The diameter of the fiber optic probe is roughly 1/4". When holding it against the crosshairs of my gg, I seriously doubt my measurement would have been more accurate with the fresnel in place. Once again, I was not interested in "metering" the image at the gg for the purposes of making an exposure. Of course a gg and fresnel would attenuate the light as compared with what would strike the film and that is covered in the operation manual that came with the probe. All I was interested in was making a comparison between the aperture of one shutter with another. The small size of the probe made this easier as I could come closer to pinpointing the exact spot from which I made my measurements.
The OP's method is completely valid and possibly the best available way to go about it, if a known trusted lens is available. All the objections in this thread are nonsense.
Science is what we understand well enough to explain to a computer. Art is everything else we do.
--A=B by Petkovšek et. al.
I have about ten lenses in Copal shutters and none of the aperture scales spreads out like that. But you got what you got.
Neat job, Robert. I haven't used a Leroy lettering set for decades. Even an old fossil like myself would have made a chart of the diaphragm diameter as viewed through the front lens group for each desired f/number and made a temporary scale as you did, using this chart. For this, viewing the diaphragm diameter should be done from a considerable distance to eliminate parallax error, or the eye can be shifted by about the apparent diameter of the diaphragm. Scanning that scale lets one make a finished scale in Irfanview or almost any other image editor. That finished scale can be scaled up or down to fit any other lens mounted on that shutter.
I think you're right, the spacing on all my Copal shutters is not like that it's equal spacing between every f-stop. I'd actually copy the f-stop scale from a 210mm Symmar/Sironar shutter, the difference between a 203mm and 210 mm lens for the same aperture is very small and insignificant, the OP could adjust for it in positioning anyway if needed.
At the moment some stop markings are quite significantly out, I'd need to check my 210mm Symmar S, plus I have a spread sheet for calculating corrected apertures for the same Shutter with different FLs fitted.
Ian
I don't understand why you went all this way of measuring with your exposure meter. After all - if the lens elements are the same in both shutters with identical spacing, all you need is just to have the same aperture opening in both shutters to get identical amount of light to go through. Why not just measure the actual aperture diameter and set it on the new shutter to the same value and mark the scale? If both the shutters have the same inner dimension then the same opening diameter must give the same result optically, regardless of the name on the shutter label!
Measuring the comparative diameter of these two shutters is not as easy as you might think. The Compur shutter has many more blades in its diaphragm creating what appears to be almost a perfect circle. The Copal shutter's diaphragm contains fewer blades and the result is more of a polygon making measurement more of a challenge. By simply measuring the light passing through a known aperture, it was pretty easy to adjust the diaphragm on the Copal shutter to achieve the 1/3 stop increments.
You can take advantage of modern times.
Have you an Smartphone ? Then you have a precision tool !!
Just download a Luxometer App from google play. It uses the front photocell that is intended to measure ambient light for screen auto level.
It is not a photographic tool in the sense that is directional (you should place a translucid dome on sensor and calibrate that in the app), but it will deliver very accurate relative readings, from some 30 to some 30000 lux in the case of my Xperia Z2. Use stored Max max reading, as you won't see screen while reading, as potocell is also in the front side.
You also can buy a cheap handheld luxometer (some $20) This goes from 0.01 lux to 300.000 lux. Also useful to plot film calibration curves.
Take the readings with no GG.
Plan B:
Just place a DSLR or SLR in the back of the view camera (without lens), you have to manufacture a simple attachment or use a second tripod for the SLR. I used my F5 for the same. You have to know the flange to sensor(or film) distance to give the right bellows extension.
Regards.
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