Rushing around South Africa at the moment I have just seen the request for data on Spot Size in a TEM, perhaps I can help?

The spot size control adjusts the first condenser lens and in combination with the second condenser lens they demagnify the virtual source to provide a certain beam "spot size" on the specimen. This spot size may be measured
by setting the magnification at 10,000x where 1 µm = 1 cm. 

The Spot Size control adjusts the first condenser lens and in combination with the second condenser lens they provide a certain beam "spot size" on the specimen.  This spot size may be measured by setting the magnification at 10,000X where 1um = 1cm.  When trying to produce quality images I would always suggest one uses a spot size smaller than 3um.

Whilst the Spot Size is used by most people at its largest setting (bright), it is not the way the designers expect the instrument to be.  Good operating techniques should always take the spot size into account. Even at lower magnifications better quality images will be attained at smaller spot sizes.

Since papers were produced in 1944 about transmission images in LM and EM, we have known that parallel beams are very important.  If you are chasing the best image quality, biology or materials, we always desire high coherence. There are some misunderstandings on how to obtain a parallel beam or high coherence.  For example setting the final condenser under focus is incorrect. The procedure for high coherence would be to use the smallest spot size you could tolerate (this probably means you must up the emission current, use at least 20 to 30 µA). Once in this condition over focus the final condenser (clockwise from crossover) the spot, whatever size it is now, becomes your new virtual source. The further over focus you go, the greater the distance between the specimen and the crossover the more parallel the beam and therefore it attains a higher coherence, which is what we are after.  You will deduce the smaller the condenser aperture the sharper the spot and the smaller the spot, the greater the coherence for a given degree of over focus.

Work with a design team and they expect everyone to over focus and use small condenser apertures and small spot sizes. They do not expect everyone to use too low an emission current because this makes the task too difficult!  Unfortunately almost everyone does use too low a current, I have talked before about filament life being the most important feature of many laboratories and it is always to the determent of image quality.

So in short you should always use a smallish spot size when taking micrographs and you should always run with the second condenser over focus.  With sheet film photography try to use 3 to 4 seconds exposure, as this will give you better coherence too.  Also remember that the denser the negative the more contrast you will build in your final print.

Hope this helps, spot size IS so important in the production of a quality image.