Darkfield - is resolution lost?

[75RR]

I think this is a good moment to bump a link to what is probably the best video ever on microscope light paths and image formation fundamentals.

viewtopic.php?f=25&t=3573&p=34254&hilit

[Hobbyst46]

...I can fabricate adapters out of wood and plastic also.

I would hesitate to make a male dovetail or male sleeve of wood. Tiny splinters might fall inside the optics; also, wood might swell or shrink and deform upon changes in air humidity. PVC (if not very old), black Delrin are OK.

[MichaelG.]

I think this is a good moment to bump a link to what is probably the best video ever on microscope light paths and image formation fundamentals.

viewtopic.php?f=25&t=3573&p=34254&hilit

Just to say that I was at mmc2019 today, and mentioned this [the fact that he is still greatly appreciated] to Peter.

MichaelG.

[75RR]

I think this is a good moment to bump a link to what is probably the best video ever on microscope light paths and image formation fundamentals.

viewtopic.php?f=25&t=3573&p=34254&hilit

Just to say that I was at mmc2019 today, and mentioned this [the fact that he is still greatly appreciated] to Peter.

MichaelG.

Glad you did!

So where is the selfie with the great man?

For those of us who do not live within commuting distance, this is as close as we get: https://www.mmc-series.org.uk/

[apochronaut]

Heather, I had in mind that you had mentioned it was for a 37mm sleeve, somewhere. Is that correct? The bottom of the picture of the condenser looks a bit dovetailish but that is probably an artifact of the way the image was created.

The skinniest part of the dovetail looks to be about 39mm so 37 would fit, and the fattest part of the dovetail (the base) is about 46mm. I can fabricate adapters out of wood and plastic also.

Thank you for your help.

Heather

Good. So it has a dovetail type mount. If you could fashion a male dovetail ring for it that had a common size interior, and three set screws with which to lock the adapter to a sleeve type condenser, then you could choose from several available condensers when they show up; and they will. It's not as immediate as buying off the shelf but the results will be better and cheaper.

There are however, several condensers out there that I know of that were built with a dovetail of quite similar proportions and by chance one of them usually sells very cheaply because it is for an older model horseshoe based microscope that used a remote illuminator. There are very few around that do DF with an older microscope like that. It is a Cardioid and the internal optical design was used in subsequent condensers for many years. It has a silvered mirror, not aluminized. To be sure that it is a fit I need some more measurements from your condenser. 1) the height of the dovetail section in mm as well as the vertical thickness of the part of the dovetail yoke that fits around the condenser dovetail. 2)the height of the condenser itself from the bottom of the dovetail to the tip.

The condenser in question has some degree of height adjustment, which was the fashion in the horseshoe microscope era because the condensers were made to fit into various microscopes with differing substage architectures. In this case too, the older and less expensive stands had sleeve type mounts whereas the higher end stands had dovetail mounts, so the condenser is in fact a sleeve type( 39mm) but was also fitted with a dovetail adapter that fit over the sleeve and locked with a recessed set screw. The dovetails were of different sizes, depending on the microscope series, with the closest one to the measurements you have provided being 46mm at the bottom and 40.5mm at the top. Check your top measurement carefully. They are hard to do and a 1mm or so error is easy to apply. 46 to 39mm is a pretty steep dovetail angle.
Nevertheless, it has a 39mm sleeve and if an existing dovetail doesn't fit , you should be able to fashion one.

[apochronaut]

...I can fabricate adapters out of wood and plastic also.

I would hesitate to make a male dovetail or male sleeve of wood. Tiny splinters might fall inside the optics; also, wood might swell or shrink and deform upon changes in air humidity. PVC (if not very old), black Delrin are OK.

If that were true emergency rooms would be full of people choking on splinters after eating salads out of wooden bowls. I'm pretty sure a dovetail spun up on a lathe out of any dense hardwood wouldn't be splintery.

[Hobbyst46]

...I can fabricate adapters out of wood and plastic also.

I would hesitate to make a male dovetail or male sleeve of wood. Tiny splinters might fall inside the optics; also, wood might swell or shrink and deform upon changes in air humidity. PVC (if not very old), black Delrin are OK.

