Depth of Focus and Depth of Field
The preceding discussion has tacitly implied that there is one exact image location. Anyone who has focused an optical instrument has noticed that the best focus is a small region, rather than a point, where the image neither improves nor blurs. This typically small but definite region is the depth of focus (Figure 1-13). We can conveniently describe it in terms of the range of effective correcting lens powers (in diopters) that permit adequate image clarity.
Depth of focus is a property of the imaging system, not of the image, and it varies from lens to lens. As discussed in the Quick-Start Guide, a pinhole has an infinite depth of focus; lenses have a far more restricted depth of focus.
Whereas depth of focus is the range of image locations over which an object remains sharply focused, depth of field is the range of object locations that will be sharply focused at a single image location (Figure 1-14). Clinically, depth of field is important for prescribing bifocals and trifocals for presbyopia. Even when the depth of focus is small, the depth of field can be large, depending on various factors.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.