The issue is that when I close down the field iris, center it in the field of view, and then move the condenser up and down to focus on the iris blades, I can only do this for low magnification objectives, like 4x and 6.3x. When I try it with the 10x and 40x objectives, I can't get the iris blades in focus even with the condenser all the way down.
Here's my Kohler procedure:
- Place slide on stage and focus the image by moving the stage up and down.
- Close down the field iris to minimum.
- Center the circle of light visible through the eyepieces in the field of view using the condenser centering screws.
- Bring the field iris blade edges into focus by moving the condenser up and down.
- Open field iris so edges of the blades are nearly at the edge of the field and re-center using condenser centering screws.
- Open field iris so blade edges are no loner visible.
- Set condenser iris to illuminate 65-80% of the exit pupil.