LOMO 2.5 Objective Focusing

[riorio44]

Hello all, I am having trouble getting my LOMO 2.5x 0.05 objective to focus on a POLAM 213. The other objectives are fine with focusing, just the 2.5 I am having issues with. When I remove the ocular, I can see that the specimen is in focus, however when I add the ocular I cannot see the specimen. Is there a truck that I'm missing to be able to focus? I have the standard setup for the POLAM.

Thanks!

[75RR]

The NA (Numerical aperture) of your condenser is probably too high for such a low magnification objective.

Most condensers will have a way to deal with this by either removing the top lens or inserting a lens under the condenser.

In the image below the removal of the top lens reduces the NA from 1.4 to 0.32

[abednego1995]

Not sure with LOMO, but some manufacturers have different parfocal distances for low power objectives, especially for those under 4x.

Cheers,
John

[MichaelG.]

Not an instrument I have ever handled ... but I have just found a short article here:
http://www.smsi.org/publications/mn199808.pdf
Which indicates that two condensers were provided
... and, crucially, refers to an auxiliary ground glass condenser lens for use with the 2.5x'

MichaelG.

[75RR]

Not an instrument I have ever handled ... but I have just found a short article here:
http://www.smsi.org/publications/mn199808.pdf
Which indicates that two condensers were provided
... and, crucially, refers to an auxiliary ground glass condenser lens for use with the 2.5x'

MichaelG.

Good catch + interesting microscope ... this is from page 16
.

[riorio44]

Thank you all very much for your replies. I'm sorry for not responding sooner, I was out of town and away from the microscope. I had seen that article before but am a little confused on what the 'ground glass condenser lens' exactly is with the Polam.

Under the condenser set up is a swing out lens which looks like this:


The lens is frosted, and swings out of the way from the light path to the condenser, so I believe this would be what the article is referring to. I have tried with and without this lens, both with no luck.

Alternatively there are two filters, which I believe go onto a circular disk above the mirror for the light source. One appears frosted and the other a neutral density filter. I have tried the frosted one both with and without the swing out lens with no luck.

I have also tried to remove the 0.85NA lens on top of the condenser with all setups as I've already mentioned and no luck there either.

When attempting Kohler illumination, I can tell that the aperture diaphragm is focusing, but it's only when the objective is nearly in contact with the top condenser lens and below the stage.

The 2.5x objective appears to be the same length as the others, not sure if that would have anything to do with my focusing issues. Attached are the objectives lined up together.

[MichaelG.]

Maybe a daft idea, but ... Have you tried using that ‘auxiliary’ without the main condenser ?

Something may have been lost in translation

MichaelG.
.

Edit: Just found another useful link, which states :

Condenser Universal : CN-UPOL
N.A.0.85/0.30 Strain-free achromatic with removable top element.

http://www.envirotools.hu/ateso_fenyu_p ... oszkop.htm

[riorio44]

I hadn't tried that before. Unfortunately it still isn't focusing. I had tried to remove the 0.85 lens of the condenser with no luck. I placed the neutral density filter over the light output and it allowed me to see a blurred image, yet it still would not focus.

I have a feeling it's something simple that I'm missing. I'm just not exactly sure what I'm doing wrong.

[75RR]

Did you place (what looks like the auxiliary lens - see below) in its holder or just hold it by hand under the condenser?

Also can you take a photo so we can see through it.

[BramHuntingNematodes]

Ground glass and frosted glass are the same here.

[75RR]

Doesn't make sense that it be frosted. Perhaps ground in this case means shaped?

[MichaelG.]

I have a feeling it's something simple that I'm missing. I'm just not exactly sure what I'm doing wrong.

May I suggest doing a simple ‘sanity check’ on the focussing.

Forget the condenser problems for a moment ...

According to the ‘envirotools’ link that I posted, the working distance for the 2.5x is 9.0mm
That gives plenty of room to try top lighting a piece of printed card, or the label on a slide.
... Just try that, to make sure that the objective does focus !!

