Ancient Leitz Wetzlar darkfield condenser

[imkap]

I couldn't resist buying this, it was on an auction so...

apochronaut you once mentioned a toric condenser, this might be similar?

I'll try to adopt it on BHS. Although I'm not sure what is the purpose of the brass thing, has no lens. Looks like an eyepiece, maybe used for centering?

20230126_201751.jpg (67.13 KiB) Viewed 1758 times

[dtsh]

My guess on the brass part would be perhaps a funnel stop to reduce the NA on higher NA objectives. The AO darkfield uses one for the 97x/100x objectives and they look similar, though that one appears larger than the AO examples, so might not be the same thing. For the AO ones, there's a field stop at the back of the objective you unthread and replace with the funnel stop.

[MichaelG.]

.
This should be of interest:

http://microscope.database.free.fr/Acce ... oscopy.pdf
.
MichaelG.

[apochronaut]

In the time frame that condenser was made the lens pack of high N.A. , short focal length objectives was down in the bottom if the barrel.. Either funnel stops or iris diaphragms were employed in order to lower the N.A. for DF. Most companies offered both options, a funnel stop was cheaper and more practical for a dedicated DF microscope and the more expensive iris diaphragm useful where both BF and DF were frequently .
A sparable barrel was not uncommon, so the lens section could easily be accessed because some early funnel stops were only long enough to fit down into the lens section. and a common upper barrel spacer section could be used for more than one objective, in order to economize priduction.
Leitz took this one step further, making both a funnel stop and an iris integrated into the hollow upper barrel. In this an objective could be converted to DF with the correct funnel stop. While this was no different than other designs where the funnel was dropped in, it was more secure and in the case of swapping over to an iris diaphragm, the same diaphragm could be threaded into many different objectives. With other designs the iris had to be integrated into the barrel. A much more costly design.
I know of no other DF condenser that was toric, other than those made by Reichert and AO. Toric DF condensers have a doughnut shaped lens in the bottom.

That should be a very nicely working DF condenser. The older ones had real silvered mirrors.

[imkap]

This should be of interest:

http://microscope.database.free.fr/Acce ... oscopy.pdf
.
MichaelG.

This is of interest, the rest of the site is great too, thanks for sharing. I think the one in the manual is a bit newer, but the principles are the same.

My guess on the brass part would be perhaps a funnel stop to reduce the NA on higher NA objectives. The AO darkfield uses one for the 97x/100x objectives and they look similar, though that one appears larger than the AO examples, so might not be the same thing. For the AO ones, there's a field stop at the back of the objective you unthread and replace with the funnel stop.

Either funnel stops or iris diaphragms were employed in order to lower the N.A. for DF. Most companies offered both options, a funnel stop was cheaper and more practical for a dedicated DF microscope and the more expensive iris diaphragm useful where both BF and DF were frequently .

Thanks, then it is a funnel stop. 1/12 is written on it. It has an RMS thread and an another thread down, so I guess one would put it inside the objective and screw it in the turret. I can't really visualize how it would look like, but

That should be a very nicely working DF condenser. The older ones had real silvered mirrors.

I'll clean it properly and try to print the adapter for BHS and try it out.
I did realize by holding it with my hand on the microscope, that unlike with what I'm used to (Olympus 1.4 with a printed stop, or a Zeiss turret condenser with DF), this one seems to give better image when not too close to the slide. Probably the best image was while I held it 2-3com down. It doesn't have a bottom lens, so maybe that's the reason. I checked for 5minutes, so not much

The main reason I got this was that the my 1.4 condenser doesn't fill the image circle on 3x and 4x objectives and that every speck of dust on top of the condenser is visible while doing DF, hmmm maybe if I'd oil (glycerin) it, it might help... I feel when it is kept a bit lower that the dust won't be visible.
The original condensers (DCD) are very expensive and rare to come by. There was one in Australia a few months ago that sold for 150€ maybe, I thought it was expensive then, but now I realize I missed a good chance

[apochronaut]

This should be of interest:

http://microscope.database.free.fr/Acce ... oscopy.pdf
.
MichaelG.

This is of interest, the rest of the site is great too, thanks for sharing. I think the one in the manual is a bit newer, but the principles are the same.

