manual zoom, giving a magnification range of 7.5×–60×. Swapping lenses extends the magnification to 480×.
When choosing a zoom microscope, check the manufacturer’s zoom curves. To maintain optical quality, the lenses must remain correctly aligned during zooming. This is achieved with a precisely engineered mechanical zoom body. However, the SteREO Discovery.V20 from Carl Zeiss Microscopy (Jena, Germany;
www.zeiss.com) uses an electronic zoom body that calculates the optimal position of the lenses at each level of magnification.
The recently released SMZ25 from Nikon Instruments (Melville, NY;
www.nikoninstruments.com) provides the world’s first zoom ratio of 25:1 (zoom range: 0.63×–15.75×), spanning spatial scales from single cells to whole organisms in one instrument.
Optics Besides magnification, microscopes have many optical specifications
that affect their suitability for different applications. These include: • Working distance • Depth of field • Object field • Numerical aperture and resolution • Optical quality.
1. Working distance: The distance from the bottom of the objective lens to the point of the sample in focus. Working distance is important since it affects the ease of manipulating the sample. However, working distance has an inverse relationship to numerical aperture (NA) and therefore resolution. At a large working distance of 171 mm, the SZX10® Research Stereomicroscope from Olympus (Center Valley, PA; www.
olympus-ims.com/en/microscope) has an NA of 0.055; at 81 mm, the NA is 0.1, while at 33.5 mm it is 0.2.
2. Depth of field: The distance between the nearest and furthest points from the objective that are in focus at the same time. When examining a very rough surface, a high depth of field is required. With flat samples, depth of field is not important. Depth of field is greater at lower magnifications.
3. Object field: The diameter of the circular area of sample that is visible through the microscope at one time. Magnification and object field are inversely related, that is, at higher magnifications a smaller area of the sample is visible. Stereomicroscopes typically have large object fields to allow for sample manipulation.
4. Numerical aperture: A unitless number derived by a complex formula that denotes the resolution of fine detail. For stereomicroscopes, the NA is dependent on the microscope objective and is not affected by eyepiece magnification. The higher the NA, the better the resolution, but, as described above, NA and working distance have an inverse relationship. Note that
AMERICAN LABORATORY • 29 • JUNE/JULY 2013
with all microscopes, the resolution varies with wavelength. Resolution is expressed as line pairs per millimeter (lp/mm).
5. Optical quality: All optical systems have aberrations that vary with wavelength since light is refracted according to frequency. Optical instruments are designed to minimize these aberrations. Broadly, the methods of correction are:
a. Achromatic—useful when geometric shape is the priority b. Apochromatic—useful when color is the priority c. Plan—field curvature correction d. PlanApo—apochromatic and flat field correction.
Illumination Stereomicroscopes require illumination. Stereomicroscopes typically
have two light sources: one from underneath the sample, and one above it. Incident light is commonly used for solid samples, and may be de- livered using ring lights or spot lighting. Samples may be illuminated with UV or IR light sources in order to view specific characteristics or reactions. Illumination techniques include brightfield, darkfield, phase contrast, differential interference contrast, and confocal.
Working lifetime and versatility vary between different illumination systems. The BS-3060 Zoom Stereomicroscope from Carltex (Nyack, NY;
www.carltex.com) has a light-emitting diode (LED) light for incident and transmitted illumination, with a life expectancy of up to 6000 hr.
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