aperture position

[Michael Dowdall]

How criticul is the position of the aperture between the lens cells? Does it have to be on the nodal point or would 1 or 2 mm in either direction mater that much?

Michael

[Paul Fitzgerald]

Hi there,

WHY?, strange question. If it is too far off you will get a focus shift as you stop down. You can find the right place by looking as the ground glass. Too far forward it should pin-cushion, too far back it should go barrel distortion (that could be reversed). Are you building a lens?

[Emmanuel BIGLER]

To the best of my knowledge, the physical iris itself is never placed at the nodal point(s) of the entire optical system. But the pupils = images of the iris can be located at the nodal points when the lens is quasi-symmetrical and when the iris in centered. More generally in a system where pupillar magnification is equal to 1, pupils are located at the nodal points due to the laws of geometrical optics... except if the lens is disassembled and re-mounted incorrectly. The question is in fact : what is the acceptable tolerance in a do-it-yourslef re-mount of lens groups ?
In a perfectly symmetric lens like an apo-repro (apo-ronar, fuji-C) the iris is located exactly at the centre of the lens ; therefore it is easy to show that its images, front = entrance pupil and back = exit pupil will be located at the entrance and exit principal (or nodal) points.
This is almost the case for most view camera lenses of quasi-unit pupillar magnification ratio; telephotos and retrofocus (almost unknown in view camera lenses except for extreme wide-angle lenses) are the exception.
My undertanding is that the position of the iris within a few tens of a millimeter is not really critical in a well-corrected lens.
However in a modern wide-angle lens, proper positioning of the lens groups with respect to the shutter (and iris) is critical. At least this is what Schneider suggests by trying to deter amateurs to swap their shutters themselves on of their modern wide angle lens products ;-);-)

[Michael Dowdall]

Thank you Paul and Emmanuel.

Why? Well I have a Protar lens set in a barrel that I've been using for a couple of years. For a while now I’ve wanted to mount this lens in a shutter to make it easier to use. Last winter, off of ebay, I bought a shutter with two Protar cells mounted in it. My cells are interchangeable with the new ones. But there was a problem, the space between the cells on the after market shutter was greater than the factory mounted barrel. 3mm longer. I asked my camera repair person to cut down the two adapters between the lens cells and the shutter to match the barrel lens spacing. He cut down only one adapter. The lens cell spacing is right on but the aperture is now closer to the front lens cell by 1 to 2 mm. So yes, "The question is in fact : what is the acceptable tolerance in a do-it-yourslef re-mount of lens groups ?". The lens cells of a Protar are symmetrical in design but have different focal lengths. Is this still considered symmetrical?

My life is very busy right now and don’t have the time to test this lens until I go on vacation in a few weeks. It would be a great disappointment to find out the negs shot 2000 miles away where not as sharp as I saw on the G.G. before stopping down. So I’ve got decide what to do. Go back to the repair person and get them to fix the lens or take it with me and take the chance on fuzzy focus.

Michael

[Paul Fitzgerald]

Hi Michael,

Short time + long distance vacation = put them back in the barrel, at least you know what you had and how it works for you. Try the second set in the barrel and see if they like that spacing, Protars should be able to mix and match for different focal lengths. 2mm would be too far off and focusing at f/64 is a pain.

Good luck and great fun on vacation.