Water immersion objectives are used not only at very large magnifications that require high resolving power, but also of moderate power as there are water immersion objectives as low as 4X . Objectives with high power magnification have short focal lengths, facilitating the use of water.
12.
Oil immersion objectives can have practical difficulties due to their shallow depth of field and extremely short working distance, which calls for the use of very thin ( 0.17mm ) cover slips, or, in an inverted microscope, thin glass-bottomed Petri dishes.
