okay, here we go…
Darkfield is not shining light from below or from the side. It is not polarizing filters. It is not staining things. Not that those things don’t have their place.
Take a lens. You have an object. Then a lens. Then the place where the image of the object is formed. If the object is very far away. the place where the image is in focus is at the focal point of the lens. Any physical lens has a fixed focal length and in this case the focal length and the distance from the lens to the focal/diffraction plane AND IMAGE plane are the SAME.
Now move the object much closer to the lens. The image plane moves further away from the lens, but remember, the focal point is a fixed value. So now the two are seperated.
So what you say?
Here is where it gets weird. Diffraction theory comes into play.
Take an “object”, like a nearly transparent cell on a microscope slide. So, you have this bright field with a barely visible cell.
Diffraction theory says this: At the FOCAL POINT of the lens, the “information” about the whole field is concentrated in the center. The information from the small details, like the cell, are spread “all over the place”.
So, you put a filter in the center of the focal point/length/diffraction plane. Basically a small dot that blocks light.
Then, you “let” whats left continue on through your series of lenses to the image plane.
You have removed the information about the whole field, but retained the information about the small details.
What you have left is a DARK field, with the little details glowing brightly, because their information wasnt removed.
Sounds flakey, but that is what the math predicts, and it is what actually happens when you do it in a lab.
It doesnt even take particulary precise/fancy/expensive optics to pull it off either.
Its one of those lab experiments thats pretty cool to see in person.