The depth of focus of the eye is of importance to the ophthalmologist primarily as it relates to visual acuity. For example, when determining the range of clear vision of an early presbyope at near, the examiner determines the range of distances from the eye within which the patient can clearly read print of a given size. Depth of focus determines part of this range. The depth of focus bears consideration in all testing of visual acuity as it relates to refraction. This is particularly true when the refractive correction is being determined for distant vision in the usual subjective manner. Depth of focus very likely plays a marked role also when miotics, such as DFP (di-isopropyl fluorophosphate), are used therapeutically in cases of convergence excess. This and other practical considerations are treated later.
There is no absolute value for the depth of focus, for it varies greatly with test
SCHWARTZ JT, OGLE KN. The Depth of Focus of the Eye. AMA Arch Ophthalmol. 1959;61(4):578-588. doi:10.1001/archopht.1959.00940090580013