The field strength required for field evaporation of tungsten and for best image conditions in the field ion microscope is measured by comparison with field electron emission of the same emitter. The evaporation field is independent of emitter radius except for radii below 150 A where the influence of free surface energy becomes noticeable. The field for the best conditions of a helium ion image of a typical emitter of 1000 A radius is 440 Mv/cm. This field increases gradually with decreasing tip radius, so that the ratio of evaporation field to best image field for tungsten decreases from 1.27 at 1000 A radius to 1.0 at about 40 A radius. The radius dependence of this ratio can be calculated by assuming that the time a hopping atom spends in the critical ionization zone is a constant fraction of the ionization time. Practical implications are that large tip radii are preferable for easily evaporating emitter materials, and small radii may be useful when field evaporation is to be studied under best image conditions.