It's as much to do with marketing as anything else, people seem to be impressed by the number of chunks of glass and plastic in their lenses. It also helps explain to customers where their money is going. To be fair though for very experienced photographers etc it also can give a good idea of the optical complexity of the camera lens.
Originally posted by kamwai how will number of elements and groups affect the performance?
This is a very complex issue and there's no easy answer! While there's no direct correlation between the number of elements and groups in a lens and it's optical performance this statement has to be qualfied by stating that the number of elements and groups is largely determined by the amount of optical correction required, lens speed (aperture) and overall optical design of the lens.
For example, the Zeiss Tessar design of 4 elements in 3 groups is one of the most widely copied designs for medium and large format camera and gives excellent results over quite a large range of focal lengths.
Some of Nikons best lenses are also quite simple designs, such as the 55/3.5 Micro (macro) lens, which is a 5 element desing in 4 groups, or the 105/2.5 which is also a 5/4 design.
Whereas the 13/5.6 Nikkor required 16 elements in 12 groups and the AF-D 20/2.8 uses 12 elements in 9 groups.
Originally posted by kamwai I guess different groups of elements have different functions? some for focusing..some for zooming??
Not always, in optics overall performance is determined by the design requiremens and its implementation. This is largely restricted by the types of materials available for the lens elements and cost of manufacture.
The use of multicoating on lens elements reduces significantly the amount of internal reflections and boosts light transmission signficiantly thus allowing optical designers to create complex optical designs without incurring massive light losses.