Simply rotate the objective housing to click into the desired level of magnification. Dual power are excellent starter microscopes. Zoom Magnification enables you to zoom, conveniently through a magnification range, similar to binoculars or an SLR camera. Some have detents or click stops at intervals through the zoom range so that you can quickly return to a precise level of magnification in the zoom range. Research Stereomicroscope System.
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Measuring and photomicrography reticles are placed in the plane of the eyepiece field diaphragm, so as to appear in the same optically conjugate plane as the specimen.
The field number of the eyepiece, usually inscribed on the housing exterior, is divided by the magnification power of the objective to quantitatively determine the field of view size. Included in the calculation should also be the zoom setting and any additional accessories inserted into the optical path that may have a magnification factor.
However, the eyepiece magnification is not included, which is a relatively common mistake made by novices in microscopy. When a wider field of view is desired, the microscopist should choose eyepieces with a higher field number.
In the lower magnification ranges, stereomicroscopes have substantially larger fields of view than classical laboratory compound microscopes. The typical field size with a 10x eyepiece and a low power objective 0. These large field sizes require a high degree of illumination, and it is often difficult to provide a continuous level of illumination across the entire viewfield.
Resolution in stereomicroscopy is determined by the wavelength of illumination and the numerical aperture of the objective, just as it is with any other form of optical microscopy. The numerical aperture is a measure of the resolving power of the objective and is defined as one-half the angular aperture of the objective multiplied by the refractive index of the imaging medium, which is usually air in stereomicroscopy.
By dividing the illumination wavelength in microns by the numerical aperture, the smallest distance discernible between two specimen points is given by the equation the Raleigh Criterion :. As an example, a Nikon SMZ stereomicroscope equipped with a 1.
Note that the resolution calculated for the 1. Objective lenses manufactured for common main objective stereomicroscopes typically vary in magnification from 0. The magnification, working distance, and numerical aperture of typical stereomicroscope objectives at varying magnification are presented in Table 1.
In the past, several manufacturers have assigned color codes to their stereomicroscope objective magnification values. Table 1 also lists the color code assignment for a series of Nikon stereomicroscope objectives having this identifying information. Note that many manufacturers do not assign a specific color code to stereomicroscope objectives, and the codes listed in Table 1 are intended only to alert readers that some objectives may display this and other specialized proprietary nomenclature.
The resolving power of stereomicroscope objectives is determined solely by the objective numerical aperture and is not influenced by optical parameters of the eyepiece. Overall resolution will not be affected when exchanging 10x eyepieces for 20x or higher magnification eyepieces, although specimen detail that is not visible at the lower magnification will often be revealed when the eyepiece magnification is increased.
The highest power eyepieces 30x or higher may approach empty magnification, especially when the total microscope magnification exceeds that available from the objective numerical aperture. In the case of the Nikon 1.
Auxiliary attachment lenses, which range in power from 0. In general, the resolving power influence is proportional to the magnification factor of the attachment lens. The field diameter is inversely proportional to the magnification factor, while the depth of field is inversely proportional to the magnification factor squared. Changes in working distance are also inversely proportional to the magnification factor, but are difficult to compute because the function is not linear. In addition, use of these auxiliary lenses will not have significant impact on image brightness in most cases.
Lenses designed for general photography are rated with a system that is based on f-numbers abbreviated f , rather than numerical aperture Table 2. In fact, these two values appear different, but actually express the same quantity: the light gathering ability of a photography lens or microscope objective.
F-numbers can be easily converted to numerical aperture and vice versa by taking the reciprocal of twice the other's value :. Numerical aperture in microscopy is equal to the refractive index of the imaging medium multiplied by the angular aperture of the objective. The f-number is calculated by dividing the focal length of the lens system by the aperture diameter.
If a millimeter focal length lens has the same aperture diameter as a millimeter lens, the shorter lens has twice the f-number as the longer. The aperture diameter is fixed in a stereomicroscope objective, similar to the situation with conventional compound microscope objectives. As the microscope magnification is increased or decreased by changing the zoom factor, the focal length is also altered accordingly.
At higher magnifications, the ratio of the aperture diameter to focal length increases, and the opposite is true as magnification is decreased. The focal length of a 2. The relative size of the zoom system aperture as compared to that of the objective functions to control the f-number and numerical aperture of the entire microscope system. In late model microscopes, such as the SMZ, objective focal lengths have been reduced in order to increase the total system numerical aperture.
