Oculars or auxiliary lens

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Grymnor
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Joined: Wed Jan 13, 2021 3:03 pm

Oculars or auxiliary lens

#1 Post by Grymnor » Wed Jan 13, 2021 3:28 pm

Hi everyone,

First-time post for me, but I've already picked up lots of useful information by lurking for a few weeks now. I'm after a little advice please as my knowledge so far is very limited.

My hobby is gemstones and my interests in this area are progressing into the "inclusions" within the stones, and I, therefore, wish to move on beyond my £75 stereo scope I've had for many years. After a bad experience with a second-hand purchase, I've gone the new route (against most advice I know, it's just a personal choice for my own reassurance). The stereo microscope I've ordered goes to 55x magnification and I'd ideally like to get to around 80x to cover my expected use, therefore I've ordered a pair of 15x oculars as well.

With the background covered, the advice I'd be really grateful for is on 3 points:

1. Do stereo microscope objectives have a fixed Numerical Aperture, and if so, would I be correct in multiplying it by 1000 to get an upper ceiling for useful magnification? Or does it work differently for zooms than for compounds?

2. I understand that as magnification increases, the field of view decreases. I’ve read that changing oculars to gain greater magnification results in a decreased field of view and is therefore a bad thing. Is this true for a specific magnification though?
For example: At their lowest zoom setting, 10x oculars would have a greater FOV than 15x oculars at their lowest magnification. But if a specific magnification is chosen for both, e.g. 30x, would there be a difference in FOV between the 2 ocular setups?
Furthermore, if there is no difference at a given magnification, what would be the drawback to using higher power oculars?

3. As I intend to use the microscope for the examination of small (5mm – 20mm) gemstones which would not be hampered by a small working distance, would you recommend achieving this through either a supplemental auxiliary lens or supplemental oculars? It's always going to be nice to have extra working distance, but not if it's at the expense of significant resolution. As mentioned, I have already ordered a pair of 15x oculars, but I’m now doubting my choice slightly.

Thank you very much, in advance, to anyone who can guide me a little.

Dan

apochronaut
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Re: Oculars or auxiliary lens

#2 Post by apochronaut » Wed Jan 13, 2021 5:25 pm

The zoom feature has nothing to do with the type of microscope. Zoom can be applied to any magnifying system. Generally though, if the zoom system exceeds a certain magnification , empty magnification occurs and the resolution will deteriorate. You see bigger things but you are left to interpret what you are seeing, often reducing the magnification subsequently in order to get a clearer image. Zoom systems above 3X at very low magnifications and 1.5X at high magnifications are a compromise, where convenience rules. To some degree these comments are lessened with better microscopes but in order to get a really decent N.A. at something like 80X you really need to spend a lot on the microscope.

Without changing the eyepieces, a magnification increase will reduce the field of view. In some cases, increasing the magnification with certain 15X eyepieces will keep the field of view that you had with 10X but the objective needs to be up to it. Using 15X eyepieces often will increase the apparent field of view as well, again depending on the eyepiece design. However , using 15X eyepieces with the higher power objectives on most stereos or the zoom full on will result in empty magnification. I don't know of any stereo microscope that you could buy for under 1,000.00, that would not enter empty magnification at 80X but maybe I am missing one somewhere. I would like to know about it, if it exists.

If working distance is not an issue, always choose an auxiliary objective lens. Some microscopes even offer apochromatic aux. objectives.

Scarodactyl
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Re: Oculars or auxiliary lens

#3 Post by Scarodactyl » Wed Jan 13, 2021 7:41 pm

Just to add to what Apo has already said.

Stereos don't have a fixed NA--the bottom objective has a maximum NA which is available at maximum magnification, and as you zoom out the zoom system stops it down like an iris in a camera lens to a lower numerical aperture. This has two main adantages: first that you have usable depth of field at lower magnifications, and secomd that your image doesn't dim as you zoom in. So if you use 15x eyepieces at 1x vs 10x eyepieces at 1.5x the field of view may be the same but the resolution is higher and the image brighter in the 10x eyepiece's view.

For some context, the Zeiss Stemi 508 is an expensive 'routine' stereo microscope and its max NA by default is 0.075. With 10x eyepieces it zooms to 50x, so with 15x eyepieces at max 75x you are already at the bleeding edge of what it can deliver.

What a 2x aux lens does is it doubles that maximum NA while halving working distance. 20x eyepieces can give the same field of view but they're just cropping the edges off a lower NA image, so the image is dimmer and at a lower resolution (but higher depth of field). In a cheaper system 2x aux will also usually introduce some distortions, especially at lower zoom settings.

