red/green edges around the field diaphragm

[mete]

What is the reason of the red/green edges around the field diaphragm image ?

[MichaelG.]

Chromatic aberration
Better-corrected condenser lenses exhibit less of it.

Ref, Figure 6 on this page : https://micro.magnet.fsu.edu/primer/pho ... rors2.html

MichaelG.

[mete]

I have an achromatic condenser but I still see the fringes. So it is even worse in non corrected ones then.

[MichaelG.]

I have an achromatic condenser but I still see the fringes. So it is even worse in non corrected ones then.

Yes … That’s why I wrote :

Better-corrected condenser lenses exhibit less of it.

.

MichaelG.

[mete]

is there an apochromatic condenser or why is it infeasible or unnecessary ?

[MichaelG.]

I don’t think I have ever seen a ‘conventional’ apochromatic condenser … but there are arrangements where another [matching] objective can be used as a condenser, for super-critical work.

I have also seen a camera lens used as a condenser, to great effect.

These variants are probably more practical now, than they once were, because LEDs can provide powerful lighting without heat.
[ of course they do produce heat; but from the back of the device, where it can be easily dissipated ]

MichaelG.

[deBult]

Olympus BH2 series has series of condensers with increasing level of correction e.g.:

BH2-CD standard Abbe condenser, 2 elements hardly any color correction
BH2-SC Achromatic correction, swing out condenser 4 elements in 3 groups
BH2-AAC Aplanatic Achromatic correction, 7 elements in 4 groups

Even my AAC shows (a little) color fringing, precise collimation does help to reduce this.

[josmann]

Olympus BH2 series has series of condensers with increasing level of correction e.g.:

BH2-CD standard Abbe condenser, 2 elements hardly any color correction
BH2-SC Achromatic correction, swing out condenser 4 elements in 3 groups
BH2-AAC Aplanatic Achromatic correction, 7 elements in 4 groups

Even my AAC shows (a little) color fringing, collimation does help to reduce this.

Question for ya - in what application does the AAC really improve performance? I can't imagine it matters much for BF at lower mags, but maybe it improves DF or higher magnifications?

[deBult]

I’m using all 3 types of Olympus condensers mentioned above.

Differences are subtle in my usage area (mainly pond dipping), but I’m relative new to the Olympus BH2-AAC, so YMMV.
Contrast is slightly increased when taking pictures using Plan Apo objectives (10, 20, 40*). Do not have the 100* Apo.

Oiling both the condenser and the objective side gives a beautiful image with the SPlan 100*, (though I mostly use water immersion between condenser and object glass, cleaning oil at the condenser top is messy job I detest), please note I’m not using the 100* all that much, so limited experience in the combination with the AAC here.

The AAC does not match the full image circle of the 4’ objective, which is a shame when doing plant cut’s, hence the use of the SC flip top condenser.

General “feeling” using the AAC is of a slight “better” image, “pleasing” to the eyes, but not sure how to pinpoint or describe.

No DF experience using the AAC.

Doing diatom pictures at 100* and using the top grade Olympus objectives is an application area where it really shines.
Would love to have a Phase contrast version.

Is it a “must have”: not really, but I’m very fortunate having one (well actually 2).

[apochronaut]

Abbe condensers suffer from extreme coma and spherical plus chromatic aberrations off axis. The cone of light exiting only achieves the specified N.A. in a fairly narrow circle covering 50 or 60% of it's total image circle. This means that an average achromat objective, which one would expect to also exhibit a degree of poor peripheral corrections, can work pretty well to spec. with an abbe condenser. Abbe condensers were never recommended for apochromats or fluorites, which are more highly corrected across the field, irregardless of whether they are plan or not. If you find an abbe condenser in use with fluorite or apochromat objectives, it is likely a case of budget restraint because abbe condensers are cheap. Abbes still work. Just not as well as they should.

With plan objectives, where correction across the field is expected, and thus consistent resolution across the field, a condenser that maintains it's N.A.. edge to edge ( aplanatic) is required. Usually this design is incorporated into an achromatic specification as well. Widefield high N.A. abbe condensers also have a wider projection of a full N.A. but they are almost as expensive to manufacture as an achromat aplanat, so they are not as common as either abbes or achromat/aplanats.

