Things I've seen from the past but haven't seen available now.

[Greg Howald]

Somewhere in the cosmos there are objective extension rings of varying thickness, like washers that you put into the objective before mounting it in the scope to add focal length.
Somewhere there is a tiny tube that you insert into a 100x objective to change the n/a for dark field microscopy.
Such things are very small and floating around in the vastness of the microscopy world.
Anybody know where?
Thanks. Greg

[Hobbyst46]

Somewhere in the cosmos there are objective extension rings of varying thickness, like washers that you put into the objective before mounting it in the scope to add focal length.
Somewhere there is a tiny tube that you insert into a 100x objective to change the n/a for dark field microscopy.
Such things are very small and floating around in the vastness of the microscopy world.
Anybody know where?
Thanks. Greg

A few years ago I purchased good quality objective extension rings, to use short barrel objectives on a DIN microscope. The maker and seller was in Spain (Earth, Cosmos). Can try and locate him if relevant. There were also Chinese-made rings on eBay. Note, that the use of such rings changes the tube length...
The formal name of those insert tiny tubes is funnel stops.

[mrsonchus]

Funnily enough a couple of weeks ago there was a 'set' of the spacers listed on e-bay, several thicknesses of course and several of each, each thickness in a different colour. They were to achieve parfocality over the few µ differences inherent in most (affordable at least) objective/nosepiece combinations. They were only about £20ish and I regret not grabbing them myself since then...
The inserts do occasionally come up for sale (I've seen them I'm sure for Leitz objectives when I had a Leitz system) - they're callen 'funnel-stops' I think.
Here's a post about them.

[BramHuntingNematodes]

I recently found this set of funnel stops, as well as having turned some on a lathe. They're not so complicated but are somewhat finicky to make as the concentricity of all the parts has to be exact.

[jjcook]

If you’re wanting to fine tune the particularity of your set of objectives, then something like these partiality shims are what you want, I haven’t found an inexpensive option yet though.

https://microscopecentral.com/products ... ms-spacers

[PeteM]

RMS parfocal shims are still out there. Just apparently not enough demand to drive the price down.

I haven't seen 25mm parfocal shims - but the Nikon and Leica objectives they'd go with are normally pretty close to parfocal out of the box. The problem is adapting other objectives. My solution has been to buy an aluminum tube with a 25mm diameter and use a very sharp lathe cut-off tool to slice it, very thin, like a salami.

A guess is that objectives with an iris have largely replaced the use of funnel stops.

[Greg Howald]

All of this would beat my crude solution of using a neoprene O ring. Greg

[apochronaut]

Maybe there two separate items being talked about here....three actually, if the funnel stops are included.

Parfocalizing shims are available in various thicknesses from a few thou. to about 1/8". Those are used to parfocalize objectives. In former times, some companies made several parfocal lengths of objectives only a mm or mm 1/2 different. That's what the thicker ones are for. Usually only a few thou are necessary to parfocalize objectives that need it.

Extension tubes have R.M.S. or other threads . Female on one end and male on the other. They are many mm long , 5 or up to 11 or so. They are used to extend the parfocal length when there is a big difference between objectives. Normally some optical compensation is required in order to extend an objective but there are instances where it is not necessary.

[Greg Howald]

Shims, extensions, stops. All three were once more available than they are now.

[BramHuntingNematodes]

I think the availability of brass shim rings is the same or better maybe just not marketed to the microscope set as much as they aren't as widely used now that manufacturers have settled on some standards and intermixing of lenses is not as easy. They got laminated brass shim rings for bearing races that you can dial in yourself within extremely fine tolerances merely by peeling away laminae. That's neat. Funnel stops are, as pointed out, outmoded. I don't know how widely available they ever were either. I think a lot more dark field equipment is being manufactured today compared to the 1940s. I could be wrong.

[PeteM]

The parfocal shims, up to about .010", are still readily available for RMS objectives. They're just expensive. Parfocal extenders (threaded both ends) are also available (Ebay etc.) and especially handy for mixing infinite objectives (say Leica 45mm to a Nikon 60mm parfocal scope).

As noted above -- it's possible to buy aluminum or brass tube with an appropriate ID and OD, chuck it in a lathe, and part off shims as thin as a couple thousandths and as thick as one could possibly want.