A Brief Analysis on the Proper Use of Objective Lenses and Focusing of Microscopes
When using a microscope, follow the focusing principle of low magnification first, then high magnification. Focusing under a low-power objective serves as preliminary focusing for high-power observation. When switching to a high-power objective, simply rotate the objective turret directly without changing the focal length preset under low magnification. Under the high-power lens, tissues can be observed with only slight fine adjustment, or even no adjustment at all. However, many operating instructions fail to clarify the specific definition of the "low-power objective".
In microscope operation, the 10× objective is the standard lens routinely used for initial focusing. On one hand, switching between the 10× objective and lower or higher magnifications causes no drastic focal changes. On the other hand, lower-power objectives have a longer depth of field, making it difficult for users to focus accurately by visual judgment alone. Direct switching to a high-power lens afterward may easily cause collision between the objective and the specimen.
1. Furthermore, the 10× objective is not only the standard lens for focusing but also widely applied in practical work. For example, many national standards for metallographic inspection reference standard charts under 100× magnification, which is achieved by combining a 10× objective with a 10× eyepiece. In actual operation, proper preliminary steps will position the specimen near the focal plane. After placing the sample correctly under the 10× objective, a blurry or even relatively clear image will appear, requiring only minor fine focusing adjustment.
2. Differences from conventional descriptions
Regarding focusing after switching from low-power to high-power objectives, practical experience differs greatly from the content in many reference materials. Thanks to improved modern manufacturing techniques, microscopes feature excellent parfocality among different objectives, especially imported models. After achieving clear focus under low magnification, the image often remains sharp without refocusing when switching to high magnification. If adjustment is needed, only a tiny increase in object distance is required. The adjustment range is far less than one to three full turns - merely an angle of 1 to 3 degrees, an extremely subtle movement.
3. Operation of the objective turret
Never push the objective lens directly by hand during lens switching. This may loosen the fixing threads and tilt the optical axis. Microscope objectives and digital imaging components are tightly mounted on the objective turret. When changing lenses, rotate the turret until you hear a soft click and feel an obvious increase in resistance. At this position, the objective is correctly locked perpendicular to the stage surface.
4. Rotation direction and object distance relationship
The rotation direction of the coarse and fine focusing knobs directly controls the increase or decrease of the object distance. Clockwise and counterclockwise directions are relative and are usually defined when viewing the microscope from the right side. Different microscope models have varying knob rotation directions for adjusting object distance. This should be confirmed under professional guidance. Before formal operation, always clarify the relationship between knob rotation and object distance adjustment; do not rely blindly on generalized clockwise or counterclockwise descriptions from manuals.