My guess on the brass part would be perhaps a funnel stop to reduce the NA on higher NA objectives. The AO darkfield uses one for the 97x/100x objectives and they look similar, though that one appears larger than the AO examples, so might not be the same thing. For the AO ones, there's a field stop at the back of the objective you unthread and replace with the funnel stop.

Either funnel stops or iris diaphragms were employed in order to lower the N.A. for DF. Most companies offered both options, a funnel stop was cheaper and more practical for a dedicated DF microscope and the more expensive iris diaphragm useful where both BF and DF were frequently .

Thanks, then it is a funnel stop. 1/12 is written on it. It has an RMS thread and an another thread down, so I guess one would put it inside the objective and screw it in the turret. I can't really visualize how it would look like, but

That should be a very nicely working DF condenser. The older ones had real silvered mirrors.

I'll clean it properly and try to print the adapter for BHS and try it out.
I did realize by holding it with my hand on the microscope, that unlike with what I'm used to (Olympus 1.4 with a printed stop, or a Zeiss turret condenser with DF), this one seems to give better image when not too close to the slide. Probably the best image was while I held it 2-3com down. It doesn't have a bottom lens, so maybe that's the reason. I checked for 5minutes, so not much

The main reason I got this was that the my 1.4 condenser doesn't fill the image circle on 3x and 4x objectives and that every speck of dust on top of the condenser is visible while doing DF, hmmm maybe if I'd oil (glycerin) it, it might help... I feel when it is kept a bit lower that the dust won't be visible.
The original condensers (DCD) are very expensive and rare to come by. There was one in Australia a few months ago that sold for 150€ maybe, I thought it was expensive then, but now I realize I missed a good chance

The standard oil paraboloid, bispheric or cardioid DF condensers usually illuminate about an 800 micron circle, so not even a 20X at a 20mm f.n. That is the reason for the toric lens. It spreads the illumination out to about 2000 microns.
Your best bet to achieve DF with low mag. objectives is to use a 40X phase diaphragm. Even a 20X might work for 4X. I should check on that.

[Chas]

You may have noticed them already, there are a couple of finely engraved rings on the front glass to help you centre it:

engraved rings with arrow 800.jpg (110.58 KiB) Viewed 1573 times

[Most old DF condensers seem have them but these ones are extremely fine and hard to see.. an oblique light across the top of the stage can help to pick them out]

[patta]

Oh my gosh so cool things those ancient setups!!

[Chas]

Something like a 39.5mm condenser mount though
Well mine is.. Imkap's looks to be quite a bit smaller diameter mount (?)

[apochronaut]

Many but not all DF condensers have centering rings. They are not always that useful as is the case with this condenser, at least the version that I have. Mine is an older version of the " CONDENSOR FÜR DUNKELFELD apert. 1.20, which is likely the minimum aperture, not the maximum.
The centering rings have to have a diameter that is outside the field of view. On some condensors they are smaller and just outside the field of view of the maximum field that the condensor will illuminate. On this one they are much larger and are only useful for a gross alignment. The fine alignment takes place using cues provided by the condensor itself, if the microscope does not have critically centered objectives, and lots of microscopes don't so the condensor needs to be centered for each objective. This is not dissimilar to the requirement for alignment of phase rings only in this case it needs to be done each time an objective is used. Those rings are pretty useless for that.
That's about the only fault that Leitz condensor has and it offers up useful centering cues. If DF is going to be used a lot, it makes sense to parcenter the nosepiece.

As the oiled condensor is raised to make slide contact, there will be a sudden flash of light in the slide at contact. With a 10X objective roughly focused and a small subject unstained slide you can easily center it because the condensor field is less than the microscope field, unless you have unnusually narrow field eyepieces. Old 10X compens eyepieces may have been narrow enough for the condensor field to cover. The 10X objective will display a bright spot of light of about 60% of a 20mm field number ( about 1200 microns across) once oil contact is made. At this time focus the objective accurately. As the condensor is very slowly raised after oil contact, a dark central section about 1/2 the size of the bright spot will be there, getting smaller as the condensor is raised until it disappears. Keep it centered until it vanishes. At this point, check the objective focus again and slowly continue to raise the condenser until the bright spot is brightest and perfectly centered with the objective focused. The condenser will be very close to the slide at this point. it may even lift it, if the oil is too thick, so keeping the slide down is paramount. With thick slides the dark spot may not disappear. I found this condenser works really well with modern thinner slides such as 1mm slides, a bit unusual for an old DF condensor.
With a 20X objective and up that Leitz condensor will illuminate a 20mm field #, again unusual for an old DF condensor.