Thus, a 0. Some manufacturers supply adapter rings that allow objectives designed for a specific microscope to be used on other usually earlier model stereomicroscopes. In several cases, two objectives having the same magnification can have different focal lengths due to variations in tube lens and zoom channel aperture specifications.
The difference between the two microscope designs is the size of the zoom system aperture, which results in shorter focal lengths for the SMZ series objectives. When interchanging objectives having the same magnification but different focal lengths, an additional factor must be introduced into total magnification calculations to correct for the focal length differences.
Depth of field is an important concept in stereomicroscopy perhaps even more so than with other common forms of optical microscopy , and is strongly influenced by the total magnification of the instrument, including the contribution from both the objective and auxiliary attachment lenses.
At a magnification of 50x, using a 1x objective numerical aperture 0. If a 2x attachment lens is added to the microscope when it is configured for operation at 50x, the new magnification will be x, but the depth of field drops to about 14 micrometers, a substantial decrease from the value 55 micrometers without the auxiliary lens. In this situation, it is wiser to change the eyepiece magnification from 10x to 20x to achieve the added magnification so as to retain the larger depth of field value see Table 3.
Increasing the objective numerical aperture through enhanced optical correction for instance, from achromat to apochromat will also produce a modest decrease in field depth. Depth of field values for a Nikon plan apochromatic 1x objective are presented in Table 3, where they are listed as a function of zoom magnification factor and eyepiece magnification.
It is clear from the data in the table that numerical aperture increases with increasing zoom magnification, while the depth of field decreases with increasing eyepiece and zoom magnification factors. Reducing the size of the double iris diaphragm positioned between the objective and the eyepieces can enhance depth of field. This diaphragm is opened and closed using a wheel or lever in the microscope body housing. There are actually two diaphragms, one for each of the channels, in the common main objective stereomicroscope design.
The role of these diaphragms is to produce an increase in field depth while simultaneously improving specimen contrast observed in the eyepieces. Depth of field and numerical aperture variations, as a function of diaphragm opening size, are presented in Table 4 for the Nikon plan apochromatic 1x objective at the highest zoom magnification factor As the diaphragm size is ramped down, the depth of field utilizing a 10x eyepiece increases from 26 to 89 millimeters, approximately a percent increase.
Simultaneously, the numerical aperture drops from a value of 0. Similar effects are observed at higher eyepiece magnifications. Closing the iris diaphragms will also produce a decrease in overall light intensity, increasing exposure times for both digital and film camera systems. In most cases, the optimum setting for the diaphragms is determined by experimentation.
As the diaphragms are slowly closed, the image begins to display more contrast as illumination intensity slowly fades. At some point, depending upon the optical configuration of the microscope, the image begins to degrade and specimen details exhibit diffraction phenomena while minute structural details disappear.
The best setting is a balance between maximum specimen detail and maximum contrast as seen in the eyepieces, on film, or in digital images. Both Greenough and common main objective stereomicroscopes are readily adaptable to image capture utilizing traditional photomicrography techniques film or through advanced digital imaging. Often photomicrography is employed as a tool for recording the spatial distribution of specimen details prior to observation and imaging with a higher-power compound microscope.
This technique is often necessary for biological specimens, where dissecting, staining, and selective mounts are performed. The principal concern with digital imaging and photomicrography in stereomicroscopy is the low numerical aperture of the objectives, and the inability to capture on film or in a digital image the tremendous depth of field observed through the eyepieces.
There are also several limiting factors that should be considered when photographing specimens through a single body tube utilizing a Greenough-style stereomicroscope. Because the microscope objective is positioned at a slight angle to the specimen, depth and resolution seen in the microscope eyepieces is not recorded on film.
Some manufacturers once provided accessories that help to alleviate these problems, but many of the older microscopes have spare and accessory parts inventories that are exhausted, limiting the choices for photomicrographers. Older stereomicroscopes can be equipped with a digital or film camera using attachments that are available over the Internet or through optics and science supply houses.
Stereo Microscopes: What's the Difference? Compound and stereo microscopes are two of the most common kinds of scopes. AmScope can help you determine which type is best for your unique needs.
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