Mitch, see also the books by D.J.Jackson, described
HERE.
Regarding your current investigations...
Mitch640 wrote:The Field Diaphragm is a dial on the right hand side of the base of the scope, and it adjusts the diaphragm for the light coming through from the bright-field illuminator at the rear of the scope. It adjusts from almost black to bright enough to make you see spots all day.
Is that control actually labeled "Field Diaphragm"??
In any case, if that's what it does then it's not the same as the field diaphragm described in connection with Köhler illumination.
Let's see if I can quickly summarize. See
Visualizing the effect of a microscope's condenser aperture. That cone of light passing through the specimen and spreading out toward the objective has four important properties. (There are others, not listed.)
- Intensity -- how bright is the light.
- Angular width -- how broad is the cone.
- Footprint -- what is the cross-section at the subject focus plane.
- Focus -- where a real image of the light source would be formed.
Each of these properties has a clearly separate effect on the image that you see through the eyepieces.
- Intensity changes brightness, and nothing else.
- Angular width essentially stops down the objective -- a narrower cone will make the image less bright while increasing DOF and diffraction. As with normal photographic lenses, stopping down a bit is likely to increase contrast by reducing certain aberrations in the objective.
- Footprint at the subject plane is almost like a lens hood for normal photography -- it has no effect on the image except to reduce flare caused by light entering the lens from outside the image area.
- Focus of the condenser changes how uniform the light is, when it passes through the subject.
In some ideal world, each of these properties would have its own separate control, with zero interactions between those controls. Unfortunately, in most microscopes they do interact, sometimes quite strongly. One of my scopes has one physical diaphragm in the base, another in the condenser, and a flip-out lens for use with high versus low magnification objectives. With the flip-out lens set one way, diaphragm A mostly controls intensity while diaphragm B controls angular width. But with the flip-out lens set the other way, they almost trade places.
You might find it instructional to play with a setup like shown at the "Visualizing..." link. That would give you some direct information about what the controls are doing. It will complement what you can see by looking through the eyepieces, and by removing an eyepiece and looking at the rear of the objective as suggested by many sources.
Oh yeah, almost forgot... The Köhler field diaphragm? It's a control for footprint -- the one that has no effect on the image except for increasing contrast by reducing flare. Makes a big difference for some objectives, almost none for others. Lots of scopes don't even have one.
--Rik
