CA and condensers

[Will Milne]

Hi

In transmitted light work bright/dark/oblique illumination via flash how much influence on sensor recorded CA does a properly adjusted condenser contribute?

Can the condenser Abbe/Aplantic/other contribute to or worse create CA recorded on sensor in combination with an otherwise CA clean objective?

If so which condenser would be the most desirable from a CA free perspective?

Will

[Pau]

In my limited knowledge, a good condenser adjustement is very important (Köhler if posible, critical if your microscope don't allow Köhler), and its importance increases whith the objective N.A.

In fact, the resolution loss and uneven illumination issues are much more noticeable in photomicrography than in visual observation

The best condensers (and the most expensive ones) are aplanatic achromatic (corrected for both spherical and chromatic aberrations)
A good website for this kind of information:
http://www.zeiss.de/c1256b5e0047ff3f/Co ... 3e00368ad2

[PaulFurman]

I've been using an overhead projector for illumination. I got it from the sidewalk for free - the type used for projecting letter sized transparencies in a classroom back in the day. I think I've noticed 'rainbow' colors in reflections with this setup. Is that possible? I have it set up to illuminate the tissue covering my subject, just a few inches from the 4x6 inch mirror. There is a removable 4x6 inch translucent arch of plastic over the stage, I place tissue (now pec pads) over that plastic and blast light on that. Plenty reaches the back side for fill, the bottom is mostly black painted parts. The overhead projector has a Fresnel lens on the letter sized transparency surface, a 300mm multi-element lens on a focusable riser bar like an enlarger and a first surface mirror near the lens. Perhaps I was just seeing rainbow refractions on an intricate subject but I wonder how the whole collimated microscopy stuff might translate into this setup.

[Will Milne]

Hi

Great link Pau thanks- I am going to have to measure my condenser holder on my Wild to see if it is a standard diameter so I can look at aplantic-achromat condensers other than the Wild offers/finds. Sounds like they have the kind of correction I was hoping for.

Paul- sounds like an interesting illumination method though I'm a little lost due to inexperience in the field as to how your approach might be integrated into a scope stand/condenser/objective/hard mounted dslr setup for transmitted bright/dark/oblique lighting setup

Will

[Charles Krebs]

Will,

As Pau says... AA...Achromatic/Aplanatic are the best for photography.

I must confess that when I did some basic side-by-side comparisons it was tough to see a difference. BUT!... this was with lower power objectives in "straight" brightfield. The drawbacks of a basic 2 element Abbe will be a much bigger issue with objectives over about 0.50 NA, and I would think that oblique brightfield could also be compromised to a degree.

Here is some more good reading:

http://micro.magnet.fsu.edu/primer/anat ... nsers.html

http://micro.magnet.fsu.edu/primer/anat ... tions.html

[rjlittlefield]

Here is some more good reading:

http://micro.magnet.fsu.edu/primer/anat ... nsers.html

http://micro.magnet.fsu.edu/primer/anat ... tions.html

Charles, those are good references but they don't quite get at an issue that I've long wondered about.

It's easy for me to understand how chromatic aberration in a condenser can affect the appearance of out-of-focus elements. That's because CA in a condenser amounts to having slightly different apertures for different wavelengths. I'm used to the concept that changing the aperture changes the size and position of OOF elements. Having different changes for different wavelengths will introduce color fringes, so it seems perfectly reasonable that condenser CA will introduce color fringes for OOF elements of the subject.

But I'm having a lot of trouble understanding how CA in a condenser can affect the appearance of in-focus elements. In the optics that I know, changing the aperture may slightly change the sharpness of in-focus elements (due to diffraction), but it does not change their size and position. Hence (in the optics that I know), CA in a condenser will not introduce color fringes around in-focus elements. Only CA in the objective can do that.

As a practical matter I'm not sure this distinction matters, because most microscopy subjects include significant OOF elements no matter what you do.

But I'd like to know whether I understand the theory correctly.

Are you aware of sources that address this issue? Is it correct that CA in a condenser introduces color fringes only around OOF elements? And if condenser CA does affect in-focus elements, can you point me to some explanation that can help me to understand how it does this?

Thanks!

--Rik

[Charles Krebs]

Are you aware of sources that address this issue? Is it correct that CA in a condenser introduces color fringes only around OOF elements? And if condenser CA does affect in-focus elements, can you point me to some explanation that can help me to understand how it does this?

No I can't. Wish I could, and I'm going to look again at some of the books I have. Will's question actually re-kindled a frustration I've had. While I am interested in the physics, I'm a little more interested in it's actual manifestation in viewing and images. So when I read about the limitations of Abbe condensers and the advantages of A/A I'm always thinking.... well OK, but what does it look like in my pictures? No one seems to tell me that, only the strong recommendations for A/A for photomicrography. Maybe I'm just too dense to see something that should be obvious.

Unfortunately (or really fortunately I suppose) it's an issue I haven't bothered to delve into too much since the condenser I've been using for some time now is a quality aplanatic/achromatic... so there's not much more I can do at that end.

But it's time to look at this again and see if we can't come up with something a little less vague. This is especially true since CA is a very big issue in photomicrography. And it is sometimes hard to determine just where a problem lies. Is it in the objective, the condenser, the relay optics, the prisms in the trinocular head? A combination of some of the above?

... this CA talk reminds me of a pithy comment I read early on, wish I could remember where. It was something to the effect of how it is awfully hard to unbend light properly once it has been bent!

[Will Milne]

Hi

Thanks for links Charles very informative.

Your and Rik's comments certainly strike at the heart of what I wondering with my initial question.

well OK, but what does it look like in my pictures? ....And it is sometimes hard to determine just where a problem lies

As mention in another post I now have a pair of Nikon CF plan objectives plus on the way is a Wild Phase Contrast condenser which I picked up at a good price with a view to using the phase annulus for oblique /Col lighting explorations . My understanding is that this condenser is an achromatic design , so I have a Wild Aplantic and soon a Wild achromatic combined with the new CF objectives perhaps I can make some comparison tests that would be revealing.

Will