Help: calibrating a prontor 1s shutter

[Dan Dubowitz]

I have a prontor professional 1s shutter that came with a rodenstock 115mm grandagon-n 1:6.8. I am shooting with a 155mm rodenstostock MC f6.8 this week, and the copal 1 shutter on it is playing up, and I wont be able to get it seen to in time. I am hoping to use my prontor shutter on the 155mm. It fits, but does any one know.....Is it directly interchangeable, or do i need to recalibrate the shutter for this lens, if so how, i cant find an instruction manual or guide on line.
Any help will be warmly appreciated.
thanks
dan

[Michael E]

I don't know if the elements will fit, but your f-stop scale will be off. For example: 115mm at f 16 has a diaphragm opening of 7.2mm (115 : 16 = 7.2). At 155mm, this diaphragm opening means f 21,53 (155 : 7.2 = 21.53). You have to mark the new f-stops for the new focal length. In this case, you are off almost exactly by one stop, so it is easy to remember.

Michael

[Dan Fromm]

Both lenses are Grandagon Ns so I think (great stress think) it is safe to assume that they have the same pupillary magnification. If so:

115 mm/6.8 = 16.9 mm

155 mm/16.9 = 9.2

An aperture scale for a 115/6.8 will read a bit less than 1 f/stop larger than true for a 155/6.8.

Michael you posted while I was calculating.

[erie patsellis]

Yeah, it's that entrance pupil thing that trips up a lot of people....

[Dan Dubowitz]

Michael Dan and Erie
thanks for this
I wont have a chance to do test shots before i go (into the amazon for 2 weeks) so am keen to be 100 percent.
so basically i should re mark f16 as f22 etc the aperture openings are linear
the assumption that the f stop opening are linear making the lenses interchangeable

i can see that the prontor has 2 accessible screws for setting the f-stop to a specific lens as well as stops for the smallest f stop.
To be sure a manual would be ideal, has any one out there got one or seen one on line?
prontor.de have nothing.
Michael re whether the optics fit, the elements screw in fine and sit similarly to how they do in the copal 1
if i cant get it sorted 100 percent another option might be for for me is to try and buy a copal 1 off the shelf in london, it looks like they are individual set up for a lens?would it also need calibrating? the copal one doesn't appear to have any user accessible adjustments ...any thoughts would be warmly appreciated

cheers
dan

[Bob Salomon]

Michael Dan and Erie
thanks for this
I wont have a chance to do test shots before i go (into the amazon for 2 weeks) so am keen to be 100 percent.
so basically i should re mark f16 as f22 etc the aperture openings are linear
the assumption that the f stop opening are linear making the lenses interchangeable

i can see that the prontor has 2 accessible screws for setting the f-stop to a specific lens as well as stops for the smallest f stop.
To be sure a manual would be ideal, has any one out there got one or seen one on line?
prontor.de have nothing.
Michael re whether the optics fit, the elements screw in fine and sit similarly to how they do in the copal 1
if i cant get it sorted 100 percent another option might be for for me is to try and buy a copal 1 off the shelf in london, it looks like they are individual set up for a lens?would it also need calibrating? the copal one doesn't appear to have any user accessible adjustments ...any thoughts would be warmly appreciated

cheers
dan

If you want to be 100% sure either a test or a repair man are the only wat to be sure there are no surprises. A test would have taken less time then the first to last comments in this thread have taken.

[Michael E]

I wont have a chance to do test shots before i go (into the amazon for 2 weeks) so am keen to be 100 percent.

You are taking pictures in the amazon for two weeks without checking such a simple yet important aspect? Are you sure?

To be sure a manual would be ideal, has any one out there got one or seen one on line?

This is simple physics: The f-stop is calculated by the focal length divided by the iris diameter.

Michael re whether the optics fit, the elements screw in fine and sit similarly to how they do in the copal 1

Is the image sharp all the way to the edges? I once screwed the elements of a 6.8/90 angulon into another shutter and got only a tiny portion of the image sharp: The distance between the elements was different.

if i cant get it sorted 100 percent another option might be for for me is to try and buy a copal 1 off the shelf in london, it looks like they are individual set up for a lens?would it also need calibrating?

Same thing. You need to mark the f-stops.

Good luck, and please reconsider travelling without making sure your equipment works.

Michael