I've been trying to look at other questions here but I cannot find to late on. I'm trying to replicate Young's original experiment, trying to find a fraunhofer diffraction, this is basically diffraction in infinity.
To solve this problem I'm using a lens. There I will have my light (collimated), striking the object, then through the lens and into the camera. He says I need f-f setup, that is the distance f from the object to the lens, and f from the lens to the camera. But when I use the thin-lens approximation, all I can get is that the length from the object to the lens should be f, what am I missing here ?