The depth of focus is greatest on the lowest power objective. Each time you switch to a higher power, the depth of focus is reduced. Therefore a smaller part of the specimen is in focus at higher power.
The amount of light transmitted to your eye is greatest at the low power. When you switch to a higher power, light (and therefore resolving power, or the ability to distinguish two nearby objects as separate) is reduced. Compensate with the light control (sometimes called the iris diaphragm
The field of view is widest on the lowest power objective. When you switch to a higher power, the field of view is closes in. You will see more of an object on low power.
