# Optics - Imaging from focal plane

1. May 13, 2015

### Wminus

Hi!

Assume paraxial rays. If I have a lens with a focal length $f$ and I place an object at the focal length to the left of the lens, the image will be at infinity. Correct? But will it be imaged in infinity to the left or right of the lens?

If I am looking into the lens from the right I will see the object as very far away, right? So i guess the image is to the left of the lens?

Thank you for all replies

2. May 13, 2015

### Staff: Mentor

I suppose you could consider it either way: as a virtual image at infinity to the left (as in your example), or as a real image at infinity to the right. It doesn't make any difference in calculations (e.g. in the thin lens equation), because "-1/∞" and "+1/∞" are both zero.