OOps!!! I forgot to mention that when point source light is used, the lens is always used wide open rather then using the f-stop for 'brightness"... which is done by regulating the voltage using a rheostat, thus you have to make sure the image is properly 'in focus' at the time of exposureDavid,
When 'transmission'.electron micrographs it is almost essential that you make the prints using a POINT SOURCE enlarger (as opposed to the negatives exposed using a 'scanning electron microscope)to ensure that the fine detail from the extremely thin sections can be 'resolved' on the print made from the EXTREMELY thin sections. I spent many days making B/W enlargements from negatives for the electron microscopist when I was employed at the nearby Agriculture Canada Research Center.
The Point-light source light in a 5x7 Durst 138S was also used for making the occsional B/W negatives from microtomed sections of the 'slide-mounted' fixed and stained soft tissue.... usually far superior to what can be recorded when using a camera mounted on even the 'highest' quality bench-top microscope.
This you have LITTLE to NO Depth of field 'freeway' when it comes to focussing the image.