
Posted: Sun Aug 21, 2011 12:08 pm
| 45PSS wrote: |
| Graflex service instructions say to focus on a target at least 5000 feet away. corner cutters say a 100 feet away is good enough. a star several light years away is fairly dim but will give a more accurate adjustment than an object a few million miles away which provides a better adjustment than an object a few thousand feet away which is a whole lot better than an object 100 feet away. |
All's good information. But I think it helps getting the rangefinder adjusted if one can see errors in image coincidence more easily. Most unaided eyes, even eyes corrected with perfect prescription glasses, don't resolve enough detail to get repeatable ranging with non-magnified concidence rangefinders with a 3 inch base length within a 5% spread of the distance to the subject.
With my Super Speed on a tripod then using an 8X monocular on the rangefinder eyepiece, it's a lot easier to see exact image coincidence. My eye doctor's have told me I've got 20/15 vision with my glasses. Using the Graflex's rangefinder on well-lit, high-contrast lines at the subject, I was surprised at the actual errors in rangefinder coincidence when using an 8X monocular on the rangefinder. With a dial indicator mounted to contact the yoke to measure its positioin, I get yoke position spreads no more than .001" using a monocular on the rangefinder. Yoke position varies more than .001" without the monocular.
A 135mm lens has to move about .001" to change focus from 100 feet to 95 feet. To refocus from infinity to 250 feet, it also has to move about .001". To focus this lens from infinity to 5000 feet requires the yoke to move forward about .0005" and that's hard to resolve in the rangefinder without magnification of its image. Which means to me it helps to get the rangefinder well calibrated for accuracy using a monocular to see better coincidence.
I wish there was ground glass that could easily resolve at least 60 lines per millimeter. Such would make exact focussing with a 20 to 40 power loupe easy. This would help in getting the front standard set and locked to match the rangefinder's accuracy as seen though a monocular. I think it would take 3000 grit or thereabouts to grind the glass to do this. Or use fine grain B&W film (developed, solid grey all over) sandwiched between two sheets of glass to hold it perfectly flat.