High power field
In microscopy, the circular area seen at the highest magnification. Without using an oil immersion objective, this is generally x40, but x20 and x25 may justifiably be regarded as "high power" too. The total area is also influenced by the eyepiece. A further point is that high power fields (hpf) are surrogate measures based on shallow cuboidal slices being used as a representative sample of a larger volume, so the thickness of the slice is important too (though generally constant at 3-5 micrometres). As a result of these factors, the size of a hpf varies from microscope to microscope with variation of up to 600% noted.
When using the term, it is recommended that the actual area is stated explicitly to allow any criteria to be used universally. This is important where the results have a significant impact on diagnosis, e.g. certain tumours such as smooth muscle tumours and GISTs considered malignant if the number of mitotic figures per hpf exceeds a set number. In breast cancer grading and melanoma staging depend on mitotic counts per area.
Calculating Field Diameter
In older microscopes, the field view was limited by objective design as these were intended for use with eyepieces capturing an 18mm view. Therefore using a larger 20mm wide-field eyepiece would not increase the field diameter. Newer microscopes have larger useable field sizes (22-28mm) allowing the use of wide-field eyepieces.
For most manufacturers, e.g. Olympus and Nikon, dividing the eyepiece field view by the objective magnficiation gives you the field view. So, a 22mm eyepiece with a 40x objective gives a field diameter of 22 ÷ 40 = 0.55mm.
Field diameter can also be measured directly using a specially-designed glass slide with measurements etched on to it or using the Vernier scale provided on most microscope stages.