# Converging Lenses with Objects at “Infinity”

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.

• The focal length is determined by the lens characteristics, not by the objects being imaged. – paisanco Nov 23 '14 at 16:07
• This is an experiment. We are trying to find the focal length. I think you don't understand what I was asking. Maybe I was not clear. – yolo123 Nov 23 '14 at 16:32
• I understood perfectly well what you were asking. @Andrew has written an answer, so I won't bother. – paisanco Nov 23 '14 at 16:44

• Hmm... @Andrew I do not understand why it is ftan($\theta$). Can you explain? – yolo123 Nov 23 '14 at 16:48
• For an "ideal" lens, I think the relationship is d = f$\theta$. Thorlabs doesn't sell ideal lenses, so I think they're trying to give a more accurate equation for that particular lens. – Andrew Nov 23 '14 at 16:52