A half covered lens Let's say there's an object in front of a convex lens. I have a piece of paper behind the lens at the right location such that it will the lens will form an image on the paper. The object is illuminating.
Now if I put another piece of paper right before the lens, and cover up half of the lens. The image, instead of only half being visible, it will actually remain the same. The only difference is that it will be dimmer.
Now, the question is what if I remove the paper at the location where the image forms, and put my eye there. What will I see? Will I see the piece of paper which is blocking half of the lens at all? Does that mean I will just "see through" the paper?
I wanted to try this out myself, but unfortunately I don't have a lens besides me.
 A: Your eye is just a lens and a screen. The lens is actually a compound lens, since both the cornea and the lens play a role in focussing the light, and the screen is your retina. So if your setup is initially:

Then if you take out the screen and put your eye there you'll get:

So you can work out what appears on your retina with exactly the same calculations as any other multi lens setup. But there are some complications because your eye can adjust the focal length of the lens, and this process is instinctive so it's hard to control. In addition the pupil acts as a pinhole camera and will produce a rough image even when objects aren't at the correct position to be in perfect focus.
Exactly what will appear on your retina depends on the geometry you've chosen, but the eye has such a large depth of focus that I suspect you would see the paper cutting off half the field of vision.
Note that the iris in your eye works by constricting to reduce the size of the aperture, and you can't see your iris. This is exactly equivalent to the fact in the first experiment that the paper doesn't appear as an image on the screen.
