Waterhouse Stop Question
Say you had a brass projection lens (see where this is going...), that you wanted to put some waterhouse stops in. Where should these sit, in relation to the lenses ? All of the old brass beauties that I have seen in person have the slot in the center of the two lenses, which is well and good as it would make my life easier. However, these were all symmetrical in construction, whereas I believe that the lens in question is asymmetrical (which was my reason for its purchase)
So, I suppose my question is this: Dependant upon lens design, where should the center of the aperture go ? I took a quick look though 'Modern Lens Design', and I didn't see this noted in the appendix.
On a side note - Im still looking for a petzval designed lens in the 210-300mm range. This one is about 5.25" in focal length, but it seems to be in excellent condition, and the price was right But, it probably won't have enough coverage for 4x5
The most efficient location would be the longitudinal OPTICAL Center of the lens.
Good luck - you'll need it in trying to find out where that is.
Otherwise, I'd just put it behind the lens, in the most accessible place.
Ed Sukach, FFP.
Ed, all that a stop placed behind a lens will do, for most lenses and certainly Steve's old crock, is vignette.
Originally Posted by Ed Sukach
If there's a slot in the lens, then that is where the stop should go.
On 4 petzvals I have the aperture is just slightly rear of center. It should be easy enough to open the barrel to get the size, make a stop from poster paper with 3 'feet' that bend 90% to hold it in place, then move it from center and look for pin-cushion =/= barrel distortion. The correct position will give little to no distortion, just measure, mark and cut carefully.
Have fun with it.
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Good idea. I think Im going to run over to my friend's machine shop and have him make me a quick piece. Im thinking of a basic doughnut, with your foot idea, so I can slide it back and forth to find the best position.
On a different note, there seems to be a simular thread on the LF board, with something real interesting, here's a link:
S.K. Grimes has a Waterhouse Stops page on their site that shows how they make 'em. It also gives the formula "Diameter=Focal Length ¸ f Number" for stop calculation.
But my original question was where to place the waterhouse stop slot.
As far as the LF discussion goes, that doesn't seem to follow 'd=Fl/fn' kind of thing...But then again, perhaps Im wrong. I suppose fundamentally it should, as 'x' amount of light to hit the film at a given f/stop should be the same, regardless of shape.
Most old brass projection lenses I've seen have a small "light baffle" somewhere inside. I would put the waterhouse slot immediately behind this, and consider putting in a second baffle just behind to keep the stops in place.
That baffle is actually part of the construction of a Petzval lens, and sits in the same place that the stop does on those that have one.
-- Ole Tjugen, Luddite Elitist