I have a very basic question in optics inspired by this article from the University of Stanford which clearly states that one should see a light source through a mask in the middle of a lens if the observer is at the focal point:
OK, you are on the Moon during a lunar eclipse. Look back at the Earth. To your eye, the Earth is a black disk with a bright ring around it. [...] Yup, the Earth [...] acts just like a big lens! [...] If you were on the Moon and the Moon was, indeed, at the focal point of our Earth-lens, when we looked back at the Earth you would see the bright ring around the black circle of the Earth but there would also be a bright spot smack dab in the middle of the black disk of the Earth. This spot is the focused light of the illuminated ring of atmosphere. Got it? Shiny!
To test this, I have bought a magnifying glass and put a mask (an obstruction) on it that is half the size of the lens. I have a light source point placed at infinity along the optical axis and, as expected, an image of it (another light point) at the focal distance on the other side of the lens. Then, I have placed my eye at the exact position of this image point and looked toward the lens. I think I re-created exactly the setup presented in the article.
First, I was happy to see that the annular lens around the mask looked uniformely lit by the source point located behind, as described by the article. But while I expected to see the point "through" the mask, the mask was just... blocking the light and there was absolutely no bright spot in the middle. My question is simple: why can't we see the point in the middle of the blocking mask when our eye is located at the focal point ? Am I wrong, or is this article wrong ?