The cemented achromatic doublet is known, understood and loved by all, and we really do not expect any problems by switching designs from old to new glass types. Here is a 100mm f/5 doublet designed for the F, d and C lines (equal weights) and a field of 1 degree.







This is a perfectly ordinary achromatic doublet. Let’s now change the glasses to the lead free forms. This is the wavefront plot now:



Without doing any re-optimisation, the performance is clearly unchanged. So here, yes, the glasses are ‘exact equivalents’.

Let’s now look at a design balanced in the blue and near-UV at wavelengths of 0.37, 0.4 and 0.46 microns [I started this example with a wavelength of 0.36 until I discovered that although it is valid with SF5, the reduced UV transmission means that it does not compute with N-SF5! Also, the f/no has been changed to f/6.67 so that the design has a similar performance to that of the previous example – at f/5 it was found that the shift in reference wavelength and the non-optimal glass selection had significantly increased the OPD]

Here is the design:



The aperture has been reduced slightly to produce a lens of similar performance to the previous design and the flint has been selected to produce the effect I wish to demonstrate (it is not the optimum flint for this design at this wavelength!)





Now, let us again change the glasses for the new lead free types, and without optimizing, the performance is now changed:






Clearly a significant change showing that, in this waveband, the old and new versions of F2 can no longer be considered exact equivalents. I deliberately did not mention the BK7 as the change from BK7 to N-BK7 makes no difference. The effect shown is entirely caused by the change from F2 to N-F2. [This, I believe, reinforces Schott’s comment that when only arsenic has been removed from an old type glass then the new glass is the exact equivalent.]

Re-optimizing the design does restore the original performance.

For a further example, you might like to design an f/5 NIR doublet of 100 mm focal length, corrected for 1.0, 1.5 and 2.2 microns using CaF2 (as the crown!) and SF1 as the flint - and then switch the flint to N-SF1. By now, the effect will not surprise you.