355 G Claron Shutter Swap

[Kevin Crisp]

I've taken a Copal 3 with proper aperture scales for the Schneider S 360mm f:6.8 lens and put lens cells from a barrel mounted 355mm G. Claron in the shutter. The distancing of the elements in shutter matches the old barrel mount exactly. Given the fact that focal lengths of these two lenses are less than 2% different, would the aperture scale then be correct the way it is? It is hard to compare the apertures of the barrel mounting with the Copal 3 since the barrel mounting aperture opening is much more star shaped as opposed to the relatively round hole on the Copal 3. Thanks.

[Ernest Purdum]

Lens design makes a difference in the diaphragm size meeded. The governing factor isn't the actual size of the opening, but the size that it appears to be when looked at through the front element. It's unlikely that the original markings would be anywhere close to what is needed for the G-Claron.



Since, as you say, it's difficult to compare the two diaphragm shapes and it is also dificult to measure the effective aperture without an expensive gadget, probably your most practical solution would be to compare the amount of light coming through the lens as mounted in the barrel and as mounted in the Copal. You can do this by pointing the camera at a blank wall or some evenly illuminated subject. Tape a piece of cardboard over your groundglass, making a hole in it large enough for your lightmeter. Take a reading with the lens in barrel, then put the lens into the shutter, adjust the diaphragm until it matches the earlier reading and mark the position of the pointer. By repeating the process you can obtain the full range of markings. Be careful that the light doesn't change between any pair of readings.

[sanking]

Ernest,

Are you absolutely sure that the size of the aperture for lenses of the same focal length varies with lens design? If so I would appreciate it if you would explain this for me using some type of graphic illustration. I really can't understand why the design of the lens should affect the size of the aperture for a given f/stop with two lenses of the same focal length. In an effort to understand this better I actually measured the opening of the aperture at f/16 with two 120mm lenses of very different design, a Goerz Dagor and a Fujinon SWD. In both cases the opening of the aperture measured about 7.5mm, as one would calculate in theory by dividing focal length by the number of the f/stop.

Please excuse my ignornance if I am missing something very simple here.

[tim atherton]

i swapped a 210 kowa graphic into the copal 1 shutter that had a 210 g-claron + associated aperture scale on it and certainly has seemed to spot on (and the kowa becomes a 6.8, so i just open it up past f9 for focussing)

[Jim Galli]

I'll pool my ignorance with the rest. I know with plasmats that are not necessarily symmetrical the aperture size of the combination is partially dependent on magnification. Since the f6.8 plasmat is non-symmetrical and the G-Claron is symmetrical there may be a slight difference. That said if it were me I'd do it (actually have done it) and never look back. After I bought my shutter speed tester I learned that shutter speeds are commonly farther afield than whatever little difference in the 2 apertures may have been. Another way I check is to close both apertures down to a fairly small size, perhaps f32 in this case, and set barrel and shutter at the same place on a light table that is warmed up. I then just stick a spot meter down inside and take a reading. It means nothing except in relation to each other. For instance if the meter gave an EV 8 in the barrel and 8.3 in the shutter I would adjust the shutters aperture intil the meter gave an 8 there also and note the difference.

[Ernest Purdum]

Sandy,

In mentioning "lens design", I didn't mean the construction arrangements, but just how much the front cell changed the apparent size of the diaphragm. Keep in mind that it is the apparent or "effective" size of the diaphragm that is divided into the focal length in order to find the f number.



Measuring the effective size isn't easy. It's done by a "traveling micrometer" a little telescope on a calibrated slide that sits crossways to the lens. You line up a hairline on one side of the diaphragm, note the reading, move the slide until the other side of the diaphragm is lined up, the calculate distance moved. Some people have improvised gadgets of this nature, but I think the exposure meter readings would be easier for most people to use.



Jim Galli's procedure seems similar, though I'm a little unclear about some of the details.



If it should happen that two different lenses magnify the diaphragm to the same degree, that's great, but you can't count on it happening.

[Kevin Crisp]

Just to follow up, I noticed that on the barrel mounting, the shape of the hole, which is less round due to fewer blades in the iris, becomes much more round on the small stops. It then becomes fairly easy to attempt to correlate one mounting to the next, if you start at the smaller stops. Based on this, I would say that it is NOT a straight swap. It appears to me that to be accurate from one to the next I need to stop down 1/2 stop exactly LESS than what is marked for the faster lens. I will check this out with measurements off the ground glass, just to be sure, but will be pretty surprised if this shortcut doesn't do it. Thanks for your suggestions. Kevin