Whenever we take the case of an object at infinity, we say that the image formed ends up being a point of light on the focal point if we had put a screen right at the focal point.
Now, for my lab, what we did was that we pointed the lens toward the far away (about 100 m away) trees outside the windows. Then, I assumed the objects were at infinity.
So, in the thin lens equation : 1/f=1/s=1/s', 1/s tends to 0.
Then, calculate the focal length, I just moved the screen forward and backward. When the image was clearest, I assumed the distance from the screen to the lens was the focal length.
However, I find this weird when I came back to study for my lab test. How come I did not see a single point on the screen? I actually saw an inverted, very small (about 5 cm tall) image of the trees.
Similarly, if we assume the sun is at infinity, should our eyes not focus it to the focal point and see it as a mere point rather than a large circle in the sky?
This sounds stupid, but I think I am missing something conceptually important.