It is easy to see why having parcentered objectives makes DF a breeze. Otherwise you have to again center the condensor with each objective and each time the condensor is used multiple times.

[Hobbyst46]

...

The Zeiss instruction manual recommends that optimization of the condenser height (AFTER contact of the oil drop with the slide)
is facilitated by 1) closing down the field aperture, 2) focusing (with the 10X-25X objective), 3) adjusting the condenser height to yield a small sharp light spot, 4) centration and 5) opening the field aperture to cover the FOV. I find this Kohler-analogous sequence very convenient on the Zeiss.

[apochronaut]

Closing the field aperture would probably sharpen the edge of the condenser field. It sounds like the Zeiss condenser has a narrower condenser field than this Leitz. Using the field diapragm with this one at 25X would be only partially helpfull.
The Zeiss may have a higher N.A.peak, which would make it more usefull at very high N.A.s. This Leitz is a bit unusual in that the back lens surface( the oil surface ) is a little wider so the optical field might be wider.

[woyjwjl]

Many but not all DF condensers have centering rings. They are not always that useful as is the case with this condenser, at least the version that I have. Mine is an older version of the " CONDENSOR FÜR DUNKELFELD apert. 1.20, which is likely the minimum aperture, not the maximum.
The centering rings have to have a diameter that is outside the field of view. On some condensors they are smaller and just outside the field of view of the maximum field that the condensor will illuminate. On this one they are much larger and are only useful for a gross alignment. The fine alignment takes place using cues provided by the condensor itself, if the microscope does not have critically centered objectives, and lots of microscopes don't so the condensor needs to be centered for each objective. This is not dissimilar to the requirement for alignment of phase rings only in this case it needs to be done each time an objective is used. Those rings are pretty useless for that.
That's about the only fault that Leitz condensor has and it offers up useful centering cues. If DF is going to be used a lot, it makes sense to parcenter the nosepiece.

As the oiled condensor is raised to make slide contact, there will be a sudden flash of light in the slide at contact. With a 10X objective roughly focused and a small subject unstained slide you can easily center it because the condensor field is less than the microscope field, unless you have unnusually narrow field eyepieces. Old 10X compens eyepieces may have been narrow enough for the condensor field to cover. The 10X objective will display a bright spot of light of about 60% of a 20mm field number ( about 1200 microns across) once oil contact is made. At this time focus the objective accurately. As the condensor is very slowly raised after oil contact, a dark central section about 1/2 the size of the bright spot will be there, getting smaller as the condensor is raised until it disappears. Keep it centered until it vanishes. At this point, check the objective focus again and slowly continue to raise the condenser until the bright spot is brightest and perfectly centered with the objective focused. The condenser will be very close to the slide at this point. it may even lift it, if the oil is too thick, so keeping the slide down is paramount. With thick slides the dark spot may not disappear. I found this condenser works really well with modern thinner slides such as 1mm slides, a bit unusual for an old DF condensor.
With a 20X objective and up that Leitz condensor will illuminate a 20mm field #, again unusual for an old DF condensor.

It is easy to see why having parcentered objectives makes DF a breeze. Otherwise you have to again center the condensor with each objective and each time the condensor is used multiple times.

I didn't find "centering rings" on DCW-W 1.4-1.2. Is that right?

If yes, how to center?

[apochronaut]

That's an Olympus, correct? It's possible there isn't one, because many condensers don't have them. They seem like a good idea in theory but only if they barely surround the circumference of the illumination field, so can be seen with the objective that is the next lowest in magnification than the lowest one that the illumination field fully covers. Using the illumination spot with a low power objective works as well to center.
Sometimes the ring is there but hard to see when looking at the condenser top with the naked eye. It shows up while observing through a low power objective. Have you tried using a 4X? The illumination spot will be small and easily centerable.
From there, you will have to tweak the condenser with each objective by carefully observing oblique lighting artifacts on the subject unless you are confident that your objectives are parcentered. Unfortunately, with very high magnification objectives a decentered condenser is more damaging to the image, so it can't be stressed too much that taking the time to parcenter your objectives and keep them there makes life a lot easier, especially if one is using DF regularly. Objectives and nosepieces with crappy machining often need to be centered using thin bevelled shims against the objective shoulder, so the objective can be orbitally adjusted. I took an over 120 micron variance out of a 60X Chinese planachro that way.

