Lens Elements and Groups
When shopping for camera gear, you've undoubtedly encountered listings detailing how many "elements" and "groups" certain lenses have. They generally don't come right out and say it, but such listings often imply that more is better. But is this really true, and just what the heck are elements and groups anyway?
In order to answer the first question, we need to start with the second. "Elements" are the individual pieces of glass used in the construction of lenses. Each one is individually ground and polished using computer controlled equipment to perform a specific purpose such as focusing the light passing through it in a specific way, or helping to correct any of several different kinds of aberrations introduced by another element. Elements are curved pieces of glass much like the lenses commonly found in ordinary magnifying glasses or in eyeglasses. Building lenses to high standards, especially modern zoom lenses, is a complicated process, and a lens designer will often employ quite a few elements in a given lens.
"Groups" are groupings of elements, cemented together using optically transparent glue and intended to act as a unit. The construction of elements often make use of special formulations of glass that include any number of rare earth elements and other minerals in order to alter the refractive properties of the glass to better perform some specific function. Gluing two elements together, often ones that employ different glass compounds than each other, creates a lens group. Think of groups as compound elements.
So, if each element and group used enables a lens designer to better control the performance characteristics of a lens, do more elements and groups make for a better lens? Maybe yes, maybe no. While it is true that each element and group is treated with anti-reflective coatings, these coatings are not perfect, and every glass-to-air interface will add to the possibility of reflections and lens flare. Beware of the lens that tries to do too much unless reviews confirm it's potential. Sometimes simpler lens designs are better, but it's all a trade off. More elements and groups can mean the lens performance is better controlled, but not always under all conditions. It depends on how well the lens maker has handled all the issues involved, not just how many elements and groups a lens has.
Additional elements also frequently mean a lens is more expensive than an otherwise similar one with fewer elements. Each of those elements requires effort to create as does the design that employs it. And adding an element to a lens also adds weight to it. The weight of a professional zoom lens can often surprise someone not familiar with such lenses. If a lens weighs more than you are willing to carry around, it doesn't matter how well it might have performed had you not left it at home. If you want to get the best performance you can, especially in a zoom lens, this will often mean a number of elements and groups, which will in turn mean a heavier lens. If this is what you want or need, just be prepared for what comes with it. The Nikon 28-70mm f/2.8 ED-IF AFS is an excellent mid-range zoom but its 15 elements in 11 groups mean that it weighs over two pounds. Personally, I love this lens. Its range works well on digital, and it is tack sharp throughout. But it is big and it is heavy.
Back in the day when professional lenses were invariably fixed focal length, most lenses had just four elements and manufacturers sometimes tried to save money by paring this back to only three. Today, modern zooms frequently have ten or more elements and even fixed focal length lenses often have five or six. Today, such statistics are just one factor among many in determining which lens is better. Ultimately, the only way to tell is to try it out, or read what someone you trust has to say about it.