Fitting an adapter on a microscope objective

trobinson41 wrote:

After watching Allan Walls's excellent video "Going Deep - Part I - Extreme macro with infinity corrected microscope objectives", I purchased the Plan 10x/0.25 infinity-corrected objective and 52mm-25mm adapter that he recommended. Unfortunately, they don't quite fit together.

Welcome to the exciting world of "objective collecting". You've just run into one of the most common problems in the world: things "not quite" fitting together.

The TL;DR if you don't want to read one of my long posts is that you've probably got an older objective that uses the infamous "RMS" mount (affectionately known as the "royal screw" by us microscope nerds). It's been around forever and was was sort of the "universal mount" from like 1860 to 1960. An RMS objective has 20.32mm threads and will drop into a 25mm M25 mount without engaging.

The "long" explanation:

There are three things that could be going wrong, roughly in this order:

1. You've got an RMS objective. Its thread section is 20.32mm (the official spec is 0.8 inch, because the spec is over 100 years old) and it will not engage with an M25. You can get RMS-M52 adapters to replace your M25-M52, or you can get an RMS-M25 adapter and then stack your adapters: objective -> RMS-M25 -> M25-M52 -> lens.
2. You've got an objective that is something other than RMS or M25. Some common sizes are M26, M27, M32, M40, and M42. Post pics and I'll ID it. (If you end up like me, you will eventually accumulate all those adapters). This could be a problem for other reasons: most Nikon M26, M27, and M32 objectives are "bright field dark field" or "BD" types, which means they have "light ports" that you have to block or you get hazy pictures.
3. Your objective really is a Nikon with M25 threads, but the "thread pitch" is wrong for your adapter. So it might "fit" but jam after 1/4 or 1/2 turn. Would you believe there are actually four different common pitches? Nikon uses 0.75mm. That means the valley of each thread is 0.75mm from the valley of the next thread. There's also a 0.5mm pitch, which is standard for filters, so a M25-M52 "filter adapter" won't fit. This is an actual problem: I have two M25-M52 adapters that are set up with "filter threads" and won't mount Nikon M25 objectives. (M25x0.5 wasn't just used for filters: it was also used for some really nice macro lenses made by a company called Schneider that was once important), 0.706mm (36 threads/inch, because some companies owned equipment geared for "imperial" standards) and 1.0mm. (Oh, and just to make things more confusing, there are adapters threaded at a 0.73mm pitch that can "usually" engage successfully with either 0.706mm threads or 0.75mm threads).

You might also consider getting an M42-M52 adapter for your lens, and doing all you other adapters to M42. M42 is sort of a "universal mount" in macro. I always adapt RMS-M42, M25-M42, M40-M42, etc. My tubes are M42, my tube lens mounts are M42, and at the other end I adapt M42 to Nikon F or M42 to Nikon Z.

I did notice a couple of differences between my objective and the one in his video. The one in the video has /- WD 10.5 after the infinity symbol.

That is an extremely common, and pretty good "metallurgical" objective (the type you want, even if your subjects are "biological" like flowers or insects. Don't think of "M" as "metal", think of "M" as "macro" because the things that make for good metallurgical microscopy also make for good macrophotography.

We'll come back to that.

It is, indeed, a "common" M25 mount. Note that I put "common" in quotes, because M25/0.75 is common in microscope objectives, but M25/0.5 is common in filters, and there's a lot of M25-M52 adapters that mount "filter stuff" and will jam if you try to mount an objective.

Mine has /0 WD 17.8. I don't know what those figures mean.

Are you sure? AFAIK, Nikon never made a 10x WD 17.8. Literally the only 17.8 I can find is a Mijii 4x.

If you post pictures of yours (of every side with printing on it) I'll break all the numbers down for you. Here's some basics.

We'll start with the bottom line, something like "∞/- WD 10.5"

"/-" and "/0" mean exactly the same thing: "no cover glass". Nikon used to use "/0" but switched to "/-" somewhere in the 2000s. Biological objectives are compensated for a cover glass at higher powers. Don't buy anything that says "/0.17". The fact that yours says "/0" makes me think it's an older objective that has RMS threads instead of M25.

The number on the "left side of the slash" is the "tube length" for "finite" objectives (like 210 or 160) or an "∞" or "INF" for infinity objectives. So yours says "∞/0", basically an older objective.

"WD" is the "working distance", the distance from the tip of your objective to your subject, in mm. All else being equal (which it typically isn't) the longer WD makes for better macro. Going from 10.5mm to 17.8 is huge, and makes is so much easier to get good light to your subject.

Now the top line, like "10x / 0.25".

"10x" of course is magnification, with the objective matched to the proper "tube lens" for infinity objectives or at the proper "tube length" from the sensor for finite objectives. "0.25" is the "numerical aperture" or "NA" number. This is like the f stop, except bigger NA = faster while smaller f = faster.

There are a ton of other words and letters that can appear on an objective, and (here's the tricky part) the mount doesn't appear in any of them nor does another important parameter called the "parfocal distance". You're expected to just "know" that a Nikon "10x/0.25 ∞/- WD 10.5" is an M25 with a 60mm parfocal length for an Optiphot 300 series scope, while a Nikon 10x/0.30 ∞/- WD 16.50" is RMS with a 45mm parfocal distance for an Optiphot 200, and a "10x/0.25 210/0 WD 9,0" is for my trusty Optiphot 88.

The parfocal distance is the distance from the base of the objective to your subject. Ideally, if focused on a subject with a 10x 45mm parfocal objective, then swapped in a 20x or a 5x, you'd still be close to being in focus. Whereas if you went from a 45mm parfocal to 60mm, you have to back everything up 15mm. This is annoying for macro, but not fatal. An actual microscope is another story: they have rotating turrets carrying anywhere from 4 to 7 objectives. A 60mm parfocal objective mixed into a turret full of 45mm objectives would crash into your subject.

Also, my objective is silver-colored, instead of gold. Have I purchased the wrong objective? Is there anything I can do to use the one I have, or am I going to have to get another one? (Fyi, I did post this question on Mr. Walls's site, but I don't know how long it will take him to respond.)

He typically won't. Allan admits he's currently 5,000 emails behind, so unless you're on his "flag" list, you're behind a six-month backlog.

Thanks!

You're welcome.

Oh, and that whole "metallurgical/biological" thing? Objectives come in two basic flavors: "biological" and "metallurgical". The distinction is actually not whether your subject is "flesh" or "metal", it's how they're used:

Biological:

• Short working distances, typically just enough to safely maneuver around the cover glass of a microscope slide.
• "Compensated" for the slide cover glass, typically 0.17mm. That's the number after the slash: "∞/0.17" means an infinity objective set up for slides.
• Big fronts. Since all the light comes from under the microscope and goes through the subject, you don't have to worry about the objective blocking light from up top. In fact, you usually want to block as much top light as possible.
• Higher "numerical apertures", the "lens speed" of the microscope world. Higher NA means higher resolution: sharper images can literally be a matter of life and death in biological instruments.

Metallurgical:

• Longer working distances, enough to keep the objective safe from a subject that might have jagged edges and hard enough to scratch an objective with just a light touch.
• No compensation: work is done in open air.
• Lower numerical apertures. That often is a side effect of increasing working distance.

Good luck. Shout if you need more help. Post pictures.