Since the compound light microscope's invention, technology has incrementally improved on the instrument, leading to the level of advancement we see today. Lens manufactures are making high quality optics and all the various controls and options that make the instrument so useful are more sophisticated than ever. As with any device or instrument, the feel and performance of a quality microscope is apparent to the user.
Eyepiece Lenses
At the top of the microscope is the eyepiece lens. (Modern microscopes usually have two eyepiece lenses; some may have viewing screens rather than eyepieces.) In a microscope with eyepieces, the eyepiece lenses serve to produce part of the magnification. They are of a set power (often 10X, magnifying to ten times the actual size of the viewing object) and work in conjunction with the objective lenses.
Objective Lenses
At the microscope's midsection is what is called the nosepiece. It is a sort of turret that houses several objective lenses. The objective lenses are of differing magnifications and they work in conjunction with the eyepiece lens or lenses to produce the desired magnification. Turning the turret allows the user to select an objective lens to increase or decrease magnification.
Coarse Focus
The coarse focus control changes the distance from the subject being examined to the objective lens that's in use. This allows the user to bring the view into focus. Typically the coarse focus control moves the microscope's stage up or down to accomplish this.
Fine Focus
Once the material being viewed is brought into general focus with the coarse focus, the fine focus allows for minute adjustments. This function moves the stage in very slight increments to give the user very fine control over what is in focus. Often the material being examined is in a medium that, to the naked eye, might seem imperceptibly thin. But under magnification it may cover a considerable span. Although the medium may be in general focus, there may be objects of interest near the top of the medium, in the middle or near the bottom. Using the coarse focus control to adjust for this can be very frustrating since the coarse focus will often overshoot the adjustment one way or the other.
Stage
Under the objective lens nosepiece is the more or less flat surface of the mechanical stage. The stage is mechanical in the sense that the focus knobs, when operated, move it up and down to achieve the proper focus. Further, the stage on modern microscopes features mechanical controls that allow the user to move the viewing medium---typically house on a glass slide---left and right and forward and back. These controls are like the fine focus in that they permit subtle movements so that the user may center an object for viewing.
Light Source
Beneath the stage is a light source that---through a hole in the stage---illuminates the object being viewed. The light is focused into a cone by a device called the substage condenser. And the condenser itself has controls that allow for centering and focusing the light, permitting adjustment to achieve optimal lighting on the subject.
Tags: coarse focus, coarse focus control, focus control, objective lens, work conjunction with, allows user, being examined