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CHOOSING A COMPOUND MICROSCOPE
Compound microscopes differ from simple magnifiers in that there are two separate magnification steps that occur instead of one. The objective lens is nearest the subject under observation and provides a magnified real image. The eyepiece magnifies the real image provided by the objective and yields a virtual image appropriate for the human eye. Remember that the eye has its own lens, which relays virtual images onto the retina.
Microscopes have been standardized over time to simplify design and manufacture. Most microscopes employ the Deutsche Industrie Norm, or DIN standard configuration, while the Japanese Standard (JIS) is less commonly used. DIN microscopes begin with an object-to-image distance of 195mm, and then fix the object distance (with respect to the rear shoulder of the objective) at 45mm. The remaining 150mm distance to the Real Image Plane sets the internal real image position, defined as 10mm from the end of the mechanical tube. Objective lens thread is the same for DIN and JIS, that is 0.7965" (20.1mm) diameter, 36 TPI, 55° Whitworth.
EYEPIECES
Microscope eyepieces generally consist of an eye lens unit and a field lens unit. Field lenses are usually plano-convex in shape, and are intended to help gather more light at the real internal image plane. Eye lens units can range from a simple plano-convex lens to a complex lens system consisting of four or five elements. Eyepiece types vary in how well they perform.
Huygenian designs employ two plano-convex lenses positioned with both convex surfaces toward the object, and are good for use with lower power achromatic objectives. Ramsden designs, useful for high power achromatic objectives, use two plano-convex lenses with the convex surfaces facing each other. Kellner eyepieces are often called Wide-Field eyepieces. Kellner eyepieces upgrade to an achromatic doublet as the eye lens; Periplan has a three-element eye lens for exceptional correction. Higher quality eyepieces must be used with semi-plan and plan objectives to maintain the flatness of the field.
For more information on eyepieces, refer to our eyepieces tutorial.
ILLUMINATION
Illumination is critical for effective microscopy. Most DIN microscopes employ some form of sub-stage illumination (transmission illumination) in which light enters the objective through the sample from below. Lower power objectives, such as 4X or 10X, can accommodate sub-stage and incident (from above the sample) types of illumination due to comparatively large working distances and lower numerical aperture. Higher power objectives, such as 40X and 100X, can realistically only be used with sub-stage lighting techniques.
QUALITY CORRECTION
Three generally accepted levels of correction/quality for microscope objectives are achromatic, semi-planar, and planar. Achromatic objectives - the most common type - have a flat field in about the center 65% of the image. Planar, or plan, objectives correct best for color and spherical aberations, and display better than 95% of the field flat and in focus. Semi-planar objectives (sometimes called semi-plan or micro-plan) are intermediate to the other two types with about 80% of the field appearing flat.
| Field Flatness Measured Radially From Center | ||
Achromatic 65% |
Semi-Plan 80% |
Plan 95% |
HOW TO READ MICROSCOPE SPECIFICATIONS
Specifications commonly associated with microscope optics assume a standard system configuration (i.e. DIN). Objective lenses higher in magnification have higher numerical apertures, shorter working distances, and smaller fields of view.
Working distance is the space between the objective and the specimen under investigation.
Field of view is the area of the specimen seen at one time.
Numerical aperture (NA), however, influences both the resolving power of the objective lens, and the amount of light the lens can gather. The higher the numerical aperture of the lens, the closer two tiny objects can be to one another and still be differentiated separately. However, since higher NA is usually for higher magnification objectives, the field of view is smaller and thus less light enters the lens. (Note that numerical apertures above 0.95 can only be achieved with immersion oil.)
