Key Specifications and Designators
The objective lens barrel features several engraved specifications that are essential for its correct use and performance. These details define the lens's capabilities and operating conditions:
Magnification and Resolution
- Magnification (e.g., 40X): This is the primary number indicating the factor by which the lens enlarges the specimen. This value is multiplied by the eyepiece magnification (typically 10X) to determine the total magnification of the microscope.
- Numerical Aperture (NA) (e.g., 0.65): The NA is a critical measure of the lens's light-gathering ability and its capacity to resolve fine detail. A higher NA directly translates to better resolution and a clearer, more detailed image.
Operating Conditions
- Immersion Medium (e.g., Oil or O): This specification indicates the required medium that must fill the space between the lens tip and the cover slip.
- Lenses marked with "Oil" or "O" require a specialized immersion oil to maximize the numerical aperture (NA) and resolution, especially at $100\text{X}$ power.
- If no medium is indicated, the lens is a "dry" objective, designed to work with air as the medium.
- Cover Slip Thickness (e.g., 0.17): This is the ideal thickness, in millimeters, of the cover glass for which the lens has been optically corrected. Using a cover slip of an incorrect thickness can introduce significant optical distortion, particularly with high-power objectives, severely reducing image quality.
- Tube Length (e.g., 160 or ∞): This indicates the design of the microscope's optical path.
- 160 mm is used for older, traditional finite tube length microscopes.
- ∞ (infinity) is used for modern infinity-corrected systems. These systems transmit parallel light rays from the objective, which allows auxiliary components (like specialized filters or mirrors) to be inserted into the light path without altering the focus.
Types of Objective Lenses
Objective lenses are classified based on their level of correction for optical aberrations (defects in the image):
- Achromat: The most basic and economical. Corrects for chromatic aberration (color fringes) in two colors (red and blue) and spherical aberration (fuzzy focus) in one color. The field of view is typically not flat at the edges.
- Plan Achromat (or Pl): Corrects for the same color/spherical aberrations as Achromat, but includes a lens design to ensure a flat field of view.
This means the entire image, from the center to the edge, is in focus simultaneously, making it ideal for photography. - Plan Fluorite (Semi-Apochromat): Uses fluorite to provide a higher NA than Plan Achromat, correcting for chromatic aberration in three colors. Offers improved image brightness and contrast.
- Plan Apochromat (or Apo): The most highly corrected and expensive.
Corrects for chromatic aberration in three to four colors and spherical aberration in two or three colors, offering the highest NA, best resolution, and the flattest field of view.
How to Properly Use Objectives
To ensure image quality and prevent damage to the delicate lens or the slide, follow the basic rule: Always start with the lowest magnification objective.
- Start Low: Place the 4x scanning objective (usually the shortest) in position.
This provides the widest field of view, making it easy to locate your specimen. - Rough Focus: Using the coarse focus knob, bring the stage and slide as close to the objective lens as possible while looking from the side (not the eyepiece) to avoid hitting the lens.
Then, look through the eyepiece and turn the coarse focus downward (away from the lens) until the specimen is in rough focus. - Refine Focus: Use the fine focus knob to achieve a sharp, clear image.
- Increase Magnification: Once the image is clear and centered, switch to the next highest power (e.g., 10x, then 40x) by rotating the nosepiece.
Because quality microscopes are parfocal, the image should remain nearly in focus; only minor adjustments with the fine focus knob should be necessary. Never use the coarse focus knob at high power (40x or 100x).