Stage and Focus

Holding the specimen still, and bringing it into focus without breaking anything.

The stage is the platform holding the slide; the focus controls change the distance between specimen and objective. On most modern stands the stage moves and the tube stays fixed.

Plain stage versus mechanical stage

A plain stage uses spring clips and the slide is pushed by hand — workable at 40×, hopeless at 400×, where the field is so small that a nudge loses the specimen. A mechanical stage moves the slide on two calibrated axes with verniers, so a position can be recorded and returned to.

Coarse and fine adjustment

Coarse adjustment moves a large distance quickly and is used only with the lowest-power objective. Fine adjustment moves a fraction of a millimetre per turn and is the only control that should be touched at 400× or above.

The safe focusing procedure

1. Start on the 4× objective.
2. Watching from the side, raise the stage until the slide is near the lens.
3. Looking through the eyepiece, lower the stage until the image appears.
4. Sharpen with fine focus, centre the specimen.
5. Rotate to the next objective. A parfocal set needs only fine focus from here.

Focusing away from the slide rather than towards it is the entire point of step 3. At 40× the working distance is around 0.6 mm, so focusing upward blind will drive the lens into the cover glass and can crack both.