If that were true emergency rooms would be full of people choking on splinters after eating salads out of wooden bowls. I'm pretty sure a dovetail spun up on a lathe out of any dense hardwood wouldn't be splintery.

I thought of sub-millimeteric splinters and wood dust particles, people do not choke on such. I agree that quality hard wood is less liable to release them.

[carlh6902]

Darkfield is inherently lower resolution than brightfield, and also is not a faithful representation of the specimen image, due to the loss of the zero-order light in the fourier-reconstructed (via interference) final image.

Carl

I should have been more clear in what I was trying to say. True, the finer details are in the higher orders of refraction. The resolution loss comes in when you have to limit the objective NA, such as is typical when using a 100x objective, in order to exclude the zero order light.

The loss of zero order light causes optical distortions, due to incomplete Fourier content affecting the resulting interference pattern.

Carl


Cheers,
John

[Sauerkraut]

I think this is a good moment to bump a link to what is probably the best video ever on microscope light paths and image formation fundamentals.

viewtopic.php?f=25&t=3573&p=34254&hilit

Just to say that I was at mmc2019 today, and mentioned this [the fact that he is still greatly appreciated] to Peter.

MichaelG.

Thank you both. I just watched this excellent video and found it very useful and inspiring.

[Sauerkraut]

Heather, I had in mind that you had mentioned it was for a 37mm sleeve, somewhere. Is that correct? The bottom of the picture of the condenser looks a bit dovetailish but that is probably an artifact of the way the image was created.

The skinniest part of the dovetail looks to be about 39mm so 37 would fit, and the fattest part of the dovetail (the base) is about 46mm. I can fabricate adapters out of wood and plastic also.

Thank you for your help.

Heather

Good. So it has a dovetail type mount. If you could fashion a male dovetail ring for it that had a common size interior, and three set screws with which to lock the adapter to a sleeve type condenser, then you could choose from several available condensers when they show up; and they will. It's not as immediate as buying off the shelf but the results will be better and cheaper.

There are however, several condensers out there that I know of that were built with a dovetail of quite similar proportions and by chance one of them usually sells very cheaply because it is for an older model horseshoe based microscope that used a remote illuminator. There are very few around that do DF with an older microscope like that. It is a Cardioid and the internal optical design was used in subsequent condensers for many years. It has a silvered mirror, not aluminized. To be sure that it is a fit I need some more measurements from your condenser. 1) the height of the dovetail section in mm as well as the vertical thickness of the part of the dovetail yoke that fits around the condenser dovetail. 2)the height of the condenser itself from the bottom of the dovetail to the tip.

The condenser in question has some degree of height adjustment, which was the fashion in the horseshoe microscope era because the condensers were made to fit into various microscopes with differing substage architectures. In this case too, the older and less expensive stands had sleeve type mounts whereas the higher end stands had dovetail mounts, so the condenser is in fact a sleeve type( 39mm) but was also fitted with a dovetail adapter that fit over the sleeve and locked with a recessed set screw. The dovetails were of different sizes, depending on the microscope series, with the closest one to the measurements you have provided being 46mm at the bottom and 40.5mm at the top. Check your top measurement carefully. They are hard to do and a 1mm or so error is easy to apply. 46 to 39mm is a pretty steep dovetail angle.
Nevertheless, it has a 39mm sleeve and if an existing dovetail doesn't fit , you should be able to fashion one.

I removed the condenser and measured the width of the condenser holder on the scope itself: 42mm
Height of dovetail section (including the filter holder) is 15mm. About 12.5mm excluding the filter holder
Overall height of condenser is 72mm and apx 57 mm excluding the dovetail portion
Vertical thickness of the yolk, if you mean on the scope itself, is less than 1mm because it tapers to a point.

BF_Condenser.JPG (48.72 KiB) Viewed 9726 times

[Sauerkraut]

I would hesitate to make a male dovetail or male sleeve of wood. Tiny splinters might fall inside the optics; also, wood might swell or shrink and deform upon changes in air humidity. PVC (if not very old), black Delrin are OK.

