looking through a lens element

[dfoo]

So ... I'm holding up a lens looking through the back element. I have the lens focused at around 3M pointing at a window 3M in front of me. Why can't I see anything other than a big blur through the back element? My eye is a similar distance from the lens as the film would be a camera, but yet everything is a big blur?

 

[antiquark]

Because your eye also has a lens in it. If you were to hold up a piece if paper behind the lens, there should be a well-focused image projected on it.

 

[dfoo]

I just tried the paper trick... very cool!