One question: why is condenser NA sometimes labeled as 1.4-1.2

[osterport]

I noticed that some condensers labeled as 1.4-1.2, especially darkfield, some just labeled as 1.4.
I've seen condensers like Leica condenser 0.90/1.25oil, then I know its NA is up to 0.90 without oil, 1.25 with oil.

Why is it 1.4-1.2? sometimes 1.2? Just do not know the condition to get the best performance out of it.

[osterport]

Olympus


Nikon

[apochronaut]

BF condensers have a maximum N.A. , almost always marked on them. If it isn't it sill exists and is in a catalogue somewhere. Closing the iris , reduces the maximum N.A. theoretically down to zero, if the iris can close right down. Some condensers have the iris position marked as N.A. on a scale. This is the light that floods a B.F. illumination field and marked as N.A. on the condenser , it is the maximum and the theoretical minimum is zero for all of them.

There is no direct light flooding the field in DF., only a thin circular ring of illumination outside the view of the objective. It has an inside diameter and an outside diameter indicated on the condenser as the low N.A. and highest N.A. of the circle. It is usually about .2 N.A. apart.

[osterport]

Thanks for reply! What I'm wondering is, if a DF condenser N.A. is 1.4-1.2, does it mean in most cases 1.2 is the NA that the condenser performs and only in some special cases NA 1.4 is possible?

I got the knowledge from this forum that objective NA should be 0.2 NA less than the condenser NA in case of DF, for 1.4-1.2, should we assume the maximum objective NA as 1.2-0.2 or 1.4-0.2?

I'll do some experiment to try, but my DCW condenser shows a normal objective with 1.25oil does not work with DCW at all. I start to wonder maybe we should take the assumption as 1.2 instead of 1.4.

BF condensers have a maximum N.A. , almost always marked on them. If it isn't it sill exists and is in a catalogue somewhere. Closing the iris , reduces the maximum N.A. theoretically down to zero, if the iris can close right down. Some condensers have the iris position marked as N.A. on a scale. This is the light that floods a B.F. illumination field and marked as N.A. on the condenser , it is the maximum and the theoretical minimum is zero for all of them.

There is no direct light flooding the field in DF., only a thin circular ring of illumination outside the view of the objective. It has an inside diameter and an outside diameter indicated on the condenser as the low N.A. and highest N.A. of the circle. It is usually about .2 N.A. apart.

[Phill Brown]

BF condensers have a maximum N.A. , almost always marked on them. If it isn't it sill exists and is in a catalogue somewhere. Closing the iris , reduces the maximum N.A. theoretically down to zero, if the iris can close right down. Some condensers have the iris position marked as N.A. on a scale. This is the light that floods a B.F. illumination field and marked as N.A. on the condenser , it is the maximum and the theoretical minimum is zero for all of them.

There is no direct light flooding the field in DF., only a thin circular ring of illumination outside the view of the objective. It has an inside diameter and an outside diameter indicated on the condenser as the low N.A. and highest N.A. of the circle. It is usually about .2 N.A. apart.

Excellent explanation.
There are still random acts of discovery that defy logic but getting to grips with some basics can be just as challenging.
DF has so much going for it.
Could be boiled down to night and day for observations.
DIC can be stunning but it's relatively complicated and expensive.

[Macro_Cosmos]

Do not overthink it.
It simply means:
The DF condenser's NA is 1.4.
The maximum allowable NA of the objective is 1.2.

Truthfully 1.15, at 1.2 it will depend on how well the preparation is.

Or better, cone of light outer NA is 1.4 and inner is 1.2, hence why at a matched 1.2, getting a good darkfield image is quite dependent on your sample.

[apochronaut]

The written mantra that you just have to have an objective N.A. lower than the condenser N.A. in order to achieve D.F. gets quoted mostly by people who do a lot of reading about microscopy, have websites where they make money from microscopy, profit from donations or are paid by companies to promote their products. Many of them talk a lot about microscopy but don't seem to do much of it. I can tell you from 30 years experience in DF microscopy that achieving DF at 1.1 N.A. with a 1.2-1.4 N.A. condenser is a lot harder than the words that say it can be done. It is a theoretical possibility if all the chips fall into place and you are using an immaculately clean system and a perfectly prepared sample but in the real world of live sampling under conditions with variable levels of control you have to be prepared to accept .2 below the condenser minimum and if you want true Dark Field and not Dusk Field or worse : Dork Field, even a greater reduction.

