one of my regular car trips there is one of those electronic bill
boards. I found it to be particularly blury and wondered why. Each time I passed the sign I did a little more investigation. The
sign was positioned high up and to the right and I found that the blur
was completely removed if tilted my head up and looked directly at it. I next noticed that the blur was really a separation of the colours caused by my glasses. After some experimenting I produced a test picture that I think best demonstrates the problem.
If I look at the picture above through the centre of my lenses I see no colour fringing of the red spot. Moving off the centre I see a black shadow on one side of the spot and a magenta shadow on the other side. It
turns out that chromatic aberration is a feature of spectacle lenses
and its magnitude depends on something called the Abbe number of the
lens material. The diameter of the central area of the lens of a
given strength that gives acceptable chromatic aberration is said
to be directly proportional to the Abbe number. The best discussion
of this subject that I have found is in the 28 Jan 2005 edition of
Optometry Today in an article titled "Materials for spectacle lenses
Optical and mechanical performance". Googleing for the title should find the pdf. Update-
When replacing my glasses I changed from polycarbonate with an Abbe
value of 30 to Trivex with an Abbe value of 44 and noticed a
significant reduction in fringing.