Celestron Advanced Stereo Microscope Microscope Basics - Page 12

DIN (Deutsche Institut Fuer Normung) standard objectives are the most popular in international standards which dictates the design capability of the objectives. DIN standard objectives from one manufacturer can be used in another manufacturer's compatible microscope. The standard refers to the 45mm parfocal (explained below) distance and RMS standard 33mm thread for the objective. The 45mm distance is the measurement from the mounting hole in the objective nosepiece to the point of focus on the specimen. JIS (Japanese Industrial Standard) is a less used international standard for objectives. They use 36mm objective distance. RMS (Royal Microscopical Society) uses a 33mm parfocal distance for the objective. RMS screw threads are 20.32mm diameter with a pitch of 0.706. Parfocal refers to the ability to change objective lenses with differing powers and have the focus remain sharp or with very little refocusing. Parcentered refers to when changing objective lens power, the specimen should stay centered in the field of view. Oil Immersion concentrates the light path and increases the resolution. A special oil is used with the 100x objective lens and usually at 1000x up to 1500x. This is a technique used to close the air gap between the tip of the objective and the top of the specimen with the oil. Once the objective contacts the drop of oil of matching refractivity, it becomes one unit. Immersion oil is the only suitable oil for this purpose and will allow high magnifications and avoid damage to the objectives. There are two basic types of oil immersion - Type A is for low viscosity and Type B is for high viscosity. Eyepiece (Ocular) Various Eyepieces The eyepiece consists of a series of lenses mounted in a tube (barrel) at the upper end of the microscope. Its basic function is to look at the focused, magnified image projected by the objective lens and magnify that image a second time before your eye looks at the image of the specimen. As with objective lenses, stay away from eyepieces where the optics are made of plastic as they will ensure very poor quality. Eyepieces come in several different designs such as Huygens, Ramsden, Kellner, Orthoscopic, Plossl, etc. and all can be good in microscopes and the differences are minor as compared to various optical designs used with astronomical telescopes. The eyepieces are usually 10x but also come in 5x, 12.5x, 15x, and 20x. The "x" refers to the amount of magnification (power) that this lens adds as a multiplier to the magnification of the objective. They are inscribed with the magnification and its field number (which is the diameter in millimeters of the diaphragm opening (aperture) of the eyepiece. The aperture limits the field of view to the useful coverage of the eyepiece. For special applications, eyepieces can have scales, pointers, crosshairs, markers, etc. on them. Eyepieces also come in wide field styles. They have a large diameter and show a wider field of view than standard eyepieces. Most are 10x although 15x and sometimes 20x are common. Eye Relief is the distance (in millimeters) between the observer's eye (cornea) and the nearest surface of the eyepiece lens - longer eye relief being better for eyeglass wearers. The eyepoint is the location (or position) of the eye from the eyepiece which allows for the best possible viewing of the image. Some eyepieces use a pointer to point at a section of the specimen. You can rotate the pointer by turning the eyepiece. An eyepiece reticle (micrometer) is a piece of glass with a pattern printed or etched which is put inside an eyepiece to make measurements of the size of objects seen through the microscope. The diopter adjustment is a focusing characteristic to match the user's eyes on microscopes with binocular heads. Most all people have a different vision in each eye. This allows the user to adjust the focus of each eyepiece separately. For eyeglass wears, it allows them to adjust for their prescription and they can use the microscope without using their glasses. Typically, the diopter is on the left eyepiece and to make the adjustment --- close your left eye and look through the right eyepiece with your right eye and focus the specimen sharply, then close your right eye and look through the left eyepiece with your left eye and focus the specimen sharply. Then you will be able to focus and examine specimens with comfort and ease. Rubber eyecups (eye shields) are typically used with stereo microscopes. They reduce ambient light and provide comfort but should not be used if you are wearing eyeglasses. Condenser Lens (Sub-stage Condenser) A glass lens or lens system located within or below the stage on compound microscopes. Its basic function is to gather the light coming in from the light source and to concentrate that light into a light cone onto the specimen. High power 12