You will notice that the f/4 P=2 lens has an Effective NA advantage on the front end just as it
does on the back end of the lens with the Effective aperture. The same applies with the
disadvantage with the f/4 P=1/2 lens. It turns out that there is an association between the NA on
the front of the lens and the NA on the rear of the lens. That will addressed in the next installment.
does on the back end of the lens with the Effective aperture. The same applies with the
disadvantage with the f/4 P=1/2 lens. It turns out that there is an association between the NA on
the front of the lens and the NA on the rear of the lens. That will addressed in the next installment.
In the previous entry we discussed the numerical aperture (NA) in similar terms to the f-number,
referring the calculation to the front focal point. Measured at the front focal point, the resulting
NA is the maximum possible for the lens. Imaging at the front focal point implies an infinite
magnification and infinite image distance - not possible with any camera that I know.
That leaves us with calculating an effective or working NA - the numerical aperture that the
entrance pupil forms with the object being photographed. Like the effective aperture on the
image side, the working NA is determined by f/#, magnification, and pupillary magnification. The
equation is very similar to the equation from the previous entry but with an added multiplier that
takes into account the magnification and the pupillary magnification.
NA = 1/(2*N*((1/m)+(1/P))) (where N=aperture setting, m=magnification, P=pupillary
magnification)
referring the calculation to the front focal point. Measured at the front focal point, the resulting
NA is the maximum possible for the lens. Imaging at the front focal point implies an infinite
magnification and infinite image distance - not possible with any camera that I know.
That leaves us with calculating an effective or working NA - the numerical aperture that the
entrance pupil forms with the object being photographed. Like the effective aperture on the
image side, the working NA is determined by f/#, magnification, and pupillary magnification. The
equation is very similar to the equation from the previous entry but with an added multiplier that
takes into account the magnification and the pupillary magnification.
NA = 1/(2*N*((1/m)+(1/P))) (where N=aperture setting, m=magnification, P=pupillary
magnification)
| The Effective Numerical Aperture |