I suppose (hope) that this will answer some of your questions (bare with me: I'm not a native English speaker):
Microscopes with build in or build on illuminators, as well as free standing microscope illuminators designed to provide illumination according to Köhler, have an extra diaphragm to control the beam of light: it's situated close to the illuminators (preferably focussable!) optical system (the "collector") and called the "field diaphragm".
The diaphragm at the bottom side of the condenser is called the "aperture diaphragm".
Illumination according to Köhler serves several purposes: solving the problem of uneven illumination of the field of view, due to the incoherent and rather crude bulb filaments of that era, preventing as much as possible stray light, thus preventing haze, shattering, lack of contrast etc. and achieving the highest possible resolution in the image. Up to this day, "Köhler" still is some kind of a golden standard in microscopy.
Setting up "Köhler" sounds like a hughe thing, but in practice it only takes a minute, anyway far less time than setting up a telescope:
- project the beam of light towards the aperture diaphragm
- focus the collector until the sharpest possible image of the bulb filament is projected unto the aperture diaphragm. The size of the bulb filament's image schould be as large as the diameter of the aperture diaphragm, to be able to use the condenser's N.A. fully
- focus on a slide with the 10x objective
- close the field diaphragm, raise or lower the condenser untill the sharpest possible image of the field diaphragm can be seen in the microcopic image (hence the need for a focussable condenser)
- open the field diaphragm untill it's just out the field of view, resulting in only the part of the object under examination being lit: no stray light entering the objective
- Take the eyepiece out of the tube and look at the back plane of the objective (a phase contrast centering telescope comes very handy here). Open/close the aperture diaphragm between 75% - 100% of the back plane image, not more! The cone of light from the condenser is limited to the diameter of the objective's front lens. Again: no stray light
When changing magnifications: repeat 5 and 6.
When the aperture diaphragm is closed (too much), it impears the resolution capability of the objective. That resolution capability is defined by the numerical aperture "N.A." of the objective, which is the sinus of half the angular aperture of the objective*the refractive index between front lens and specimen. In short: N.A. = N*sinµ.
For a microscope with a condenser the formula 0.61*wavelength of light used/N.A. is often used to determin resolution.
Take for example an objective 20x, N.A. = 0.40. Resolution = 0.61 * 550nm (white light)/0.40 = 839nm = 0.84µm.
Now let's say you close the aperture diaphragm too much, resulting in a restriction of the cone of light entering the objective, to an effective N.A. of 0.20. Resolution in that case will be around: 0.61 * 550nm (white light)/0.20 = 1678nm = 1.68µm, half the resolution of what the objective is capable of.
On the other hand: contrast and depth of field will be higher, which is sometimes an advantage, for example when observing low contrast, showing hardly any color specimens such as (often) live samples.
On a sidenote: I read often in high school biology courses and such that the aperture diaphragm's function is to "control image brightness", an absolute NONO. And I know plenty of high school biology teachers who can't set up a microscope the way it should...
Edited by Microscopy, 05 September 2019 - 06:17 AM.