Poor resolution? Beware of condensers!

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GaryB
Posts: 521
Joined: Fri Jul 29, 2016 12:18 pm

Poor resolution? Beware of condensers!

Post by GaryB »

Here's an interesting puzzle I only just figured out after a lot of brain mashing. I wasn't getting the resolution I was expecting, especially as my 44x objective lens has an NA of 0.85. Very high for a 40 - 44x objective and yet the clarity, while good, wasn't 0.85 good. I fiddled for ages trying to get a good sharp view. I figured the lens maybe poor quality so I bought a Leitz 40x 0.65 off ebay hoping I would get decent quality with that lens. Sadly it was just as bad in other ways. Lower resolution but better contrast... where to look next.

*AHA! Condenser*

Condensers are very useful for getting the light condensed onto your subject matter, setting the diaphragm for the optimal contrast/resolution mix and getting rid of excess glare etc. However, one big problem I didn't realize I had with my scope, a beautiful brass 1903 Bausch and lomb, was that the condenser has an Numerical Aperture (NA) of 1.2 because the microscope came with an oil immersion lens with an NA of 1.3. Not a perfect match but close enough. So what, you say...

Here's the problem, in general, a condenser is supposed to be used racked up to almost touching the bottom of the slide. The adjustment to raise and lower is not for focusing, but for access to filters, inserts and the like, but it's 'use' position is normally up. Ideally it should focus the light at the top of the slide where the specimen lies. Unfortunately, with an NA of 1.2 it cannot do that because unless you use oil between the condenser and slide, it can never focus because the NA is higher than air (NA 1.0). It's exactly the same reason you can't focus an oil objective without oil. If you look down your eyepiece and lower the condenser, the quality of image goes down with it. At a high NA, it's effectively always down and never focused where it needs to be for dry objectives.

This hit me like a bolt of lightening when I realized what was going on. I pulled the condenser out to test my theory and found I couldn't focus on anything, even touching the glass with a needle tip. I removed the lower lens of the Abbe condenser and could then focus on things. It still needed the lower lens for light collection so I added a plano-concave lens to lower the overall NA. Now I can focus the light on the subject and just like that, an almost miraculous transformation occurred.

This 113 year old objective is magnificent! It's so far above the much newer Leitz objective it's not even funny. The level of detail is pretty astonishing now I figured out what the problem was. So there you have it, a cautionary tale if you have a high NA condenser without a flip top lens or don't use oil objectives. Unless the condenser light can focus properly it will look more muddy than it should, similar to closing the iris too much. You lose resolution and clarity.

Pau
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Post by Pau »

Nice that you realized the importance of the condenser!

I ignore the condenser details of your venerable ancient microscope, but I find some of your affirmations confusing, for example:
Here's the problem, in general, a condenser is supposed to be used racked up to almost toughing the bottom of the slide. The adjustment to raise and lower is not for focusing, but for access to filters, inserts and the like, but it's 'use' position is normally up. Ideally it should focus the light at the top of the slide where the specimen lies. Unfortunately, with an NA of 1.2 it cannot do that because unless you use oil between the condenser and slide, it can never focus because the NA is higher than air (NA 1.0). It's exactly the same reason you can't focus an oil objective without oil. If you look down your eyepiece and lower the condenser, the quality of image goes down with it. At a high NA, it's effectively always down and never focused where it needs to be for dry objectives.

This hit me like a bolt of lightening when I realized what was going on. I pulled the condenser out to test my theory and found I couldn't focus on anything, even toughing the glass with a needle tip. I removed the lower lens of the Abbe condenser and could then focus on things. It still needed the lower lens for light collection so I added a plano-concave lens to lower the overall NA. Now I can focus the light on the subject and just like that, an almost miraculous transformation occurred
- Aside other utilities, in fact the condenser rack is to focus the field diaphragm (the one in the light source) at the subject plane for Köhler illumination

- With a normal condenser like the widespread Abbe you can focus with any objective. If NA is very mismatched you could have lower resolution and excessive contrast and DOF (too closed or too low) or too glare (too open) but you could achieve focus even with a high NA oil objective oiled.

- A Abbe condenser 1.2 not oiled to the slide will provide about 0.9 NA, more than enough for a 0.85 objective. For bright field you need to close it a bit to have good contrast

- I don't understand why you want to lower the condenser NA changing lenses. The normal way to do it is just closing the condenser aperture diaphragm

There are lots of good resources online on the subject
google
"microscopy from the very beginning" filetype:pdf
to get a most useful Zeiss booklet

This recently linked video also explains the basis in an excellent way:
https://www.youtube.com/watch?v=60_jgZtyR6U
Pau

GaryB
Posts: 521
Joined: Fri Jul 29, 2016 12:18 pm

Post by GaryB »

I'm always happy to be proven wrong in my assumptions as long as I learn something. :D

I was instructed that the condenser was supposed to be in it's up position, and on my scope, any adjustment down made things really ugly, so from that I assumed that was the correct info. I don't have Köhler yet, still collecting the parts to build the lamp.. one day it'll all arrive, but I am close.

