high-power field high-power field a high-power field (hpf), when used in relation to microscopy, references the field of view under the maximum magnification power of the objective being used. often, this represents a 400-fold magnification when referenced in scientific papers. area area per high-power field for some microscope types: olympus bx50, bx40 or bh2 or ao: 0.096 mm2 ao with 10x eyepiece: 0.12 mm2 olympus with 10x eyepiece: 0.16 mm2 nikon eclipse e400 with 10x eyepiece and 40x objective: 0.25mm2 leitz ortholux: 0.27 mm2 leitz diaplan: 0.31 mm2 examples of usage the area provides a reference unit, for example in reference ranges for urine tests. used for grading of soft tissue tumors: grading, usually on a scale of i to iii, is based on the degree of differentiation, the average number of mitoses per high-power field, cellularity, pleomorphism, and an estimate of the extent of necrosis (presumably a reflection of rate of growth). mitotic counts and necrosis are the most important predictors. the following grading is part of classification of breast cancer: mitotic count per 10 high-power fields (hpfs) area per hpfscore 0.096 mm20.12 mm20.16 mm2>0.27 mm20.31 mm2 0-30-40-50-90-111 4-75-86-1010-1912-222 >7>8>10>19>223