[viktor j nilsson]

Objectives and nosepieces with crappy machining often need to be centered using thin bevelled shims against the objective shoulder, so the objective can be orbitally adjusted. I took an over 120 micron variance out of a 60X Chinese planachro that way.

Never heard of that approach, thanks for the tip! Do you have an image of those shims?

[apochronaut]

Objectives and nosepieces with crappy machining often need to be centered using thin bevelled shims against the objective shoulder, so the objective can be orbitally adjusted. I took an over 120 micron variance out of a 60X Chinese planachro that way.

Never heard of that approach, thanks for the tip! Do you have an image of those shims?

They are just standard parfocalizing shims. I ended up with a bunch that came from a dealer of various thicknesses. I put a flat file that is wider than 20mm on them and start a bevel on them, finishing on a sandpaper pad.
Firstly, I rotate the objectives around the nosepiece to find the position where the two highest magnifications are closest to each other. Usually with quality microscopes there is at least one position where they are very close if not dead on. They can be brought into center if needed, using a tiny amount of bevelled dressing on a very thin shim as above. The other objective receives a similar undressed shim or thereabouts to maintain parfocality. So let's say that the 100X and 40X are parcentered then. I then do the 20, if it is too far off, then the 10. Each gets a shim that is very close in thickness to the others to maintain parfocality. Then the 2, if needed.Then the 4. The critical ones are the two highest and whichever objective is used to align the condenser. On my Diastar for instance, that is the 4X, so the 4, 40 and 100 are critically parcentered. For the 2, 10 and 20, I just used a little strip of aluminum foil on one side between a parfocalizing shim and the objective. Everything is quite close and since I have 3 nosepieces , the DF set never comes out. I have that DF/BF set, a phase contrast/low magnification DF set set and am working on a full glycerin set, which also will be DF capable.
If you have to remove objectives from the nosepiece and put them back, in order to use others, you might consider adhering the parcentering shims to the objective somehow. I haven't thought too much about that. They at least should be marked so they go back in the same holes.

[imkap]

You may have noticed them already...

I didn't notice, mine has entering rings too, thanks for the tip Chas. What microscope are you using your condenser with?
Mine is smaller than yours 29mm threaded inner condenser tube and 36mm the inner ring also threaded.

I just did some experiments, the DF is good, but the DIY darkfield patch on AAC 1.4 yields better results. Mostly because we can get a lot more light with it. The only advantage is that the dust isn't visible, although today it wasn't visible with the AAC either.

A lot of interesting info here, as always.

[apochronaut]

It is hard to compare a DF patch under a refracting condenser to a cardioid or paraboloid because an oil DF condenser really doesn't come into it's own until the N.A. of the system gets larger than about .70, perhaps even higher. They are designed to parse out details at higher N.A.s that cannot be observed in BF.

[viktor j nilsson]

They are just standard parfocalizing shims. I ended up with a bunch that came from a dealer of various thicknesses. I put a flat file that is wider than 20mm on them and start a bevel on them, finishing on a sandpaper pad.
Firstly, I rotate the objectives around the nosepiece to find the position where the two highest magnifications are closest to each other. Usually with quality microscopes there is at least one position where they are very close if not dead on. They can be brought into center if needed, using a tiny amount of bevelled dressing on a very thin shim as above. The other objective receives a similar undressed shim or thereabouts to maintain parfocality. So let's say that the 100X and 40X are parcentered then. I then do the 20, if it is too far off, then the 10. Each gets a shim that is very close in thickness to the others to maintain parfocality. Then the 2, if needed.Then the 4. The critical ones are the two highest and whichever objective is used to align the condenser. On my Diastar for instance, that is the 4X, so the 4, 40 and 100 are critically parcentered. For the 2, 10 and 20, I just used a little strip of aluminum foil on one side between a parfocalizing shim and the objective. Everything is quite close and since I have 3 nosepieces , the DF set never comes out. I have that DF/BF set, a phase contrast/low magnification DF set set and am working on a full glycerin set, which also will be DF capable.
If you have to remove objectives from the nosepiece and put them back, in order to use others, you might consider adhering the parcentering shims to the objective somehow. I haven't thought too much about that. They at least should be marked so they go back in the same holes.

Excellent, many thanks for taking the time to describe your approach.