Determining the Correct Aperture Setting
Infinity instruments which are equipped with a variable aperture iris diaphragm
can alter depth of field and light intensity by moving the lever control. Since
Infinity instruments are designed on microscopical principles, closing the iris
will result in a quick "jump" in contrast. In effect, the optical system
"seeks" its best performance level when this "jump" is observed.
This is the position also known as "Nelson's 3/4-aperture Rule"—a
classic condition of optical microscopy; this is the aperture at which contrast
and resolving power are mutually optimal. Closing the aperture further may increase
depth of field (if needed), but some loss of resolution will inevitably result.
By designing our instruments to be compatible with "Nelson's Rule," you
can be sure that optimum performance can be set—regardless of the subject's
own optical characteristics.