What makes a good lens?

There was an interesting post on Squidu.com, that got me thinking:

[quote=interstellaryeller]I do not understand this. Anyone that has 100 or 200 lens in my eyes is a form of spam in itself. nobody can be a expert in that many fields and so I just sift over one or two of their top lens. I really think anything over 10 lens is just nonsence. But squidoo rewards this with lens rank and purple stars. But of course that my opinion.[/quote]

I find that an interesting opinion, but it got me thinking about why I make lenses and what I look for in a lens. I've written well over a thousand reviews. Technically I could turn each of them into lenses (although I prefer to write original content each time) but it would not change the fact that that would be one thousand lenses written from a position of knowledge.

It is certainly possible for one field to generate more than one lens. Astronomy for example has its history, photography, how to take the pictures, the best places to camp to avoid light pollution, observatories, famous figures, equipment etc. so someone who is an expert on those can do lenses on those fields where it's almost certain they know more than their visitors. That's seven fields with space for multiple lenses already. And to rebut the idea that people can't be an expert in more than one field, look up polymaths (http://en.wikipedia.org/wiki/Polymath). People like Leonardo da Vinci or Rupert Gould were experts in several. To a lesser extent it used to be expected of the upper classes.

Rather than an expert's guide I tend to see my lenses more as telling enough of the story to involve the visitor, more of a portal site or guide than a complete detailed view. An expert would write an entire book on some of these subjects, and lens don't have the word count to cover everything to that level - nor do most readers have the inclination to go into that depth on their first encounter with a topic (I certainly don't).

In general I'd say a lens is not spam if the user comes away knowing a bit more than they did, having enjoyed the read, and not feeling as though they wasted their time - ideally with more of an interest in the field.

That's just my view, other people may disagree. Any comments?