For routine gem work it's fairly normal to just plop in 15x eyepieces to get up to 60x or so and not worry about diffraction. You can get away with it to some extent because stereos give your eyes real 3D information on top of the resolution in a single image so it will tend to look better than what you see in a single image. But I would always go with the auxiliary lens instead if it's possible.

Grymnor
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Joined: Wed Jan 13, 2021 3:03 pm

Re: Oculars or auxiliary lens

#4 Post by Grymnor » Thu Jan 14, 2021 12:15 am

Thank you both for taking the time to type out answers for me.

I think I understand what you're saying, it's at the edge of my level of understanding so far.

Would I be following you if I said that without knowing the NA of my objective it's a bit of guesswork, based on known systems at equivalent or higher price points? As I've spent roughly £1800 on a Euromex with a gemological base (NZ.1703‐GEMF), it sounds very likely that my target of 80x magnification is well into empty magnification territory and my resolution would plateau somewhere below the hoped-for maximum figure.

I play around a bit with macro photography, so the explanation about the zoom system stopping down the aperture as I zoom out is a useful concept for me. It seems similar to macro photography in that it is a balancing act between lighting, lenses, apertures etc. (at least we get to dodge exposure worries!). The biggest take away lesson for me is to try and utilise the auxiliary objective route rather than oculars. There is a 1.5x lens available for my microscope which would seem to balance working distance with magnification for me.

Is it correct that making changes closer to the object being viewed, is better than further away? I think I read that, but I've been unable to find the page again to re-read it.

Another thought has just struck me, at some point in the future I was hoping to take a few photographs, hence the trinocular head, but would this be compromised in some way with the eyepieces not matching the magnification of the lenses in the camera tube? I'll go and do some reading so that you don't have to spoon feed it all to me!

Once again, I'm very grateful. :)

Greg Howald
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Re: Oculars or auxiliary lens

#5 Post by Greg Howald » Thu Jan 14, 2021 5:55 am

The camera is going to do what it is going to do. I have four of them. Most digital microscope cameras give a size relative to what you would see at 40x. But that's the size of the image not the magnification of the image, so at 2x you get a 2x image. Even though it seems quite large using the camera you usually see no more detail than with your eyes at 2x.
If you go with an auxiliary Barlow type lens under the objectives you will do best not to exceed 1.5x. If you go with eyepieces you will do best at 15x. And yes the two may be able to be used together if you are lucky. But then you have most likely reached the limit. Using the objective at lower magnification you should get plenty of light with a 1 5x Barlow and 15x Eyepieces.
Since you have ordered the 15x Eyepieces I would try that first. But by all means have fun, play with it, learn.
Greg

MicroBob
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Re: Oculars or auxiliary lens

#6 Post by MicroBob » Thu Jan 14, 2021 7:33 am

Hi together,
for my 1970s Leitz Großfeld Stereocmicroscope TS I have these numeric apertures:

1,0x 0,05
1,6x 0,62
2,5x 0,8
4,0x 0,8
10x 1,0

The 1x gives an exceptionally crisp image, the 4x quite a good one, the 10x perfectly usable but not overwhelming. I don't own the others.
Since the camera can resolve higher than the eye the objective should have something to spare when observing by eye. For the three lower powers I would expect acceptable performance with a camera and a good usability with 15x eyepieces.
So this is probably one of the highest resolving affordable stereo microscopes.

When using lower resolving stereo microscopes I can see the limitations in the way tiny highlights are visible: They appear like tiny bright discs of a size that is bigger than my eyes could resolve.

There are newer stereo microscopes with n.a. of 0,2 and even 0,6 0,35 , with luck the former may be affordable for the better off amateur. Someone in this forum has shown such a stereo microscope 1 or 2 years ago, I think from Nikon or Olympus.

Bob
Last edited by MicroBob on Thu Jan 14, 2021 5:30 pm, edited 1 time in total.

Scarodactyl
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Re: Oculars or auxiliary lens

#7 Post by Scarodactyl » Thu Jan 14, 2021 4:39 pm

An NA of 0.2 with a 2x objective isn't too out of reach (the sz7 reaches that in the center of view). I don't think anyone has made a full stereo with a resolution up to 0.6--the current top end offerings all cap off around 1000 lp/mm plus or minus, so around 0.33.

MicroBob
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Joined: Sun Dec 25, 2016 9:11 am
Location: Northern Germany

Re: Oculars or auxiliary lens

#8 Post by MicroBob » Thu Jan 14, 2021 5:34 pm

Hi Scarodactyl,
you must be right. I googled and Leica says their M205 has the highest n.a. of all at 0,35:

https://www.leica-microsystems.com/de/p ... -m205-fca/

What n.a. had the most powerful stereo microscope you used and how useful did you find this?

Bob

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