1. Ch. Pendleburry: Lenses and System of Lenses, treatged after the manner of Gauss. (Forgotten Books 2018)
2. https://en.wikipedia.org/wiki/Zoom_lens
3. https://www.bing.com/search?q=camera+zoom+lenses&FORM=QSRE5
4. W.T. Welfored: Aberrations of optical systems (CRC-Press 1986)
5. https://en.wikipedia.org/wiki/Optical_aberration