Dark Field issues..

[Joe Henry]

No matter what I try I cannot get a nice deep dark dark field view, all I get is gray and I have try numerous size black dots on the filter. I am using an old Leitz Wetzlar Laborlux (1950's) equipped with achromatic objectives etc. Any ideas on what the problem is? Thanks.

[apochronaut]

Usually that is caused by the N.A. of the objective being a little too big relative to the N.A. being provided by the condenser. Scattered light bleeds into the objective causing a kind of dusk field. The scattered light causes interference which degrades the resolution of the image as well. An out of focus stop will do pretty much the same thing.
When you establish a DF stop , exterior to it is a circle of light. That circle has an o.d. and an i.d. which needs to be in focus. The light at the i.d. enters the slide at a certain angle or N.A. which reflects inside the slide to illuminate the object(s) but with a dark background. If the angle is too small, the light will scatter at the edge of the objective's view and some will enter the objective directly at the periphery, lightening the dark field.
Can you move the stops in the z axis at all?
My guess is that your stops are out of focus and creating a blurry diffracted edge. The stops need to be as close to the front focal plane of the condenser ( near to the iris) as possible.

[Frankiev]

I didn't post this query but thanks apochronaut for your explanation.
Your posts have been helpful on many occasions.