Follow up on Saul's Dark Field Adapter

[Harold]

I decided to try my hand at dark field microscopy so I bough the kit for my Olympus BH2 from µStuff and started playing with it yesterday. Now I've got a couple questions.

First, the supplied dark field stops can be mounted on the top of the insert or in the filter tray at it's bottom. Which position is correct?

Second, the stops come in a variety of sizes, and I'm not sure which one would be best. I'll be using the adapter with 10x and 20x S-Plan objectives.

So far, I've been able to get some very interesting views. In the process, I've learned that dark field is exquisitely sensitive to any dust or other contaminants in the optical train. Any further tips on technique would be very welcome. Thanks

[dtsh]

Darkfield can be really nice, but as you're finding it is very sensitive to debris and precise centering of the condenser (or stop in your case) will greatly affect the result.
My DF setup is with a cardioid condenser, but the result is similar with stops (my One-Fifty Phase uses a stop). You may have to adjust the condenser height as well.

I found the procedure for centering in the AO darkfield manual helpful, the centering parts begin on page 4 and it *should* work much the same even if the tech used to get the cone differ (the One-Fifty Phase is factory-aligned (or in my case, user re-aligned)).

https://user.xmission.com/~psneeley/Per ... ctions.pdf (thank you, carlh6902)

[PeteM]

The stops will likely work best mounted at the top of the adapter - near to the condenser iris.

It's trial and error to figure out which ones work best for you. You should find examples that work very well in the range of 4x to 20x and pretty well at 40x objective magnifications. Beyond that you'll likely needed a dedicated darkfield condenser.

And, yes, you want everything (slide, mount, cover slip, optics) scrupulously clean to avoid glowing bits if side-lit debris.

[Harold]

Thanks both of you for your advice. I figured the best way to show what I've got so far is to post a picture. This one was taken thru the 20x SPlan with a stop about 12mm in diameter placed in the filter tray on the bottom of the adapter. Don't understand the color change of the Stentor, but it's not objectionable. The rotifers that got into the frame are a nuisance.

https://imgur.com/jK30Slr

[dtsh]

That doesn't look too bad to me, especially for early results with a new illumination technique.

[Harold]

That doesn't look too bad to me, especially for early results with a new illumination technique.

DTSH,

Thanks. That shot was cherry picked out of maybe 30 attempts. Still too much artifact. Things look great visually, but getting really good focus on photos is hit or miss. Seems like the depth of field is even more constrained that bright field. I've got an S-Plan 40x objective on the way, and I'm thinking about Amscope dark field condenser with the 37mm mounting flange. Not sure if I want the oil or dry version.

[PeteM]

As you probably know, the dry and oiled darkfield condensers are destined for different ranges of numerical aperture. Might want to first decide what magnification you want - and then the darkfield condenser?

High power oil immersion darkfield typically requires a 100x oil immersion objective with an iris - the iris to move its numerical aperture (and angle at which light is accepted) to below that of the condenser. And then the proper condenser oiled to the bottom of the slide - and the slide oiled to the objective. The results can be spectacular, but more work is involved.

Your image looked great to my eyes. FWIW, I suspect the majority of stunning darkfield images seen on the web have taken a detour through Photoshop before presenting themselves in public with uniformly black darkfield backgrounds.

And then there is focus stacking to get the desired depth of focus.

[Harold]

As you probably know, the dry and oiled darkfield condensers are destined for different ranges of numerical aperture. Might want to first decide what magnification you want - and then the darkfield condenser?

High power oil immersion darkfield typically requires a 100x oil immersion objective with an iris - the iris to move its numerical aperture (and angle at which light is accepted) to below that of the condenser. And then the proper condenser oiled to the bottom of the slide - and the slide oiled to the objective. The results can be spectacular, but more work is involved.

Your image looked great to my eyes. FWIW, I suspect the majority of stunning darkfield images seen on the web have taken a detour through Photoshop before presenting themselves in public with uniformly black darkfield backgrounds.

And then there is focus stacking to get the desired depth of focus.

The image I posted was run through GIMP to be cropped. It got a slight touch of noise reduction but is otherwise unadulterated. I'd like to use enough magnification to make focusing an image on my video screen easier for my weak old eyes. Usually using the 20x and 40x objectives. I haven't tried 100x mainly because the FOV is too small.

For now, I'll stick with Saul's DF adapter, but I am considering the oil version of the AMscope DF condenser. The dry one doesn't allow for centering, and that appears to be critical. Meantime, I am just amazed at the detail DF renders visually. It's like a whole new little world.