Help understanding a Darlot Portrait sliding aperture

I finally picked up a Darlot Cascet set and have been pleased with some of the shots I've taken (mostly of my wife who has not approved their posting to the internet yet.) What I'm not understanding is why the waterhouse slot can move. It's part of a cascet, so I thought it might be for different lenses to keep it centered, but all the lenses appear to be the same distance apart in any configuration. I took test shots with it fully forward and fully back in both wide open and fully necked down. Between the four shots I see no difference aside from what you would expect in reduced aperture. No difference between the forward or back positions. Does anyone have insight into the purpose of the movement? And what the threaded knob is for? I thought it would lock the waterhouse slot, but it appears to have no affect at all.

Your Universal Darlot casket set can be one of two versions.

The early ones had pre-aplanat/RR symmetrical doublets - whilst the later ones approached the Dallmeyer/Steinheil optics design! The early versions can provide an astonishing range of RV (rapid view) focal lengths!

Back to the adjustment! This is designed to be of use with the Petzval lens mounted. The knob justs releases/Locks the two halves of the barrel. The movement does alter the distance between the front and rear cells. According to Darlot, this controls the degree of "Flou" in imaging! This can be loosely translated as softness increase! At the time (1865), Dallmeyer was developing his alternative rear Petzval cell design , whose main purpose was also softness/diffusion control. There was a lot of discussion about Dalmeyers claims at the time - but even French makers eventually produced their own Dallmeyer versions.

I no longer have the casket set myself, but think I have a set of english language instructions somewhere.

And they appear in a post in this short thread:

https://www.largeformatphotography.i...rlot+universal

Thank you very much. You answered pretty much every question I had. Not sure exactly how old this set is, but it's slightly different than your instructions. The lenses are 11, 13, 17, 25, 38, and 45. I really can't see the difference the instructions talk about by pulling it fully out, but then I'm also using it on a 4x5 right now. I still need to make a plate adapter to put it on my 5x7 Scovill and see if I can spot it there. I do love what I've managed to do with this on my Speed Graphic and it's becoming one of my favorite lenses to play with.

I pulled the portrait lenses out of the mountings and there are no pencil marks on the edges, but they're definitely symmetrical RRs. I can see pencil marks on the edges of 3 of the 7 interchangeable lenses that appears to be 820 written one direction, and then 71 written next to it if you turn the lens over on all three. I can't discern what's written, if anything, on the other 4. My attempts to get a good photo of the writing have failed so far as it doesn't appear those lenses can be removed.

The instructions I posted in the old thread were basically for the second model and printed in the period 1880/1890. That is, for the aplanat casket set, but the seller has corrected the newer instruction for the old set he was selling to a customer.