As to the specifics of your reply, as I said, this condenser cannot focus on the subject on my scope. I went through a barrage of tests off the scope and on it with acetate paper to check focus points, beam spread etc. It would only focus on an object or focus a light beam above the stage with the concave lens added to extend it's focal length a little. Technically, no oil may mean an effective .9, but it still cannot focus on it's own front plane. It may be a bad design but B&L were working with Zeiss and Schott glassworks at the time on it's optics and I believe Ernst Abbe was running Zeiss then.

Lowering the overall NA (rather, lengthening it's focal point) allowed the light to focus a couple of mm above the top of the condenser where it needed to be. Just shutting the aperture compounded the problem and looked overly contrasted With thick dark detail and not very clear. If I now lower the condenser, it has the same murky look it had before the modification. This was very much a process of trial and error over a good few hours. The fact that I'm now getting a much sharper and cleaner view without having to shut the iris more than 10-20% of view is great. The details are startlingly clear, before they were heavy and undefined. It had much less of an effect on the Leitz .65 because it's not highly resolving to begin with.

Contrast is still a little issue as the old lens is pre coating days, it has virtually zero aberrations, again, helped by the clear focus so less fringing from the condenser. Getting a sharp focus wasn't difficult, getting it clean and seeing the subtle details was an entirely different matter. The best thing is none of it is permanent so I can still try and optimize it more. This was a first run at it as proof of concept, and in my case it worked marvelously.

One very nice addition of this particular condenser is that it has a top and bottom iris, one for normal use, the top one closes over the top element which really helps with glare more than the lower one. I close the lower one until it's a little in the objective back plane, then adjust the top one to match then put the eyepiece in and presto! Pure amazing.

All I can take from this is my own empirical evidence after literally weeks of being disappointed. If I had any idea this lens could perform like this I would never have bought the Leitz objective.. I was ready to junk the B 'n' L. Theory is all well and good, but this is not my vivid imagination, it literally is that good now. Even with it's lower contrast I can see so much more, frustules are no longer lines with some vague texture, now they're ridges with pores inbetween, what were circular holes are now hexagonal.

Really, I wish I could show you. I've spent the last few hours going over my tester slide of ocean debris at 550x and even at that magnification the diatoms here are tiny yet still bursting with detail I've only seen on the web until now. It may be that my condenser is incorrectly set but I don't have the tools to make the adjustments I'd like to try and it's very old and the threads are gummed up with some sticky compound to keep them in place, only the lower lens is easy to remove, the top one won't shift. It could be a simple case of adjusting the distance between the two lenses via their screw threads but that top lens just won't move and the lower one has a seat.

Anyway, I'm glad you pointed out my errors and I'm going to read that Zeiss book. Here's comedy for you, I was the one that posted the link to the video :lol:

Oh, one other snippet, I just mounted a bunch of dried diatoms and tiny plant matter with a blob of CA glue (superglue) and set a cover glass on it. It basically sets as acrylic and really shows off diatoms and greenery nicely. Just make sure it's mostly dry first, CA glue sets with moisture. Too much moisture and it will crystallize in an ugly way, but it does make a really nice and quick permanent mount, optically, very clear and hard. Accidentally hitting it with an objective has little effect as it's now a solid from top to bottom.

Perl
Posts: 294
Joined: Tue Oct 03, 2006 11:43 pm
Location: Sweden

Post by Perl »

Well you need to know how to use the condenser
and have the equipment matched againt its parts
Regards
Pär Lundqvist
****** Seeing is Believing ******

GaryB
Posts: 521
Joined: Fri Jul 29, 2016 12:18 pm

Post by GaryB »

I'm learning, slowly figuring it out... bit by bit. Over the last couple of days I managed to finally get some decent images from my camera with it after a lot of fiddling about with lighting etc.

I'm having a fun time messing with everything and seeing what happens. Sometimes good, sometimes not. All of it enjoyable, a whole new world of exploration and discovery while learning about optics, lighting and adjustments.

I haven't spent a lot of money on it but having a great time making do with old gear and DIY, two of my favorite things :D

Perl
Posts: 294
Joined: Tue Oct 03, 2006 11:43 pm
Location: Sweden

Post by Perl »

Nice To hear !
****** Seeing is Believing ******

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