Question

Why must you center and focus the object in the field of view under low power before switching to high power ?

Answer 1

The reason to focus in low power first and then switch to high power is that, in low power the field of view is larger which we can see more of the object. But when change it to high power the field of view become centred thus only less of the object can be seen.
So it is better to view on low power, centre the object and then change to high power.
In other simple words when using high power the field of view become smaller thus if we not move/ focuse the field of view we cannot appreciate it on the high power.

Thank you...