With many average quality oil condensers and certain objectives , DF doesn't occur until .85 or .80 but surprisingly good imaging can take place through an objective working at .80, especially if the condenser is working at a higher N.A. and certain aberrations and distortions such as incoherence, s.a., ca and N.A. falloff are minimized in the condenser. In BF a 1.25 N.A. objective can't really fo a great job at 1.25 N.A., if the sample is ragged and the condenser flabby and or decentered. Those are principle causes of a lot of oil immersion work in BF being a let down or even outright depressing to amateurs. With an oil DF condenser, right off the bat you are starting with a performing condenser, because it is made that way. That's why they are expensive.
In microscopy there is a misbelief that the condenser imposes a total limitation on the objective by limiting it's N.A. to that of the condenser. It isn't surprising that the defacto basic microscope peak resolution set up is a 1.25 N.A. oil immersion condenser and a 1.25 N.A. oil immersion objective. This has evolved over centuries and allows the objective to theoretically achieve 1.25 N.A. and the system can be done cheaply. Good results inexpensively. That 1.25 N.A. theoretical peak occurs under the limitations of a pretty lousy condenser by most other metrics other than the N.A. however. Eventually a better default condenser crept in to most mfgs. systems : the .90 dry achromat aplanat. Yikes but doesn't this then limit the N.A. of the objective to .90? No. It becomes roughly mid way between .90 and 1.25 due to condenser defocusing and if you use the Rayleigh Criterion for microscope resolution. Plus there is the added benefit of controlled ca, sa and coma in the dry achromat aplanat condenser plus the freedom from using oil immersion in order to kick start the condenser, something hardly anyone was probably doing anyway by the time the achromat aplanats came along. The resultant imaging quality in most cases is superior with plan objectives and very close with high mag. non-plan objectives to that obtained with an oil immersion abbe.

Now , on to DF where the situation is reversed. Is the stopped down objective putting a practical limitation on the condenser? If you use a 1.4 N.A. condenser with a .80 objective in BF does the combined N.A. equal somewhere in between ?. Obviously, no because the objective N.A. circle excludes all light outside it.
What if it doesn't exclude all of the light outside it as is the case with DF? At this point it becomes a bit confusing because the resolution laws, Abbe's, Rayleigh's, Sparrow's and even the less perceptually based FWHM are all based on diffraction limitation as a resolution measurement, so the combined N.A is a factor in that but diffraction plays a more limited role in DF. DF functions due to extreme N.A. angles of light being relayed into the objective with very few diffraction points. I have actually found very little to explain the exact mathematics of the condenser/objective N.A. relationship in DF because resolution in DF is not directly related to a point spread function between Airy discs as it is in BF.
It seems apparent though, that the maximum and minimum N.A. measurements of DF condensers provide the following.
The maximum N.A. gives a specification for resolution potential, which in combination with the N.A. of the objective used, determines the resolution of the whole. I don't think the resolution is limited by the N.A. setting of the objective alone and may be calculated based on a formula that includes both N.A.s as numbers in the equation.
The minimum N.A. gives a specification above which an objective's N.A. cannot be in order to achieve DF. It thus has an indirect effect on resolution because it limits an objective's N.A. to a maximum.

[Phill Brown]

The effect of the condenser diaphragm is similar but reversed for the NA available outside the DF.
Broadly floodlighting from all sides might give a brighter image but much of the resolution is reduced by flare at the boundaries of the subject.
Not always the case but something to take into account,oiling might not improve the resolution as relayed by the image sensor.

[Chas]

In some cases could it also be that the 1.2 or 1.25 number sometimes refers to thickness of the slide (aka slip) that the condenser is optimised to use ?
(Maybe more so when the numbers cannot be read as an ascending sequence).

The AO oil DF on this site has 'SL 1.25' on the slide.
https://microscopecentral.com/cdn/shop/ ... 1563572148

I have a Watson Zonal Darkground that says 'SLIPS 1.2mm' and a Watson Paraboloid that says '1.1mm' on the side.
A Cooke Troughton Simm's catalogue mentions that theirs "is adjusted for slides of 1.25mm thickness"

(???)

[apochronaut]

The slide thickness specification can be confused with the N.A. specification because they both are in pairs. The slide thickness usually has something like sl.th. after the spec. The two are interrelated because the N.A. is affected by the slide thickness when it is outside of specification.

[Phill Brown]

Watson zonal oil is intended for the FL50x .95 oil or stopped down 100x 1.25 to around .9.
The head is 1.5x so with 14x eyepieces it's still acceptable as the limit, it's 60+ years old so fov is not super wide and the smell is vintage.
I haven't been able to get DF at .95 with subjects on 1mm which is what modern slides seem to be mostly.
Old stock is still out there.
Happy days.