MichaelG.

[MichaelG.]

Doesn't make sense that it be frosted.

Why not ? ... it’s a common method of producing even illumination over a wide field.

MichaelG.

.
Edit: See p429 for a brief overview: https://books.google.co.uk/books?id=Yab ... &lpg=PA429
.
Edit: This example on ebay looks to be frosted:
https://www.ebay.co.uk/itm/LOMO-condens ... 3761728674

[75RR]

Doesn't make sense that it be frosted.

Why not ? ... it’s a common method of producing even illumination over a wide field.

MichaelG.

.
Edit: This example on ebay looks to be frosted:
https://www.ebay.co.uk/itm/LOMO-condens ... 3761728674

Problem is not even or wider illumination it is focus.

On the other hand, given that it is much shorter than the 10x and has a working distance only 1mm longer (9mm vs 8mm) perhaps it is worth lowering it to 9mm by eye (i.e. looking at it from the side).

[MichaelG.]

Problem is not even or wider illumination it is focus.

On the other hand, given that it is much shorter than the 10x and has a working distance only 1mm longer (9mm vs 8mm) perhaps it is worth lowering it to 9mm by eye (i.e. looking at it from the side).

... Hence my suggestion of the top-lit ‘sanity check’

I fear that two separate issues are being conflated.

MichaelG.

[BramHuntingNematodes]

Doesn't make sense that it be frosted. Perhaps ground in this case means shaped?

Our own Leitzcycler has documented the "absolutely necessary" use of auxiliary ground glass (frosted) filters in the LOMO Biolam below the condenser, seen here:


It might mean "shaped," but it would be both unidiomatic and highly confusing to any potential users that also have experience with the Biolam. Anything is possible in LOMOworld though!

[75RR]

.
In the image you show (which I hope you don't mind me adapting) there is a frosted filter 'A' and a auxiliary lens 'B'. It is this lens 'B' that I was referring to, I do not think it is frosted.

Also the word ground in my experience does mean to shape a lens (See Hubble image below)

https://www.nasa.gov/content/hubbles-mirror-flaw
.

[BramHuntingNematodes]

Yes we do grind lenses but a "ground glass" lens would be an unidiomatic way of referring to an unfrosted lens shaped by grinding, unless a philologist can point us to some canonical examples of this usage. To use it this way when describing an optical system that does contain, as described above, at least two actual ground glass (frosted) elements would be unfathomable recklessness on the part of localizing team for the manual or reviews.

Two bits of frosted glass seems redundant though; perhaps one of them has been switched with the correct element.

[BramHuntingNematodes]

It occurs to me also that the substage could be eliminated with a 2.5 objective by doing some rough reflected light microscopy by placing something flat and opaque on the stage and shining a flashlight on it. I can do examinations this way with a 10x objective quite easily. EDIT: Oh michael G has already suggested this. Well, I concur!

Did LOMO have a variety of tube lengths?

[riorio44]

Thanks for all the replies. I will do my sanity check in a few hours when I get home .

I tried manually holding the condenser and raising it up from the light source to the bottom of the stage, no luck.

The auxiliary lens photo I posted was from the internet, not my actual lens. Mine is frosted and is also doubly convex, one side more so than the other, if I can remember right. I'll take some photos of the auxiliary lens also.

[MichaelG.]

Also the word ground in my experience does mean to shape a lens (See Hubble image below)

.
Fair comment ... but that is a stage in the manufacturing process, usually followed by polishing.

More commonly, ‘ground glass’ refers to a surface like this:
https://petapixel.com/2019/12/23/how-to ... n-at-home/
... and what’s good for displaying an image is also good for diffusing a light.

MichaelG.
.

P.S. ___ Did you look at photo 2 of 4 on the ebay listing that I linked ?

[75RR]

More commonly, ‘ground glass’ refers to a surface like this:

Agree, but the text actually says ground glass condenser lens, one would think they would say ground glass filter if that was what it was.

