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Stephen Vaughan
14-Jan-2004, 02:09
I have a 210 G Claron in a Copal 1 shutter. I also have a 240 G Claron which I would like to mount in the same shutter. If I do this can I use the same aperture scale? i.e. do the apertures correspond in size between the two lenses? Or, do I have to get a scale specifically for the 240? Many thanks as always........

Frank Filippone
14-Jan-2004, 05:35
You can do what you wish. There is a correction factor of 1/3 of a stop. The 240 len is 1/3 of a stop more closed down than indicated. ie, When using the 240 in this instance, if the iris says F11, the actual stop is F12.5. Having even more fun, the equal focal length 1/3 stop increments are.... 210mm, 240, 270, 305. Sound familiar?

Stephen Vaughan
14-Jan-2004, 07:56
Thanks Frank - at the moment the 240 lens is in a Copal Press shutter and the 210 is in a standard Copal shutter. When I compare the apertures between the two it seems the 240 is at least a 1/2 stop wider (possibly slightly more - but definetely more than a 1/3). How do I standardise the two lenses? Thanks......

Frank Filippone
14-Jan-2004, 08:25
The factors between the 210 and 240 are a mathematical and physics solution. In other words, they are correct. If you think otherwise by looking through the glass, go right ahead. Let me suggest trying a test... shoot a blank sky with both lenses. Develop the negs, then measure the transparent density of the negs. Then you have the scientific answer. I had a complete set of G-Clarons. 210, 240, 270, and 305. It works.

Emmanuel BIGLER
14-Jan-2004, 08:44
Stephen. If you are ready to swap your shutters, then you are also ready to measure the actual diameter of the iris without glass. Measure the actual diameter of the iris with the 210 scale at f/9. Screw the front element of the 240 and try to find the position of the aperture control that opens to the max aperture of the 240. Say : this would correspond to f/6.3 in between 5.6 and 8.
I assume that you can actually open the iris beyond the F/9 engraving designed for the 210, and that there is no mechanical limit preventing to open wider than that.
Now you know the physical diameter of the iris for the 240 at F/9 and the ratio between both physical diameters @f/9 on the "210" scale and on the future "240" scale to be recomputed. But this is just for cross-checking the procedure, in fact. F-numbers are inversely proportionnal to the physical diameter of the iris. So if actual f/9 (max aperture) for the 240 reads f/6.3 on the scale designed for the 210, all engravings have to be re-computed by applying the 9/6.3 correction factor to all "210" readings. So now f/11 on the "210" scale will correspond to 11*9/6.3 ~ 16 on the new "240" scale. You can cross-check this conversion ratio with the ratio of physical diameters, since you have to unscrew both lens elements, you can measure it easily.

If somebody needs to estimate the real f-stop number without un-screwing the lens elements, the optical engineer will make an image of the iris as seen from the front, this is called the entrance pupil. Measuring the actual diameter of the entrance pupil yields the true f-number equal to the focal length divided by the diameter of the entrance pupil. You do not have access physically to the entrance pupil which is located inside the glass, however you can make an image of this pupil with a known magnification ratio and measure the final image.

Arne Croell
14-Jan-2004, 09:17
For a lot of lenses (but not all*) the wide open aperture is often not only defined by the aperture diaphragm, but also by parts of the lens cell mount. For plasmats, like G-Clarons, this is the mount rim of the innermost lens of each cell. One can see this when looking through the mounted lens from the front, shutter open, and closing the aperture. Essentially, you look at the entrance pupil. Initially (outside the engraved aperture scale) the diaphragm blades are not visible. They should just became visible when the stated open aperture is reached on the scale, f/9 in your case. At that point the diaphragm opening and the stop defined by the lens mount are identical. If this is the case, and you have the same type of lens, as with your G-Clarons, just swap the lens cells, and note at what position of the aperture lever the blades just appear seen through the front cell for the new set. That is your new f/9. If the difference to the old position is not too big it is often enough to loosen the screws holding the f-stop scale and shift the scale a bit. Or, if that is not enough, lengthen the holes for the screws with a needle file.

*This method works for most lenses, nearly all modern ones, but not for all, since it really depends on the fact if the manufacturer provided a "stop calibration" with the lens mount. Notable exceptions are often wide angle Gauss lenses (WF Ektar, G-Claron WA). If the original mount had an additional mechanical stop (e.g. a screw) to keep the aprture lever from opening all the way, it certainly does not work.