When calculating total magnification, a 15x eyepiece with 40x objective gives, and field number 18 mm, what is approxima
FOV = FN / objective mag. FN 18 mm /40 = 0.45 mm. Eyepiece mag does not affect FOV diameter formula, but affects total mag (600x). High WF eyepieces increase FN.
Ref: NCBI Bookshelf, Molecular Biology of Cell: Resolving power, numerical aperture, and Abbe limit.