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I've been studying scanning electron microscopes quite a bit but can't seem to get my head around why we need objective lenses.

On paper I don't see a reason, if we would want to focus the electron beam in 1 small spot, why we cant put our specimen directly in the focal/crossover point beneath the condenser lens.

Why do we bother with the lens aperture and the objective lens behind it?

After reading in about 3 different sources I think I finally understood a basic concept of optics. That the condenser lens and objective lens are supposed to work together. I can't quite say why but it seem that neither lens could do the job alone. We always (? not sure here) need both to focus the beam nicely in 1 spot. Can anyone give me some insights into this? Source

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  • $\begingroup$ Because subjective lenses would bring their own biases into the images. $\endgroup$ – BioPhysicist Oct 4 '19 at 18:41
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There are really two issues that enter through practical design needs:

  • The aperture cleans up the beam a lot, making it possible to make a clean spot

  • The combination provides larger “relief” to the spot, so that it’s further from the lenses. Since these are magnetic fields, not glass, it’s best to give them some room.

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  • $\begingroup$ So the main advantage is the aperture and the 'distance'? Could you maybe explain why it's good to 'give them some room'? $\endgroup$ – Caito Oct 5 '19 at 6:59

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