What's the principal difference between aperture iris diaphragm and field iris diaphragm?
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(Condenser) Aperture Diaphragms are placed at or close to the the back-focal plane of the condenser. They control the numerical aperture of the condenser and their conjugate plane is at infinite. Closing it, you reduce the illumination at the sample plane homogeneously (like turning up or down the light source).
Field Iris Diaphragms are placed in the illumination path, in a plane conjugate to the sample plane. Closing it, you reduce the illumination at the sample plane not homogeneously (clearly seeing the edges of the iris).
(both effects are very clear in the links)
