There are many rules of thumb in optics, and we will summarize the most useful ones here. While they may be discussed elsewhere in this book, this chapter is a compendium of the most important and useful rules of thumb and hints.

• Diffraction-limited Airy disk diameter = 2.44 ? f/# in units of ?.

  • In the visible the Airy disk diameter is approximately equal to the f/# expressed in micrometers.

• Diffraction-limited angular Airy disk diameter = 2.44 ?/ D rads, where D is the entrance pupil diameter in the same units as ?.

• Resolution of a diffraction-limited system in the visible is approximately 136/ D in arc seconds, where D is the diameter of the entrance pupil in millimeters.

• A system will provide image quality, which is nearly indistinguishable from perfect, if the optical path difference from a nearest reference spherical wavefront reaching the image departs from sphericity by one-quarter of the wavelength of the light or radiation. This is the Rayleigh Criteria.

• The depth of focus for one-quarter-wave peak-to-valley optical path difference = (2 ? f/#) ₂ in units of ?.

  • In the visible, the depth of focus is approximately equal to the ( f/#) ₂ in micrometers.

• The spatial frequency where the MTF goes to zero = 1/( ? f/#) = 1000 line pairs/mm for an f/2 lens at a wavelength of 0.5 m.