P.S. ___ Did you look at photo 2 of 4 on the ebay listing that I linked ?

I did, might just be me but it looked clear.

riorio44 says that the lens he showed, which also looked clear, is not his but downloaded from the internet :)

Hopefully the issue is just the lack of parfocality and once he lowers the objective enough it will focus.

[MichaelG.]

We are obviously going round in circles, so this is my last comment until we see real pictures of the actual ‘auxiliary condenser’ from riorio44’s POLAM

A ‘ground glass condenser’ typically describes a plano-convex lens with a ground finish on the plano face
... Which is exactly what I think I see in ebay photo 2

MichaelG.
.

Edit: It is now evident that riorio44’s has a double convex

[Leitzcycler]

I am very pleased to see my photos has some importance here Maybe I exaggerated it is absolutely necessary. However, according to my experience, it helps a lot. Sorry my poor English as I am not so familiar with all terminolgy. The round glass plate (A in picture) is a diffuser, would the better expression be matte? The lens (B) is a normal lens, so it is clear.

[Leitzcycler]

And yes, it looks exactly like the glass plate in the video.

[riorio44]

The sanity check failed. Top/side lighting the slide and trying to focus, specifically at the 9mm mark was unsuccessful.

Here is my auxiliary lens:

aux lens.jpg (43.24 KiB) Viewed 7892 times

Here are views showing the convex top and bottom:

aux lens bottom convex.jpg (55.86 KiB) Viewed 7892 times

aux lens top convex.jpg (56.58 KiB) Viewed 7892 times

The auxiliary lens over light:

aux lens light.jpg (37.87 KiB) Viewed 7892 times

I attempted to drop the slide all the way down to the light source and back with no focusing.

When I remove the ocular/eyepiece and observe the slide, it is in focus. As soon as the eyepiece is reinserted the image becomes blurry. The eyepiece works fine with the other objectives, so I don't believe that is the issue. It is a 10x eyepiece.

Also of note, I experimented a little with the condenser. When I close the aperture diaphragm all the way, I can faintly see the specimen in decent focus. It is by no way sharp, but you can see it somewhat.

Is there something simple in my setup that I'm missing?

[MichaelG.]

The sanity check failed. Top/side lighting the slide and trying to focus, specifically at the 9mm mark was unsuccessful.
[…]
Is there something simple in my setup that I'm missing?

Sorry to hear that ... I’m struggling to think what the problem might be.

But it does seem to suggest that there is something wrong with that 2.5x objective.

MichaelG.

[75RR]

When I remove the ocular/eyepiece and observe the slide, it is in focus. As soon as the eyepiece is reinserted the image becomes blurry. The eyepiece works fine with the other objectives, so I don't believe that is the issue. It is a 10x eyepiece.

Clutching at straws here but I wonder if the intermediate image is at a different height when using the 2.5x.

After confirming that you can see the specimen with the eyepiece removed, place the eyepiece back and then slowly raise it by hand.

+ Thanks for the image of your "ground glass condenser lens", it is as you say "frosted and is also doubly convex"

I suppose it is also possible (actually quite likely) that it might need a combination of the 0.85NA condenser and the ground glass condenser lens.
.

[MichaelG.]

I suppose it is also possible (actually quite likely) that it might need a combination of the 0.85NA condenser and the ground glass condenser lens.
.

Perhaps even more likely is that it needs a combination of the 0.85NA condenser with the top removed to make it 0.30NA, and the ground glass condenser lens. [ref. posts #7 and #8, above]

Edit: But, for the moment, this is largely irrelevant if the top-lit ‘sanity check’ failed.

... Oh how I wish for sight of an Instruction Manual, written in English.

MichaelG.

[MichaelG.]

Nothing is ever certain ... but this appears to be the specification for the 2.5x
https://rafcamera.com/catalog/product/v ... polarized/

MichaelG.