All lenses are made up of at least one lens element. Most modern lenses have multiple lens elements, in groups, with a specific orientation.
For those that want to take apart lenses for cleaning purposes you will, at some time, encounter an opps.
You’ve taken apart a lens and the elements are removed. You clean them nicely and then put it back together. Then later on you realize that your images come out soft, or edges are out of focus, or you cannot focus at all. Opps.
What happened is that one or more of the elements are in the wrong orientation. What do you do ?
You hop onto the web and search for images that show diagrams of the element/group arrangement. There are many resources for these on the web and a lot of them can be found in older user guides or handbooks.
Search for “xxxx lens diagram” or “xxxx lens elements” … and hopefully you get something.
Another resource is through Forums. Many Forums that specialize in specific camera types will often have members who have collected this type of information and will gladly provide this to others.
Having a mirrorless camera like my NEX-3 (tool) helps in quickly checking a lens after you have serviced it.