaking suspicion that the sharpness would be different between them. So I decided to put it to an extreme test.

1. I mounted the camera to a tripod.
2. Mounted the 150mm lens and focused it as close as it would go.
3. Mounted three macro rings between the lens and the body. I need that DoF to be *tiny*.
4. Set up a tape measure going at a 45 degree angle on a table.
5. Shoot with the first back. Then change the back and shoot with the second (without change the lens, focus or camera).

Here is a close up of the results:

Notice the rightmost corner of the '5' is not so sharp in the image on the left. But it gets sharper in the image on the right.

Now, in the grand scale of things, I won't be shooting like this very often and it may not have such an impact here. However, I know little about optics or optical theory. But I'm wondering how this impact would scale to other scenarios.

Let's say the focus is off by 1/16 of an inch here (the tape measure is in inches). The whole DoF is maybe 3/16th inch or so. Since the focus is off by 1/16th inch, that's 30% of the DoF, does that mean if the DoF grows to 6 inches, the focus will be off by 2 inches? Does it scale in that fashion or am I incorrect in that?

If the focus remains off by 1/16th of an inch and I am shooting with a DoF of 2 feet, then I obviously have no problem but if it's the former, then it may be an issue.

Thanks for the help and Happy Memorial Day (for those in the USA).

 

[charjohncarter]

I really don't think the Super 23 or the Mamiya Press were designed to be a macro camera. Although the Super 23 has the option to collapse the 100mm lens and extend the bellows back it still in not macro. So as you say the grand scale of things, maybe you should have bought a different camera. I find my Super 23 a difficult camera because of the size and other ergonomic problems, but when I get that 6x9 negative under normal not weirdo use it is so great.

 

[farlymac]

If you increase the depth of field, it's not going to change the amount of focus differential between the two backs, you will still have a 1/16th inch difference. So the greater the DOF, the smaller the percentage of 1/16 is to the field, as in 1/16 to the foot, which is 0.0625. Besides, you can't be sure just changing the backs changed the focus unless the whole rig was mounted to an optical bench. It's likely well within the factory specs anyway.

PF

 

[JChrome]

I really don't think the Super 23 or the Mamiya Press were designed to be a macro camera. Although the Super 23 has the option to collapse the 100mm lens and extend the bellows back it still in not macro. So as you say the grand scale of things, maybe you should have bought a different camera. I find my Super 23 a difficult camera because of the size and other ergonomic problems, but when I get that 6x9 negative under normal not weirdo use it is so great.

Definitely agree about it not being a great macro setup. Although I can use the ground glass prior to check the focus. The lenses are not modern, so color photography isn't as appealing. BnW macro isn't so much in my world.

In this experiment, I'm less interested in the macro capabilities as I am just in judging the focal differences between two film backs.

 

[JChrome]

If you increase the depth of field, it's not going to change the amount of focus differential between the two backs, you will still have a 1/16th inch difference. So the greater the DOF, the smaller the percentage of 1/16 is to the field, as in 1/16 to the foot, which is 0.0625. Besides, you can't be sure just changing the backs changed the focus unless the whole rig was mounted to an optical bench. It's likely well within the factory specs anyway.

PF

Muy bueno! Thanks for the note. Yea, I suspected as much, I was just concerned the focus was off moreso. You are correct about it not being bolted to an optical bench.

 

[Spanik]

AFAIK adding macro rings won't change the dof as that is only determined by the lens. But apart from that I wouldn't expect a Mamiya press to have those tolerances either. If focus is that critical, use the ground glass and hope you don't disturb anything when changing to the back.

 

[JChrome]

AFAIK adding macro rings won't change the dof as that is only determined by the lens.

I would respectfully dispute this 🙂.

If you are shooting a subject 5 feet away and put on macro rings and focus on that subject at 5 feet away, only then is the DoF the same in both scenarios (because the distance between the lens and the film plane are the same).

But if I put on macro rings and focus on a subject 3 inches away then the DoF is much narrower. Couple that with f5.6 and you get 3/16th of an inch of DoF (something I can't achieve by shooting at 5 feet distance).



www.stillthrill.com

 

[oftheherd]

If you increase the depth of field, it's not going to change the amount of focus differential between the two backs, you will still have a 1/16th inch difference. So the greater the DOF, the smaller the percentage of 1/16 is to the field, as in 1/16 to the foot, which is 0.0625. Besides, you can't be sure just changing the backs changed the focus unless the whole rig was mounted to an optical bench. It's likely well within the factory specs anyway.

PF

That was my first thought as well. One would need to be very careful when changing backs to be sure there was no disturbance movement of the camera. I don't consider it impossible, but care must be taken. If I wanted to check distance of the film plane from the camera back, that is what I would test; with a micrometer of some sort. I think Harbour Freight sells useful ones for not a lot of money.

The back on the 23 can indeed be extended with macrophotography, but it is primarily intended for perspective control.