Compound vs. Zoom Microscopy


as the Field of View increases, both
Observable Feature Size and Depth of Field increase

The optical microscope can be found in 3 basic configurations based on the size of the area to be viewed and the size of the detail that must be resolved. For a stereoscopic view (3D) of sample areas from 100mm wide down to 1mm wide, a Stereomicroscope is used. For a two dimensional view of sample areas from 100mm wide down to 1mm wide, a Zoom system is used. And, to view areas of samples 1mm wide down to 0.045mm wide, a compound microscope is required. The three basic parameters that define the characteristics of each of these are:

Resolution -- minimum size of observable detail
Depth of Field -- height of vertical area in focus at one time
Magnification -- width of the image formed by the lens divided by the width of the area of the sample imaged.

The performance specification that defines the Resolution and Depth of Field capability of a lens is the Numerical Aperture. When used with the Lens Magnification, it determines the applicability of the lens to the imaging requirement. 

The range of Numerical Aperture that can be obtained for each of the three basic microscope configurations:

The fundamental resolvability of a large field-of-view system will be the pixel size of the CCD chip in the camera. If one uses the state-of-the-art Sony 2/3 CCD chip camera, the pixel size is 6.7 x 6.7mm on a chip 6.6mm x 8.8mm. A lens that images an area 66mm x 88mm reduces the area under observation to the area of the CCD chip, which represents a reduction of 10 times. Consequently, the observable picture element size would also be limited to 10 times the CCD chip pixel size per imaged picture element or 67 x 67mm. In this case, the overall magnification of the system would be the area on the monitor divided by the area imaged, 310 x 230mm divided by 88 x 66mm, or 3.5X. For intermediate and small area imaging, the characteristics of the optical components will usually limit the resolvable feature size. 

Resolution can be estimated by:


Depth of Field is a function of Numerical Aperture and can be estimated by:


The Next-Generation of Optical Microscopy
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