A special type of 2 lens condenser is the aspheric condenser, based on the 2 lens principal of the abbe condenser but with one lens surface correcting for spherical aberration and coma due to it's aspheric curvature. These are usually known as abbe aspheric and they perform about the same as an achromat/aplanat.

I have a d.i.y. apochromat condenser. I have not finished evaluating it yet and at this point it seems to be best as a water immersion apochromat for higher N.A. objectives

So, the main achievement of more highly corrected condensers is consistent N.A. over a wider field.

[mete]

@deBult:
I have (Zeiss) aplanatic achromatic and I was thinking color fringes are not much visible, hence my question, but it seems it is not the case.

I think Zeiss does not have achromatic only, it is either standard or aplanatic achromatic, but I dont know maybe the standard or some of the standard ones are corrected to some extent.

What do you mean by “precise collimation does help” ? a build quality attribute or something user adjustable ?

@apochronaut:
I think I cant appreciate your answer enough because I am new to the hobby, but thanks for the detailed info. I had the gut feeling I need achrom.aplan. to fully appreciate semi-apos, it seems I was correct.

[BramHuntingNematodes]

Some collimation of the light source is usually available. Adjustment of the condenser is somewhat more difficult. Zeiss standards have centerable condensers while old B&Ls require some unscrewing and fiddling with. There is a trend among new lab scopes to factory adjust and lock everything down real tight, so there may be some role for QA to play.

[mete]

In Axiolab 5, the light assembly from the light source to the field diaphragm seems like sealed at the factory.

[Hobbyst46]

@deBult:
I have (Zeiss) aplanatic achromatic and I was thinking color fringes are not much visible, hence my question, but it seems it is not the case.
I think Zeiss does not have achromatic only, it is either standard or aplanatic achromatic, but I dont know maybe the standard or some of the standard ones are corrected to some extent.

When setting up Kohler illumination, and closing down the field diaphragm, the difference between the basic Zeiss condenser (with the flip-out top lens) and the achromat-aplanat is obvious: strong color fringes vs uniform, single color spot, respectively. The fringes diminish when the field diaphragm is opened according to the NA of the objective.

[apochronaut]

Some collimation of the light source is usually available. Adjustment of the condenser is somewhat more difficult. Zeiss standards have centerable condensers while old B&Ls require some unscrewing and fiddling with. There is a trend among new lab scopes to factory adjust and lock everything down real tight, so there may be some role for QA to play.

Your giving a bad impression of B & L. Most companies( including B & L) had a range of condensers and condenser bodies. Remember that the condenser is the lens system and the thing that the lens system fits on is the condenser body , which also usually carries the diaphragm(s). Older systems usually had a sleeve or yoke that the condenser body fit into. There were two or three options of condenser bodies at different price points. One was a type fixed to the stage and factory centered. Sometimes it would be in a sliding sleeve or spiral so it could be raised or lowered manually. The next type was fitted in a mount that could be raised or lowered on a rack. This was also factory centered, usually by 3 screws. If you only do BF, a factory centered condenser is fine and if for some reason it gets decentered it can be adjusted and tightened. The above systems were put on standard small lab scopes and student scopes usually but the condenser system was a flexible option.
The third type used a condenser body that had built in xy centering controls, usefull if several condenser types were swapped in and out.

By the 1980's the idea of having the xy controls on the condenser body disappeared and those were transferred to the yoke itself. Condensers became more integrated with the diahragm and fewer of them could have the condenser section swapped around.

[BramHuntingNematodes]

Oh and with the Dynazoom you can move the head around with an Allen wrench

[apochronaut]

That was fairly common but also an option. It keeps prying fingers in institutions where many use an instrument, from willy nilly messing with stuff.
The series 4 had a drop out nosepiece on a circular dovetail but it doesn't drop out unless you use an allen wrench to loosen two of the centering adjustors. Cumbersome. When you replace those with thumbscrews, it drops out easily and recenters when reinstalled. Allen screws or thumbscrews are a choice.