Field of view number
The field of view is for eyepieces of optical microscopy the diameter of the object field multiplied by the magnification of the objective used in mm on. With better eyepieces it is often engraved next to the indication of the magnification. For eyepieces in which a real image is created in front of the first lens, e.g. B. With Plößl eyepieces, there is the field stop with this diameter.
For example, with a field of view of 20 mm and a 40-fold magnifying objective, the result is an object field diameter of 0.5 mm and an area of 0.196 mm 2 (according to the formula for the circular area a = πr 2 ). If there are intermediate magnifications in the beam path (e.g. using a 1.5x lens), the calculation must be adjusted accordingly.
Except for distortion effects at the edge of the image field, half the field number divided by the focal length of the eyepiece is equal to the tangent of half the field angle. In the case of interchangeable eyepieces for telescopes, instead of the field number, the field angle is usually given, since the angle magnification and not the objective magnification is considered there.
In histological diagnostics , the number of fields of view of the microscope used is important, since limit values for the number of core division figures per main field of view (HPF, corresponding to 40 times objective magnification) are specified for the grading of some malignant tumors . Since more or fewer core division figures are to be expected with larger or smaller HPF, the limit values are adapted to the size of the HPF and thus to the number of fields of view. In order to convert the usual specification of core division figures per ten HPF into core division figures per square millimeter, as required in the diagnosis of malignant melanoma , the number per ten HPF must be divided by 1.96 (with 20 field of view) or 2.38 (with Field of view number 22) or by 3.07 (with field of view number 25).