I suspect that not all the readers will be familiar with apochromatic lenses. The theory is available in several places – Herzberger’s book ‘Modern Geometric Optics’ comes to mind.

In achromatic lenses, the focus versus wavelength curve is essentially parabolic. In designs for the visible region, the foci at the blue and red wavelength will coincide with the green wavelength falling slightly short (for simple positive power lenses). The amount that the focus falls short is known as the secondary spectrum. Secondary spectrum is a ‘property’ of achromatic designs made from ‘ordinary’ glasses and where each lens group is colour corrected.

However, using anomalous-dispersion glasses such as the short flints (KzFSN4 for example), fluorite-crowns and flints in certain combinations can produce lens designs where the variation of focus with wavelength is essentially cubic - in a visible region design, for example, the red, green and blue regions will have the same focal plane with only small deviations of focus for wavelengths in between.

It was while designing an apochromatic lens that I discovered that serendipity and lens design do, just once in a while, go together! Let’s look at an ordinary apochromat. This one is a ‘split – triplet’ – i.e. one where an element has been split off to give coma control. It is easily derived from one of the ZEMAX sample files.

The design is for a very wideband apochromat of 100mm focal length, f/10 color corrected from 0.36 to 1.0 microns:



and looks like this:



With the following OPD performance and focal shift plot:





And, so far, absolutely no surprises…But now just replace F2 with N-F2, and without optimising we get:





These changes show clearly that the partial dispersion in the blue/near-uv region are very different between the old and the lead-free glass types. It is well known that apochromats are very sensitive to the exact melt data of the glasses so this large change in performance is not unexpected.

But what is unexpected is the result of re-optimising the design with the N-F2 glass:



The residual aberrations are clearly smaller. But why? We have developed a super-apochromat! Just look how well the focal shift is corrected now...



I do feel like one of the Princes of Serendip!

But will it make my fame and fortune? No, probably not. And why not? Well, I would expect the designs to be extremely sensitive to exact melt data. And at these unusual wavelengths the glass manufacturers charge a lot extra to undertake the measurements.

I leave the reader to explore further.

If anyone would care to study the effect of substituting new glasses for old in thermally stabilised lenses,  it would make an excellent Knowledge base article ;-)

I’m sure I have made the point several times already but I would like to emphasise once again that the new, lead-free glasses generally are not the exact equivalents of the old glasses that they replace.