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When we use a usual biology class compound microscope, we need to focus the stage (place the object) to the "correct" position. It is just in that sweet spot that the image is sharp and clear. So we are deciding the object distance.

Here is the ray diagram of a compound microscope.

enter image description here

So, when we are focussing, we move the objective lens which tweaks the image distance. My doubt is that, shouldn't the image be seen clearly, wheresoever the first real image forms, if within Fe (Focus of the eyepiece lens). Then, that would make it a range instead of a single point (shouldn't we be able to see a clear image not at a single point but a range?). Whatever, the distance of the virtual image, the eye can accommodate it onto the retina, right?

If the first image (real image of the objective lens) goes behind Fe, or is on Fe, what happens?

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The eye will see a sharp final image as long as the final image is between infinity (real image in focal plane of eyepiece) and the near point (25 cm) of the eye (real image between $F_{\rm e}$ and eyepiece).

Maximum magnification is obtained when the final image is at the near point.
However it is often the case that the final image is made to be at infinity to reduce eye strain.

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  • $\begingroup$ So why do we usually focus the microscope only to one particular point? $\endgroup$ – Polisetty May 19 '17 at 13:58
  • $\begingroup$ I have added to my answer. $\endgroup$ – Farcher May 19 '17 at 14:00

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