Posted by
Barbara Foster on
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Dear Craig
This is one of the easiest and most useful of calculations:
The full field of view of an objective is dependent on two factors:
the magnification of the objective and the FIELD NUMBER of the
eyepiece. The field number of the eyepiece is determined by the
distance across a baffle which is about about 2 cm up, inside the bottom
of the eyepiece. However, you don't need to measure it: the value,
in mm, is engraved on the eyepiece. Older or simpler microscopes
typically have field numbers on the order of 18mm. Newer
microscopes and research stands typically range from 22mm to 25 mm; ultra
wide eyepieces may go as high as about 35mm.
Please note that I said "full field of view" because the field
aperture iris (located at the light port in the base of the microscope
can be used to decrease the size of the field of view. This control is
valuable if you have highly scattering features: simply move the
object of interest to the center of the field and close the image of the
field iris around it. For most samples, you will be surprised at how much
clearer and crisper the image becomes... all without any loss of
resolution.
As for the calculation: Field of view = field number/magnification
of the objective*
Note the asterisk. The if there is any other INTERMEDIATE
magnifier, multiply the Mobj x Mintermediate. The eyepiece is NOT
part of this equation.
On a more empirical note, you can actually measure the REAL diameter of
the field of view directly using a stage micrometer. This approach
works fairly well for everything except the very lowest
magnifications. In that instance, you need some reference point in
the eyepiece (cross hair or an eyepiece micrometer). Align the
stage micrometer to one side of the field and take a reading to the
reference point then align the stage micrometer to the other side of the
field and measure to the reference point. Simply add the two values
together. This approach is especially useful with cameras because
the addition of the phototube and the camera chip frequently reduce the
field of view seen through the microscope.
Hope this was helpful.
Best regards
Barbara Foster, President
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At 03:00 PM 9/23/2007, Craig Brideau wrote:
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at
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I was wondering how I estimate the field of view for a given lens in
a
laser scanning microscope? I know it will depend on the
entrance
angle of the beam into the back aperture of the lens, but I don't
know
how to translate this into lateral focal point displacement for a
given working distance/NA/magnification of lens. Can anyone point
me
towards some good references?
Thanks,
Craig