On my favorite newbie friendly photography website, ProDSLR there is a thread where a poster mentions that a particular lens has “15 elements in 11 groups.” My first thought when I read the post was, “What did you say about my mother?” Surely the post represented some sort of photography-related hieroglyph and that I should make my way to the nearest camera shop to purchase my very own Rosetta Stone. I didn’t have to do that at all. The original poster, Kayne, was kind enough to clarify what he wrote with such detail that there could be no misunderstanding. He said that he didn’t mind if I reposted what he wrote in my blog. Here it goes:
15 elements in 11 groups is the lens construction Dee. Take a look at this illustration…
If you count the pieces of glass in the lens you will see there are 15 of them. They are referred to as elements. The 11 groups are the number of those elements that move for focusing. A group is a piece of glass that has a bit of space in between it and the next closes piece of glass to it. Or two pieces of glass that are fit together that have space between them and next single piece of glass or two pieces of glass that are fit together. Now… in the illustration you can see 4 pairings of elements… these being 2 pieces of glass that are very close together in the illustration. They are the ones that look like they have just a sliver of space in between them. Inside a lens these pieces of glass touch each other. Those 4 pairings move in conjunction with one another. These pairings are considered 4 groups. The rest of the individual pieces of glass make up the other 7 groups. Altogether they make up 11 groups.
This lens has 14 elements in 11 groups.
Now take a look at this next picture… its the 14-24 cut in half. You can clearly see the 3 pairings(groups) of elements that are touching each other. There are two right at the lens mount. Then three individual pieces of glass. Then another pairing.
Kayne’s pretty smart, huh? If after reading this explanation, you don’t ‘get it’, consider another hobby. If by chance the explanation provoked more questions, I’ll venture to say you’re on the right track.
Naturally, I had more questions after Kayne’s explanation, so I asked him the following:
1. Is there a standard? Meaning, Is an excellent lens one that has lots of pairings and lots of elements?
2. What’s minimum number of elements or pairings that you look for in a lens?
3. What is the function of the pairings (the ones close together)?
I know you don’t build the things, but if you happen to know, please tell me .
In looking at the close pairings, I can’t help but wonder why they simply don’t use ONE larger piece of glass instead of two.
His responses were as follows:
1. There is no standard to the amount of elements and groups that comprise what would be considered an excellent lens or any lens for that matter. A lens is constructed in such a way as to minimize or eliminate as many optical aberrations as possible. Whether the lens is a prime, wide angle prime/zoom or telephoto prime/zoom is just one of the determining factors of a lenses construction. Others being its focal length(s) and zoom range. If there are too few elements and groups the final image may have many optical aberrations. If too many elements and groups there could be a lot of flaring in the final image. The engineer must find a good medium between too few and too may elements and groups.
2. Personally… there is no minimum/maximum number of elements and groups in a lens that I look for. As long as the lens produces sharp images, renders color nice and true and produces good(in my eyes) bokeh I would be interested in it.
3. The pairings of elements(just like single elements) are designed in such a way to minimize or eliminate a specific optical aberration. For instance… Chromatic Aberration or “Color Fringing”. To get rid of this an engineer may take two elements made of different glass compounds and different refractive indices and cement them together to form a pairing or group designed to do this. This may also be done to minimize or eliminate Spherical Aberration, Coma problems or any other optical aberration.
So for your comment about only using one larger piece of glass instead of two… if that could be done I imagine engineers would do that. But it can’t. To get rid of specific optical aberrations different compounds of glass with different refractive indices, molded in a specific way, cemented together and with certain coatings must be used.