If that were true emergency rooms would be full of people choking on splinters after eating salads out of wooden bowls. I'm pretty sure a dovetail spun up on a lathe out of any dense hardwood wouldn't be splintery.

I thought of sub-millimeteric splinters and wood dust particles, people do not choke on such. I agree that quality hard wood is less liable to release them.

Challenge accepted!

[apochronaut]

The picture I am seeing, is of your abbe condenser. Is the DF one pretty much the same height or were the measurements taken from the DF condenser? The DF condenser I have in mind as a possible inexpensive replacement( picture in the link) is quite a bit shorter and would need a bit of modification to get that high, even with the condenser nose screwed as high as possible.
By making a dovetail or using the existing detachable dovetail and fitting it under the condenser( there are 3 existing threaded holes spaced at the 3 apexes of a 40mm equilateral triangle on the bottom), the height would then be about 45mm from the top of the dovetail to the top of the condenser.

Your condenser track would need to make up about 12mm of further vertical movement. Is that possible?

[apochronaut]

Oh, yes. That important bit of detail. The one I was looking for had been up for a while at 40.00 but I can't find it. It had the advantage of having a built in 6v. 18 watt bulb. I have swapped those out in such a condenser for a 6v. led signal light bulb with good results. That replaces the microscope illuminator and adds in the possibility of 60X and up DF, if wanted.

However, that condenser is the same as this one , only including a built in illuminator. This one listed at 75.00, or best offer. Has a case too. The dovetail ring is removable and could be fitted to the bottom. You can screw the nose out about 3/4 cm., maybe more if it has good tight threads.
https://www.ebay.com/itm/American-Optic ... gbdcNTnkis

There is also this paraboloid type made by B & L. This is the entire condenser optical system, which threads into a sleeve mount type condenser body fitted with centering screws. These were used up until about 1965 by B & L. The thread section allows for a precise adjustment for height.
Since most microscopes capable of DF made more recently have centering screws built into the dovetail yoke, the actual centering body for this condenser is not required. A capable diy'er could make an appropriate custom plain dovetail mount to fit their scope and use these condenser optics. The price is right. Paraboloids are slightly less good than cardioids when it comes to colour correction at high magnifications but have a slight edge in the size of the illumination circle. With this one, I think you can get down to 20X at a 20mm f.o.v.; 10X with a narrower f.o.v. but I haven't checked out what that actually is. Many typical cardioids can fill a 20mm field at 25X and up but there is a dark corona with a 20X. This may be why , certain mfg. issued 25X objectives, rather than 20X. AO and Reichert did a different thing. They used a toric lens under the condenser , which increases the image circle but reduces the intensity. In this way, you can use a Cardioid Toric condenser for 10X-100X, only the illumination intensity is a little low at the high end. This shouldn't be of consequence, since for any use of DF over 600X, it is advisable to use 50 watts or more of a filament illumination source, or a self illuminated DF condenser , as described above.
https://www.ebay.com/itm/Bausch-Lomb-Pa ... SwohdZmMhv

[Sauerkraut]

The picture I am seeing, is of your abbe condenser. Is the DF one pretty much the same height or were the measurements taken from the DF condenser?

Sorry about that. The Abbe shows close to max measurements. The DF condenser height is shorter at 62mm total with 10 mm of that being for the dovetail portion. I feel confident I can make any mod needed to address size differences as long as the unit is not too tall to fit in the space. Or at least it would be fun to try even if it chalks up as another failure, aka learning experience.

The cardioid one looks easier to mod in some ways. Looks like the seller is away for a few days so no hurry on making that decision.

Just curious what you think of this one too. Another member pointed it out a while back and I've wondered if it would work:

https://www.ebay.com/itm/Carl-Zeiss-Kar ... SwVVdcdjDJ

Thank you for the time and expertise you have put into this.

[Sauerkraut]

As an alternate idea, there are a few Swift DF/PC scopes on eBay. The link below is for one such unit and the same seller has the less expensive version - the regular Quodmaster - without the 100x phase set up previously listed at $240 + shipping with had best offer option. The listing expired but the seller likely still has a few. My only real issue with this scope is it is not trinocular and has 15x eye pieces that I don't know if can be swapped out easily for 10x (standard sizing or not?).

