A compound microscope uses several types of objective lenses -- or "objectives" -- to increase magnification beyond the 10x magnification typically provided by the microscope's eyepiece. These objectives range in magnification power from 4x to 100x and may be used either dry or with immersion oil. However, the high power objective has several benefits over both the lower power objectives and the oil immersion objectives.
Magnification and Resolution
Compared to the lower power objectives, which magnify 4x to 10x, the high power objective allows you to see a smaller portion of the specimen, but at a higher magnification -- up to 40x. While an oil immersion objective can provide 100x magnification, it also requires the use of immersion oil to increase the resolution of the image viewed in the microscope. The high power objective lens offers the best combination of magnification and resolution without having to use immersion oil.
As the magnification power of an objective increases, so does the length of the objective: the low power objective is the shortest and the oil immersion objective is the longest. Because higher magnification correlates to closer physical proximity of the lens to the slide, it is easier to accidentally push the objective lens into the slide while adjusting the focus at higher levels of magnification. However, it is easier to focus the high power objective without hitting the slide than to do the same with the oil immersion objective, which has a very narrow plane of vision and therefore requires more adjustments.
The high power objective has a narrower field of view than the lower power objectives, so it is difficult to view fast-moving living organisms such as protists with the high power objective. However, the high power objective is good for viewing nonliving specimen mounts, especially those with details that are not visible at lower magnifications, and wet mounts of living organisms that move slowly. The high power objective also has a larger plane of vision than the oil immersion objective, which often allows you to focus on structures within the specimen that are on different levels of view.
Ease of Use
Oil immersion objectives provide a higher magnification power, but they require the use of a special viscous oil to increase the resolution of the image. This oil must be removed from the objective lens using a special lens paper to avoid scratching or fogging the lens. Oil immersion lenses also require a wet mount slide, where the contents of the slide are left exposed rather than covered as they typically would be with a high objective dry lens. The high objective lens requires neither oil nor a wet mount, so less preparation and cleanup is necessary.