TABLE OF CONTENTS It is tempting to present an MTF versus frequency plot for each lens design. Such a plot represents all the information necessary to evaluate the performance, and it is excellent for comparing two lenses which are evaluated under identical conditions for the same application. Modern software makes an MTF plot only a mouse click away, and the reader of this volume can readily get the MTF for these lens designs at focal lengths and under conditions which are appropriate to his application. Unfortunately, an MTF plot provides no information about which aberrations are adversely affecting the performance, or how one should go about improving the design. And unless a lens is effectively perfect, a single MTF plot at an arbitrarily chosen focus may well give a very misleading evaluation of the lens. An MTF evaluation is correct only for the focus at which the MTF has been calculated. To be valid, an MTF analysis (and the choice of the "best" focus) must take into account the following:
the application,
the relative importance of the axial image quality vs. the off-axis image quality,
the relative importance of resolution vs. contrast,
the focal length at which the lens is to be used,
the relative aperture setting,
and several other factors.
Note well that an MTF plot cannot be scaled when the the lens is scaled (except in the highly unlikely coincidence that the wavelength is going to be scaled by the...
