This document summarizes an optical design for a 100 mm focal length f/1.25 lens with a 20 degree field of view that is diffraction limited from 0.4861u to 0.6563u. The design uses expensive specialty glasses with anomalous dispersion to achieve color correction over a broad spectrum while maintaining diffraction limited performance. All positive lenses are OHARA FPL51 glass, while the negative lenses use Schott KZFSN2, KZFSN4, or equivalent OHARA glasses. The polychromatic MTF curve shows extremely high performance over the wide field of view and wavelength range. This fast f/1.25 design achieves an unusually high level of color correction and has a 210

1. A 100 mm focal length f/1.25 curved image design with a 20 degree
field and diffraction-limited correction from .4861u to .6563u
David Shafer Optical Design
Fairfield, CT 06824
203-259-1431
shaferlens@sbcglobal.net

2. A curved image surface allows for much higher
performance designs than a flat image requirement does.
Here is a monochromatic example – a f/1.0 design with a
20 degree field and a 150 mm focal length. It is
diffraction-limited at .5000u over the 52 mm diameter
field. That provides an enormous number of Airy disks
over that image area.

3. It is a real challenge to make this monochromatic
curved image design be corrected for color over a broad
spectrum while still keeping it diffraction-limited. That
can only be done by using very expensive special glasses
with anomalous dispersion. I have done just such a
design, where it is 100 mm focal length and f/1.25 with a
20 degree field diameter. It is diffraction-limited from
.4876u to .6563u

4. All the positive lenses are the very low dispersion
OHARA glass FPL51. The first two negative lenses and
the last two negative ones are Schott KZFSN2, or the
OHARA equivalent. The 3rd
negative lens is Schott KZFSN4
or the OHARA equivalent. You can see from this
polychromatic MTF curve that the performance is
extremely high over the 20 degree field and the .4861u
to .6563u range.

5. This is a very unusually high state of color correction for
this fast f/1.25 design. Notice the relatively large
distance to the curved image. The axial length along the
optical axis is 210 mm for this 100 mm focal length
design.
As the lens drawing shows the curved image surface
has a rather mild sag over the image diameter and this is
quite achievable with today’s technology, as indicated by
published results.
Consider, for comparison purposes, the classical
monocentric ball lens with several elements and glass
types. It cannot achieve this fast f/1.25 speed and that
design has a large glass path and weight that would be
completely impractical in a design with this 100 mm focal
length.