Leitz Orthoplan darkfield: dedicated DF or phase condenser?

[Rorschach]

Hello,

Does someone know if there are any benefits to image quality or otherwise in using a dedicated darkfield condenser (i.e. condenser no. 88) as opposed to using the darkfield annulus that exists in the Orthoplan Zernike phase condenser (i.e. condenser 402a)? It lookse like no. 88 would allow objectives of aperture 0.7 or lower whereas the 402a phase condenser allows aperture 0.65 or lower. That seems like a small difference, unless some really useful objectives are ruled out by it. Are there such? What about any other pros and cons between these two. Obviously I am talking about working with low to medium aperture objectives here.

An additional question relates to the replaceable top lens on the 402a. I have the basic Achr. 0.9 top lens on it but also have the Apl. Oel 1.25 top lens. I am assuming there is no big benefit of using the Apl. Oel 1.25 in phase contrast work? It's use apparently would require oiling the top lens / bottom of specimen slide in addition to using an oil immersion lens (and oil there too), which seems like a pretty annoying work flow. So I guess just stick with the Achr. 0.9 top lens, even with large aperture oil immersion objectives?

A third question relates to the adapter optic K4 which is required for condensers to properly function on the Ortholan in brightfield (maybe also darkfield?), due to it being a largefield microscope. However, I assume the K4 is not needed in phase contrast work where Leitz seems to recommend only objectives of a normal field (i.e. not Plan but Nplan and even regular)? Wouldn't the only drawback be when the 402a brightfield and darkfield annuli are used? That is maybe a compromise worth it for some users, as the K4 seems really pricey.

[Alexander]

Hi,

changing the top-lens makes phase-contrast unusable. The phase rings must match the top-lens. You may use the 1.25 Apl. für bright-field and dark-field.

A dedicated dark-field condenser provides a somewhat better image quality but the difference is quite small.

[apochronaut]

Refracting condensers are never free of ca or sa both of which are more readily apparent in DF than BF. An achromat aplanat condenser has corrections for those aberrations but to only such a degree, whereas many DF condensers, especially the quite common cardioid type , are reflecting types and aberration free.

It is therefore not really possible to achieve high resolution with a refracting DF condenser..

[Rorschach]

Ok, thanks guys for the informative comments.

What about using the dedicated DF condenser that is intended for larger aperture objectives and oil immersion with those dry high aperture objectives? I would like a DF system without using oil immersion. However, I do happen to have a set of Leitz water immersion objectives that are probably 37mm parfocal, judging by the markings (the 22:1 A 0.65 Oel + W, the W 50:1 A 1.0 S= 0,5 and the W 90:1 A 1.20). No idea what the S=0.5 might mean, though. Unfortunately they have no corrections I guess, because no 'Plan, Pl or Apo' markings.

[Hobbyst46]

Ok, thanks guys for the informative comments.

What about using the dedicated DF condenser that is intended for larger aperture objectives and oil immersion with those dry high aperture objectives? I would like a DF system without using oil immersion. However, I do happen to have a set of Leitz water immersion objectives that are probably 37mm parfocal, judging by the markings (the 22:1 A 0.65 Oel + W, the W 50:1 A 1.0 S= 0,5 and the W 90:1 A 1.20). No idea what the S=0.5 might mean, though. Unfortunately they have no corrections I guess, because no 'Plan, Pl or Apo' markings.

Some time ago I compared the Zeiss oil-immersion DF condenser performance: oil vs glycerine vs water immersion (and dry objectives if I recall) on the forum.

[apochronaut]

You have to use oil immersion with a DF condenser in order to achieve an N.A. performance approaching 1.0 or above. due to the grey or dusk zone that exists between the minimum condenser N.A. and the objective N.A. It would have to be a very high N.A. condenser as well. Getting the condenser N.A. as high as possible is the key to high resolution DF., since the condenser produces a ring of illumination outside the N.A. or view of the objective. You can use any high dry objective, with .65 N.A. being roughly the limit for use with a non-oiled DF condenser and depending on the oil immersion DF condenser employed .85 to .95 is the limit. In my experience .95 is difficult to achieve. Having iris diaphragms in all objectives over .80 helps a lot because the difference between greyfield and darkfield is often .02 N.A. , even less. You will get it at .83 but not .85 sometimes.
Oil DF condensers also have limitations of field at the low magnification end irrelevant to N.A. The circle of illumination with oil DF condensers is small compared to an abbe or achromat condenser of equivalent N.A. thus many of them put up an illuminated field if only 800 microns +-, so limited to use with no less than a 25X objective, and in some cases 40X. There are high resolution DF condensers that have extra optics in the illumination beam that widen the field, some integrated such as the AO and Reichert toric type and I think others that can receive an aux. lens for sub. 40X performance, then removed for above 40X.

Using water immersion objectives has no bearing on the condenser used except that it has to have an N.A. higher than the objective and therefore must be oiled for water immersion objectives with N.A.'s in excess of about .85.

Using water or glycerin as an immersion medium for the condenser really just makes matters worse. You distort the condenser performance and lower the condenser N.A., necessitating a lower objective N.A. in order to achieve a mediocre DF.
No need to be afraid of oil. In microscopy, if you want to achieve the highest performance with limited and inexpensive equipment , oil is your friend.

[Rorschach]

Thanks, a lot of good info there, despite much of it being bad news

I will have to find out the particularities of the various Leitz DF condensers regarding the low end mags. It is really quite disappointing if the limit there turns out to be 25x or thereabouts. As to high end, point taken. I haven't seen too many Leitz objectives with an iris so that's probably going to limit my usage.

For the Polyvar, there's always the DF option on the universal condenser. Plus the two high end plan apo objectives both have an iris! Also, Reichert made a dedicated DF condenser for the Polyvar but I haven't seen one for sale at a reasonable price

[apochronaut]

The Polyvar DF condenser would have been a toric type and possibly with a wider front toric lens than earlier versions, for instance that used on the Zetopan, which only had an 18mm f.n. However, in order to save the cost of design and patenting , they might have split the DF function in two and used an existing toric condenser.
The eyepiece f.n. affects the low end magnification to which a condenser is functional by vignetting the condenser image circle with the field stop. In the case of the Polyvar, putting a DF patch in the universal condenser would cover the bases for the 4 lower magnifications and the pre-existing production toric condenser used by Reichert and AO, for the 3 upper magnifications. Thus without engineering a completely new DF condenser they would have achieved a 24mm f.o.v. coverage throughout the magnification range.
Does the condenser nose or lens pack section on your universal condenser thread out or is it fixed in place?

[Rorschach]

The Polyvar DF condenser would have been a toric type and possibly with a wider front toric lens than earlier versions, for instance that used on the Zetopan, which only had an 18mm f.n. However, in order to save the cost of design and patenting , they might have split the DF function in two and used an existing toric condenser.
The eyepiece f.n. affects the low end magnification to which a condenser is functional by vignetting the condenser image circle with the field stop. In the case of the Polyvar, putting a DF patch in the universal condenser would cover the bases for the 4 lower magnifications and the pre-existing production toric condenser used by Reichert and AO, for the 3 upper magnifications. Thus without engineering a completely new DF condenser they would have achieved a 24mm f.o.v. coverage throughout the magnification range.
Does the condenser nose or lens pack section on your universal condenser thread out or is it fixed in place?

I see that I forgot to reply to this. Good info again, thank you! By 4 lower mags you mean up to 25x and 3 upper mas are 40x and upwards, I assume?

Yes, the condenser has a lens pack that can be unscrewed. I had to do that already to gain access to the internals in order to fix a completely stuck iris system.