Problem with a dark field

[konrad2137]

Hello everyone!
I decided to do dark filed microscope like here on a film
[youtube]https://www.youtube.com/watch?v=pDjWb2NhHbk[/youtube]

On 200x the effect is good, the field is really dark and the objects are bright.

DSC_0017.jpg (396.49 KiB) Viewed 5110 times

However, on 800x there is no such effect.

DSC_0022.jpg (324.02 KiB) Viewed 5110 times

What is the cause and what can I do with it?

[75RR]

Each magnification requires its own sized Darkfield Stop. The higher the magnification the larger the Stop needed.

Up to about 20x it is quite easy to do*, past that one needs more accuracy and eventually for the highest magnifications an oiled Darkfield Condenser is required.

* It is actually related to the objective's NA, but it is simpler to think of it in terms of objective magnification.

See link for two pdf files that you can print on acetate:

viewtopic.php?f=12&t=134

[MicroBob]

Hi Konrad,
what apertures do the objectives have and what type of condenser are you using?
For high n.a. dark field you need a special oil immersion cardioid dark field condenser and it has to fit the thickness of the slide.

Bob

[Hobbyst46]

Example discussion of darkfield, and the critical importance of the numerical aperture (NA) of the objective for achieving a good DF, along the thread in the post:
viewtopic.php?t=5786
Example from the internet, google: "Chapter 9 Dark Field Microscopy © C. Robert Bagnell, Jr., Ph.D., 2012"

[75RR]

This is a link to an extract on Darkfield Microscopy that I found useful:

https://www.dropbox.com/s/v12is8h24qh14 ... y.pdf?dl=0

Note: No need to create an account to download from Dropbox.
Just scroll to the bottom of the invitation window and click on 'No thanks, continue to view'
Then press 'download' at top right of screen.

[konrad2137]

Thank you for help! I use normal condensor, not a darkfield one. Objectives are 4x, 10x, 40x and 100x and I use 20x oculars. I know about immersion oil and I use it for 100x objective.

[75RR]

Objectives are 4x, 10x, 40x and 100x and I use 20x oculars.

See link for an explanation on Empty Magnification and why it is better to let the objective provide most of the magnification.

viewtopic.php?f=25&t=4984&p=45460&hilit#p45460