Mamiya RZ67 - how to calculate focal distance?

cestlefun17 · Sep 17, 2016

When you extend the bellows on the on the Mamiyz RZ67 there are curved lines corresponding to each focal length lens so you can find the focal distance. However, there is also a ruler along the top that measures how far you have extended the bellows in millimeters.

Is there a formula using this ruler to precisely determine the focal distance? I'm trying to figure out some formula that incorporates:

*focal length in mm
*bellows extension in mm
*focal distance in mm

For example, with a 90mm lens extended 8mm it is focused at 1.2m (1200mm). What is the formula that relates 90mm, 8mm, and 1200mm?

Thanks!

CarlH · Sep 17, 2016

I would have thought you would need the distance from the film plane to the lens, not just the amount you have moved it out for any formula to work?

cestlefun17 · Sep 17, 2016

Well the film plane never moves, so is there a constant then that I am supposed to add to the bellows extension in mm?

The problem with using the curved lines as a guide is that sometimes your focal distance isn't marked. I can focus at exactly 1.2 meters because that is marked--I follow the 90mm lens curve on the graph and line it up with the 1.2 meters marker. But if I want to focus 1.3 meters away, I can't be as precise because 1.3 meters isn't marked on the chart.

That ruler at the top has to be there for a reason.

480sparky · Sep 17, 2016

In terms of exposure, the difference between focusing between 1.2 and 1.3 meters is negligible.

Dave442 · Sep 17, 2016

I think that Mamiya put the mm scale there to do the calculation for adjusting exposure for the bellows extension, and the chart includes the .5 and 1 stop adjustment ranges so you don't have to do the math.

I don't think the scale is accurate enough if you want to set it to a distance and then set the camera film plane at exactly that distance from the subject and expect perfect focus, depending on your DOF it may be acceptable focus. Most of the time photographers focus and take the picture and never give more than glance at what the focusing distance was, I guess you have some special case where you want to set the distance ahead of time. That distance scale on the Mamiya is really just useful enough to know what to dial in on the lens to see what it gives as far as DOF.

I think what you want is to take the bellows extension distance in mm and use the lens focal length in mm and from that arrive at the focusing distance. I had this link that talks about some of the math, but may need more information than just the bellows extension:
Lens Tutorial

compur · Sep 18, 2016

That's why they made these things:

SoulfulRecover · Sep 19, 2016

I have never used any bellows comp with my RZ and have never had an issue with exposure

480sparky · Sep 19, 2016

It's usually only an issue when focusing very close. Landscape, architecture etc. aren't really affected that much by it.

SoulfulRecover · Sep 19, 2016

Ive had it completely out with the 110 lens and slide film and it was fine?

I don't know. Ive shot a lot with it completely extended and never had an issue

fmw · Sep 22, 2016

I used an RZ-67 system for years. I never once used these scales. Actually I seldom used the internal metering. Of course a simply solution is to bracket exposures for closeups.