Understanding Microscope Objectives
NOTE: For an introduction to microscopes, see Microscope Basics.
A variety of microscope objectives are available. All objectives use lenses to focus light. Light is broken down into various wavelengths (colors) as it travels through a lens. The various wavelengths have different focal points. That means that red, green and blue appears to focus at different points. This is called chromatic aberration. Spherical aberrations are focal mismatches caused by the shape of the lens. Quality lenses are designed correct for chromatic and spherical aberration to bring the primary colors to a common focal point. These terms may help you determine the best objective for your application:
Achromatic objectives–This objective brings red and blue light to a common focus, and is corrected for spherical aberrations for green. It is excellent for black and white viewing. If an objective is not labeled, it is achromatic.
Fluorite or semi-apochromat objectives–These lenses are chromatically corrected for red and blue, and the green focus is also close. They are spherically corrected for blue and green. This objective is better suited for color viewing or recording than achromatic objectives.
Apochromatic objective–This is the most expensive objective. It is chromatically adjusted for four colors (deep blue, blue, green and red) and spherically corrected for deep blue, blue and sometimes green. This is the best choice for color viewing. These have a higher numerical aperture (N.A.) than achromats or fluorites.
Plan objective–These objectives produces a flat image across the field of view. The three objectives discussed above all produce a curved image. A plan-achromat, plan-fluorite or plan-apochromat are corrected.
Infinity Correction–When measuring from the back end of the objective to the primary focal plane, many microscopes are limited to a specific distance (160mm). More expensive microscope use a different series of lenses, prisms and mirrors to allow for an "infinite" distance between those two points. This is called infinity correction.
Labeling of an Objective
Each objective is labeled with the following information:
- ∞ for infinity correction
- Cover glass thickness (usually 0.17mm)
- OIL, HI (homogeneous immersion) or OEL if the objective is designed for a drop of oil between the lens and the specimen. If it is not labeled as an oil immersion objective, it is a dry objective
- Numerical aperture (N.A.)
- Color ring (red–4X, yellow–10X, green–20X, blue–40X or 60X, white–100X)
Eyepieces and Objectives work Together
The magnification of the image depends on the combination of the eyepiece and the objective used. This combination also affects the field of view. This example shows how these factors inter-relate.
Problem: The PZMIII or PZMIV stereo zoom microscope normally comes with a 1.0X objective and a 10X pair of eyepieces. The magnification is 6X to 50X, however the concept of magnification is difficult to visualize. Let's discuss what can be seen at the two zoom extremes. Imagine the visual circle to be a range of 34–4.2 mm. This microscope has a working distance of 100mm. Researchers working with small animals will have difficulty working in this tight space.
Solution: Instead of the standard configuration, setup the microscope with a 0.5X objective to increase the working distance to 187 mm. The result of using this lower power objective is that the magnification range decreases by one half and at the same time the field of view double. To restore the microscope system to the original condition (magnification and field of view), replace the 10X eyepieces with 20X eyepieces. The use of these two options restores the visual field of view and magnification range back to the original condition with the added benefit of a larger working distance.
TIP: On the trinocular version of the PZMIII or PZMIV stereo microscope with the standard configuration (1.0X objective, 10X eyepieces) and with the optimal camera adaptor (0.5X on a ½” CCD camera) the video capture field of view is up to 40% less than the visual field. By using a 0.5X objective with 20X eyepieces the video capture area doubles, and the resulting video capture more closely matches the visual field of view.
The first image shows the eyepiece view when using a 1.0X objective with a 10X eyepiece. It has a 34mm field of view. The second image shows the video field of view of about 16–4.7mm (COLCAM-NTSC camera with a 0.5X coupler). The third image shows the video view that approximates the eyepiece view. It uses a 0.5X objective with a 20X eyepiece.
NOTE: If a 1/3” inch camera (6mm diagonal) is used on the 0.5X microscope adaptor you can apply the ratio of 6/8 for the reduction in the captured field.