In Figure 3.4 it can be seen that the angle of incidence at this surface is much larger for the upper ray than for the lower.
The Merte ’
surface A strongly curved,collective cemented surface with a small index break (to the order of 0.06) has an effect which can be used to reduce
the undercorrected zonal spherical aberration.The central doublet of the Hektor lens shown in Fig.3.5 illustrates this principle.
3.6 Vignetting and Its Uses
Vignetting,which is simply the mechanical limitation or obstruction of an oblique beam,is usually regarded primarily as something which reduces the off-axis illumination in the image.However,vignetting
often plays an essential role in determining the off-axis image quality as well as the illumination.
3.7 Eliminating a Weak Element;the Concentric Problem Occasinally an automatic design program will produce a design with an element of very low power.Frequently this means that the element can be removed from the design without adversely affecting the quality of the design.Often a straightforward removal will not work;the design process may simply “blow up.”
3.8 Balancing Aberrations
The optimum balance of the aberrations is not always the same in every case;the best balance varies with the application and depends on the size of the residual aberration .In general,for well-corrected lenses, the aberrations should be balanced so as to minimize the OPD,that is to say,the wavefront variance,but there are
Significant exceptions.
Spherical aberration
If a lens is well-corrected and the high-order residual spherical aberration is small,so that the OPD is to the order of a half-wave or less,then the best correction is almost always that with the marginal spherical corrected to zero,as illustrated in Fig.3.6b.However,when the zonal spherical is large ,there are two situations where one may want to depart from complete correction of the marginal spherical.
If the lens will always be used at full aperture(as a projection lens,for example),and if the spherical aberration residual is large(say to the order of a wave or so),the diffraction effects will be small when compared to the aberration blur;then the spherical aberration should be corrected to minimize the size of the blur spot rather than to minimize the OPD.This will produce the best contrast for an image with relatively coarse details,that is to say,for a resolution well below the diffraction limit.As an example, at a speed of f/1.6,a 16-mm projection lens has a diffraction cutoff frequency of about 1100 line pairs per millimeter(lpm).But its performances is consided quite good if it resolves 100 lpm,an order of magnitude less than the diffraction limit.Such a lens can advantageously be corrected for the minimum diameter geometrical blur spot.This state of correction occurs (for third-and fifth-order spherical)when LA z=1.5LA m,or TA z=1.05TA m;the result is a high-contrast, but low-resolution,image.This correction is illustrated in Fig.3.6a.
The three correction states shown in Fig.3.6 also indicate the manner in which the spherical aberration is changed when the third-order aberration is changed.
Chromatic aberration and spherochromatism
Here the question is how to balance the spherochromatism,which typically causes the spherical aberration at short (blue) wavelengths to be overcorrected and that at the long (red) wavelengths to be undecorrected.
Astigmatism and Petzval field curvature
In a typical anastigmat lens the fifth-order astigmatism tends to become significantly undercorrected (i.e.,negative) as the field angle
is Increased.A typical state of correction is shown in Fig.3.7,Fig.3.8.。