From past posts at least one member acquired a similar Swift scope and seemed initially pleased with it.

https://www.ebay.com/itm/Swift-M4000D-B ... 3931558507

[MichaelG.]

Slight digression, and apologies for the cluttered photo

Does anyone know how this works ?

Planktonkondensor.jpg (140.39 KiB) Viewed 9660 times

... it could be an interesting DIY project

MichaelG.

[75RR]

Slight digression, and apologies for the cluttered photo

Does anyone know how this works ?

Might be something like this:

https://micro.magnet.fsu.edu/primer/tec ... field.html

[apochronaut]

As an alternate idea, there are a few Swift DF/PC scopes on eBay. The link below is for one such unit and the same seller has the less expensive version - the regular Quodmaster - without the 100x phase set up previously listed at $240 + shipping with had best offer option. The listing expired but the seller likely still has a few. My only real issue with this scope is it is not trinocular and has 15x eye pieces that I don't know if can be swapped out easily for 10x (standard sizing or not?).

From past posts at least one member acquired a similar Swift scope and seemed initially pleased with it.

https://www.ebay.com/itm/Swift-M4000D-B ... 3931558507

The Zeiss cardioid would be as good as any other. There isn't too much to choose between various condensers, as long as the design is a cardioid type. The only thing I would clarify from the seller is just how wide that collar is. It looks like it might be 40mm or more so, you have to make sure there is clearance. The collar section on the AO/Spencer is 38 3/4mm and the projecting nose section is about 25mm wide X 25mm high, projecting above the collar.

Regarding the Swift. Yes 10X eyepieces are available( I have a pair for very little, if you need them). However, the objectives are not plan and although the seller says that Quadmasters also do BF and DF; that would be true of just about any microscope; if one had the equipment to do that. Although you can do BF with phase objectives, there is some loss of contrast, so the quality of the Bf is less than using dedicated BF objectives. For DF, you would still need a DF condenser to effect DF with the higher magnifications, although you can get pretty good results using a 100X phase annulus as a stop for the 4X and 10X objectives.

If you are looking for another microscope that had phase and DF capability , I would be looking for one of the 100 watt microscopes that allows you to use a 100X objective for DF; unless you feel you would be happy with a 40X limit. Should be about the same tab as the Swift, which is showing me a price of 400.00. I think for that much , you could do much better.

[MichaelG.]

Slight digression, and apologies for the cluttered photo

Does anyone know how this works ?

Might be something like this:

Yes, quite possibly, altough I thought those were just diaphragms, for use with an existing condenser.
Perhaps I was being excessively literal in my interpretation of "configuration of diaphragms"

MichaelG.

[Hobbyst46]

Slight digression, and apologies for the cluttered photo

Does anyone know how this works ?

Might be something like this:

Yes, quite possibly, altough I thought those were just diaphragms, for use with an existing condenser.
Perhaps I was being excessively literal in my interpretation of "configuration of diaphragms"

MichaelG.

A quick survey of the ancient German literature that mention "planktonkondensor" (by Zeiss Jena or Leitz) indicates that their purpose was low power objectives.

[MichaelG.]

A quick survey of the ancient German literature that mention "planktonkondensor" (by Zeiss Jena or Leitz) indicates that their purpose was low power objectives.

That seems reasonable, given that the caption card mentions up to 8mm field of view.

MichaelG.

[Sauerkraut]

The Zeiss cardioid would be as good as any other. There isn't too much to choose between various condensers, as long as the design is a cardioid type. The only thing I would clarify from the seller is just how wide that collar is. It looks like it might be 40mm or more so, you have to make sure there is clearance. The collar section on the AO/Spencer is 38 3/4mm and the projecting nose section is about 25mm wide X 25mm high, projecting above the collar.

Regarding the Swift. Yes 10X eyepieces are available( I have a pair for very little, if you need them). However, the objectives are not plan and although the seller says that Quadmasters also do BF and DF; that would be true of just about any microscope; if one had the equipment to do that. Although you can do BF with phase objectives, there is some loss of contrast, so the quality of the Bf is less than using dedicated BF objectives. For DF, you would still need a DF condenser to effect DF with the higher magnifications, although you can get pretty good results using a 100X phase annulus as a stop for the 4X and 10X objectives.

