NavList:

Ken you wrote:

"In video microscopy it's common to go beyond the diffraction limit and
pick out features (but not "detail") that should be too small to see."

 

It just occurred to me that we're probably talking about two different things here. There's a diffraction limit at the target --you can't image things much smaller than a wavelength of light. And in microscopy, when people talk about the diffraction limit, that's what they're usually referring to. 

 

Separately, there is a diffraction limit in the optical system. The human eye can't resolve things smaller than about 0.8 minutes of arc, more or less depending on various circumstances, because of this diffraction limit in the eye (and also because of the spacing of the cones in the fovea).

 

When you look through a microscope, what's the smallest detail you can resolve? At low magnifications, the limit is determined by the smallest features that the human vision system can resolve (which is dependent on the diffraction limit of the eye). As magnification increases, you eventually run into diffraction at the target. Targets that are smaller than a wavelength of light may be visible with special observing methods, but not literally resolved. And of course, the solution is to image the target with something that has a wavelength much shorter than visible light --hence, the electron microscope.