Microscope Objective Thread Adapter Suitability?

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Barrelcactusaddict
Posts: 72
Joined: Wed Sep 29, 2021 10:04 am
Location: Rexburg, Idaho

Microscope Objective Thread Adapter Suitability?

Post by Barrelcactusaddict »

Hi,

I have a question regarding a microscope objective adapter I purchased a couple months ago; it allows a Mitutoyo objective to be used on an RMS thread microscope, or in this case, to the RMS female thread on an objective-to-camera lens filter thread adapter. I wasn't thinking about this at the time, but now I'm concerned that the image will be cropped when using an infinity-corrected Mitutoyo objective. Since the Mitutoyo optics have a greater diameter than an RMS objective, and the light would be exiting the objective in parallel lines, shouldn't some of the image be lost when the light reaches the RMS objective-diameter hole? I would think that if this is true, the final image resolution would be less than if using the Mitutoyo directly on an M26 female thread-to-camera lens filter thread adapter.

I'm very confused, and I'm now worried this adapter would be of little use in a paired lens setup (infinity-corrected objective w/ telephoto lens as tube lens, onto an APS-C sensor). I'm currently planning to use a dedicated ultra macro lens rather than a paired lens setup, but if I wanted to use the latter setup in the future, I want to know if this adapter would create an issue.

The first two images are of the adapter itself, and the third is of the adapter attached to an RMS female thread-to-camera lens filter thread adapter. Thank you, any explanation that can be provided is most appreciated.

-Kaegen
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Adalbert
Posts: 2427
Joined: Mon Nov 30, 2015 1:09 pm

Re: Microscope Objective Thread Adapter Suitability?

Post by Adalbert »

Hi Kaegen,
I'm now worried this adapter would be of little use
I'm afraid you are right.

For the tele you need some cheap step down rings e.g. 67mm->26mm

Best, ADi

Scarodactyl
Posts: 1617
Joined: Sat Apr 14, 2018 10:26 am

Re: Microscope Objective Thread Adapter Suitability?

Post by Scarodactyl »

It depends. Some of the objectives have a big back element, but many don't--the mount is designed as a one size fits all solution after all, so if one objective needs the wide back element and they really want it in their lineup then every objective needs a big thread. Seiwa offers an rms version of their mitutoyo copies.

Barrelcactusaddict
Posts: 72
Joined: Wed Sep 29, 2021 10:04 am
Location: Rexburg, Idaho

Re: Microscope Objective Thread Adapter Suitability?

Post by Barrelcactusaddict »

Thank you.

Well, for step-down rings, I just have three female RMS thread adapters that fit the filter thread of two different Pentax telephoto lenses, and the Canon 60mm f/2.8 Macro (which I found out was pointless after the fact, as macros are unsuitable as tube lenses); I also have a Pentax lens K mount > Canon EF mount adapter, since I own a Canon EOS 500D. I also have an RMS female thread > Canon EF mount adapter, but this would only be good for use with an EF mount bellows or extension tube. Overall, I have everything I need, except one of the Pentax lenses and a microscope objective.

I looked at the Seiya site, but it doesn't specify if their M Plan Apos are infinity-corrected or not, but looking at the tube lens they recommend is described as "Compact deisgned tube lens of infinity f=200mm"; so maybe they mean that the tube lens focuses the objective (infinity-corrected)? That is nice that they have thread options for their objectives, but I don't really trust copy brands, even though they're far more affordable.

It looks like the Mitutoyos and their copies do have some sort of rectangular black filter on the thread-side (there are very few pics of the thread-end out there); I also just now saw that the Mitutoyo M Plan Apo 5x exit pupil diameter is 11.20mm: I'll have to measure the adapter, but maybe the RMS thread-end hole is a bit larger than that (looks like RMS thread is 20.32mm diameter). I hope the exit pupil diameter on the Mitty is what I assume it is.

Maybe my question was a waste of others' time, since it was a major failure on my part to see that one bit of info on the objective's specs: I'm sorry about that. I do appreciate Adalbert and Scarodactyl's advice.
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Scarodactyl
Posts: 1617
Joined: Sat Apr 14, 2018 10:26 am

Re: Microscope Objective Thread Adapter Suitability?

Post by Scarodactyl »

Any time you see an objective specify a focal length like that it means they're infinity corrected (not counting antique objectives that list their effective focal length instead of magnification). The seiwa objectices are pretty good, though don't have the same coverage as a mitutoyo.

There isn't a rectangular filter or baffle on the objectives, just for the record. There's a round black element though. From eyeballing the main options none of them really fill the threads up, and I don't think an rms adapter would cause any issues.

Barrelcactusaddict
Posts: 72
Joined: Wed Sep 29, 2021 10:04 am
Location: Rexburg, Idaho

Re: Microscope Objective Thread Adapter Suitability?

Post by Barrelcactusaddict »

My bad, again I didn't look closely enough; indeed, it isn't a rectangular filter on the rear of the objective. I hadn't seen a listed price for the Seiwa, so I'll need to request a quote, I think; hopefully it'll cost about the same as the Laowa 25mm lens.

The 5x M Plan Apo seems to have a smaller exit pupil diameter than the HR version (5.6mm difference), but both versions would still clear the adapter's RMS diameter. I'd never be able to afford an HR version, that's for sure.

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