If you are looking for another microscope that had phase and DF capability , I would be looking for one of the 100 watt microscopes that allows you to use a 100X objective for DF; unless you feel you would be happy with a 40X limit. Should be about the same tab as the Swift, which is showing me a price of 400.00. I think for that much , you could do much better.

Thank you for this information. I'll snag a good used DF condenser Like that A.O or Zeiss cardioid for my scope then. Should be fun to tinker with.

Heather

[apochronaut]

The four main physical specifications to consider when adapting any condenser from one microscope to another are.

1) does the mount fit or can it be converted to fit without an undue amount of alteration. There are basically two types of mounts. Sleeve and dovetail. Sleeve mounts come in various incremental sizes and are almost always in mm. Occasionally, two brands will have the same size. Sleeves can be shimmed up but only trimmed down with a lathe or by removal and replacement , if possible. Dovetails are mostly proprietary in size and shape, although there is some uniformity in budget scopes. Sometimes the dovetail can be removed in it's entirety and replaced with another, sometimes not. Sometimes a dovetail can be added to a sleeve type condenser, sometimes not. Theoretically, a dovetail could be altered with a lathe. 3-D printing has expanded the options.

2) Can the condenser be made tall enough to provide proper contact with the underside of the slide.

3) Is the condenser narrow enough to clear any substage parts or protuberances.

4) Can any controls , such as an iris diaphragm lever be clear and accessible.

5) Will the installed system have a facility to center it. Some microscope condenser yokes have centering controls and some don't. If there are no centering controls, then those need to be on the condenser. Microscopes with simple non-centering substages are designed to be pre-centered. Installing an accessory condenser is not really feasible because it them would have to be centered using tools and then changed again, when the other condenser was called for. You could install a condenser upgrade, however , center it, and leave it there.

Optical specifications are another issue. Sticking with oil immersion DF condensers, there are four main types, all frequently found in the marketplace. They are not always marked as to what type they are. There are distinguishing features.

1) Toric Cardioid. Not any different than a cardioid, except that they have a wider circle of illumination and can be used with lower magnification objectives( down to about 20X, possibly 10). They can be distinguished by having a bi-spheric lens in the bottom and a hemispherical mirror up inside, above the lens. High freedom from chromatic aberration.

2) Cardioid. Has a hemispherical mirror up inside, when looking from below. High degree of freedom from chromatic aberration. Can be used down to about 25X.

3) Paraboloid. Has a noticeable DF stop visible when viewed from below. Moderate to high freedom from chromatic aberration. Can be used with lower power objectives, down to about 20X, possibly 10, with some.

4) Stop type. Noticeable DF stop visible. These are standard condensers fitted with a DF stop. The freedom from ca will be as good as the condenser is capable of. You could for instance have a 1.4 N.A. achromat aplanat fitted with a DF stop and it would probably be pretty good but it would also probably cost about as much as a cardioid. Unless otherwise claimed or can be verified, I think it can be assumed that most oil type DF condensers being advertised for budget prices are plain condensers with a DF stop built into them. O.K. for low mags. but at much over 40X, the performance will lag far behind the above three types.

There are a few other types, mainly higher end but these are rare and seldom found. When they are, they are usually expensive but the performance might reflect that.

The field that that each condenser will illuminate is fixed. If your microscope has an 18mm f.o.v., then the condenser will cover a greater percentage of that field than if it was 22mm. If it covers the field with a 10X objective at an 18mm field, it most likely won't at a 22mm field. That's why the minimum field coverage for the various condensers is a bit of guesswork. Even if the manual for the condenser says it will work down to 10X, you need to know at what field number that is, in order to determine what lower magnification it will work at as an adaption to your system.

[Sauerkraut]

Thank you Apochronaut, for this comprehensive summation of all the information needed to create add-on dark field. My favorite magnifications are 10x and 20x, so this gives me more to think about in targeting the right style of condenser. My scope has beautiful BF images through 20x. By 40x there is enough loss of depth of field that I don't use it much. It's just ok. And by 100x - forget about it. That objective got demoted to the drawer. Wish 30x objectives were commonly made.

