Parfocalling-do it yourself?

[GerryR]

I have seen many posts where it was suggested that replacing an objective will result in it possibly not being parfocalled. However, I have also read some articles on parfocalling, and it seems rather straight forward:
1) After removing the lower objective tube covers on the higher power objectives (or the one you want to parfocal), you remove the spot of "glue" holding the adjustment sleeve for the lens using acetone. 2) You use a slide (calibration slide ?) with a very fine line(s) and sharp focus with the 10X objective (usually no adjustment sleeve on the 4X or 10x objectives). 3) You then switch to the objective you want parfocalled and adjust the sleeve to get sharp focus (without touching the stage focusing knobs!) 4) Put a dab of clear nail polish on the sleeve to prevent further movement. 5) Reinstall the lower tube covers and you're done.

Has anyone done this, and is it as simple as described?

[Greg Howald]

I wouldn't consider it. Seems a bit scary to me.. I have been known to adjust parfocal capabilities using spacers or o-rings on objective threads but have not tried to glue anything. Greg

[apochronaut]

Not.
There are some designs such as the Bausch & Lomb patented design that allows for the optical section of the objective to be moved up and down in the threaded objective housing by loosening or tightening the objective rear diaphragm against a moveable outer shroud that also encompasses a front bezel. There is no glue , just threads.
There is a fairly standard objective design that has the optical section centered in an outer tube and this can be moved if you want to risk decentering but mostly parfocalism exists as a design feature in families of objectives and is adjusted in the factory. In many older designs it meant that the objectives were mounted on a microscope in sets, because each objective was precision adjusted in the factory by internal shimming for s.a. , which also affects focus. This was due to small batch differences in a specific glass type's n.
While the instructions you have provided might be applicable to an objective design, they are far from universal and many systems have no adjustable feature. Some can be fudged a bit.
When dealing with imprecise hobby grade instruments, or with infinity corrected systems, the best way to parfocalize is by shimming at the objective shoulder(s). Special ring washers are available in incremental thicknesses.The thinnest are impossibly thin , only a few thou. You stack them in order to achieve the desired thickness, or use one of the correct thickness if that works out. The shim thicknesses are verified with a micrometer and the difference between objectives is determined by using the fine focus micrometer.
The objective with closest focus is treated as the standard and the other objectives are shimmed to match.

[PeteM]

Only some objectives be adjusted that way. As Greg says, RMS parfocal shims are available and work well.

I haven't found parfocal shims for 25mm objectives, but those can be made up using thin brass sheets and shim punches. Shims can also be turned off a hollow brass or aluminum tube, but it gets tricky to get them just a few thousandths thick.

[GerryR]

Not.
There are some designs such as the Bausch & Lomb patented .......
The objective with closest focus is treated as the standard and the other objectives are shimmed to match.

Interesting that they start with "the objective with closest focus," which makes sense, as that would seem to be the most critical. The use of shims, as Greg Howard and PeteM mentioned, also seems more sensible rather than messing around with the internals of the objective.

Thanks for your inputs!

[zzffnn]

I also shim at objective shoulder using o rings or rubber bands.Tension provided by screwing action and rubber bands is enough to hold objective at the exact shim height. No glue is needed.

[GerryR]

I also shim at objective shoulder using o rings or rubber bands.Tension provided by screwing action and rubber bands is enough to hold objective at the exact shim height. No glue is needed.

Excellent solution! All this works really well if you have to bring the objective closer to the specimen; what happens if you have to go the other way??

[PeteM]

. . . what happens if you have to go the other way??

As Phil said, if you start with the objective that needs focusing the closest to be parfocal, you're always adding shims.

The only times when I've wanted a "negative shim" have been when adapting a single RMS infinity objective to a set of 25mm mount (Nikon CFI or Leica). In that case you need to add an adapter and these add more space than you'd want to shim all the other objectives. I've carefully turned the thread a bit deeper on a lathe to regain parfocality, but it's not for the faint of heart and you have to be sure not to cut too deep.

One way to quickly get close to what size shim is needed is to count what fraction of a turn is needed to get things in focus (leaving the objective loose, say, a quarter turn out to get it in focus). Then, convert turns to fractions of an inch for an RMS thread. A 36 tpi thread is just under 28 thousandths of an inch per turn. Should your objective be off, say, 1/4 turn then you need about a .007 shim.

[GerryR]

Thanks for all the inputs; great suggestions, and I definitely learned a better way of doing this!!

[Scarodactyl]

I haven't found parfocal shims for 25mm objectives

You can get shim washers with the appropriate ID from mcmaster iirc. Unfortunately never ended up in a situation where they helped me.

[GerryR]

Parfocalling is exactly what you have explained. Some people sometimes are still wrong about it.

Please clarify; who explained it properly and what do people get wrong about it? Much appreciated. Thanks.

[viktor j nilsson]

Parfocalling is exactly what you have explained. Some people sometimes are still wrong about it.

Please clarify; who explained it properly and what do people get wrong about it? Much appreciated. Thanks.

It's a spam bot.

[GerryR]

Parfocalling is exactly what you have explained. Some people sometimes are still wrong about it.

Please clarify; who explained it properly and what do people get wrong about it? Much appreciated. Thanks.

It's a spam bot.

You might think people would have something better to do.