Table 1 - Resolution and Numerical Aperture by Objective Correction The following table (Table 1) provides a list resolution ( r) and numerical aperture ( NA) values by objective magnification and correction. Other factors, such as low specimen contrast and improper illumination may serve to lower resolution and, more often than not, the real-world maximum value of r (about 0.25 µm using a mid-spectrum wavelength of 550 nanometers) and a numerical aperture of 1.35 to 1.40 are not realized in practice. In some instances, such as confocal and fluorescence microscopy, the resolution may actually exceed the limits placed by any one of these three equations.
These equations are based upon a number of factors (including a variety of theoretical calculations made by optical physicists) to account for the behavior of objectives and condensers, and should not be considered an absolute value of any one general physical law. Notice that equation (1) and (2) differ by the multiplication factor, which is 0.5 for equation (1) and 0.61 for equation (2). Where r is resolution (the smallest resolvable distance between two objects), NA is a general term for the microscope numerical aperture, λ is the imaging wavelength, NA(obj) equals the objective numerical aperture, and NA(cond) is the condenser numerical aperture.
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