It will be fun to try a few things and figure out what works best.

[Hobbyst46]

Thank you Apochronaut, for this comprehensive summation of all the information needed to create add-on dark field. My favorite magnifications are 10x and 20x, so this gives me more to think about in targeting the right style of condenser...

I will try and check how my "new" antique Zeiss (West) cardiodid condenser performs with 10X0.25, 10X0.30, 16X0.40 and 25X0.45 objectives (I am afraid it does'nt..). Perhaps a dry condenser will be a better choice for your microscope. Or a turret phase contrast condenser, if Omano carry such condensers, because the highest NA phase anulus enables DF.

[Hobbyst46]

...

So here is a small test. My very old Zeiss (West) cardiodid condenser is not in a great condition. Some of the paint has chipped off, and I had to cover it with black insulation tape to minimize flares. But anyway, these are quick and dirty images of a (too dense) slide of diatoms in Pleurax. Since I am already satisfied with it performance with high NA objectives, I tested it with low NAs: 10X0.22, 10X0.30 Neofluar, 16X0.32 Plan, 25X0.45 Plan. and 40X0.65 Plan.
All dry of course.
Images are only resized for upload, no other PP.

It works, and DF is obtained. The main problems IMO are :
(1) uneven illumination across the FOV. I tweaked the centration and height of the condenser, and it could be improved, but had to re-adjust for every objective. I am not sure that it will be like this with other condensers, but overall, it is not bad.
(2) for the 10X and for a 6.3X0.16 objective (image not shown), the condenser cannot fill the whole field. The smaller the NA, the less it can be filled - quite like what happens with an ordinary BF condenser when the swinging-out top lens is out, and no auxiliary lens (underneat the condenser) is there to widen the illuminating beam. For the cardiodid as well, an auxiliary lens can widen the beam, but it seems to introduce eneven flares etc so, not recommended.

Zeiss did not make a 4X lens for my microscope. So the lowest mag was 6.3X.

For the 40X0.75 Neofluar Ph2 (dry) objective, beautiful DF is obtained, and with higher NAs (oil) too, at least up to NA=1. The 100X1.3 (no plan no phase, no nothing) yielded a quasi-DF, still lovely {images not shown}.

Edit: comparison with a dry condenser - the phase contrast no 3 port on a turret condenser. This produces fairly similar images with the 6.3X and 10X objectives; across-the-FOV illumination can be obtained with the auxiliary lens. Yet the dark background is inferior than that obtained with the oil condenser. Furthermore, the Ph3 port does not give DF at all with objectives of NA>0.30. {images not shown}.

Hope it helps.

[PeteM]

I think this is a good moment to bump a link to what is probably the best video ever on microscope light paths and image formation fundamentals.

viewtopic.php?f=25&t=3573&p=34254&hilit

Just wanted to thank you for that. It's outstanding and new to me.

[MichaelG.]

So where is the selfie with the great man?

No selfie ... but here are the important bits, from my 1:1 demo session:

Evennett_demo.jpg (134.5 KiB) Viewed 9512 times

Evennett_rig.jpg (196.18 KiB) Viewed 9512 times

The man and the kit are both still doing a great job.

MichaelG.

[apochronaut]

(2) for the 10X and for a 6.3X0.16 objective (image not shown), the condenser cannot fill the whole field. The smaller the NA, the less it can be filled - quite like what happens with an ordinary BF condenser when the swinging-out top lens is out, and no auxiliary lens

Although it seems like it might be N.A. dependent ,it isn't. The size of the illuminated circle is fixed by the condenser, and as the real f.o.v. gets smaller with increased magnification, at some point it matches the illuminated circle or to put it in reverse, the objectives in declining magnification observe a progressively larger field and eventually at a certain field size the illumination circle matches it, then passes inside the f.o.v.
It wouldn't matter if the N.A. of a 10X objective were .90, the illuminated circle would still fall short of the f.o.v.

[Hobbyst46]

I stand corrected. Thanks.