How can one focus position on a lens focus light from an infinite range if objects in that range are at different distances from the lens?
Bonus: how can lenses focus past infinity?
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“infinity” in this case just means light rays come in parallel. Things don’t actually need to be infinitely far away, a mountain range 50km away and a building 1km away are basically both “infinitely” far away.
If you imagine a small object very close to the lens, the light rays from the object are diverging. Spreading outwards. The lens needs to bend them inwards to get them to hit the film/sensor/you retina straight. See [here](https://www.learnopencv.com/wp-content/uploads/2017/12/focal-length.jpg)
If you imagine an object across the room, the rays from it to the lens are almost straight. Not quite straight, but very close. Those that previous image, and image moving the focal point out and you’ll see how the rays get straighter, less angled and less diverging. The lens only needs to bend them a little. That’s not to say there’s aren’t wildly diverging rays just like an object up close, there absolutely are. But we don’t care about those, they miss the lens entirely.
If an object is really far away, the rays come in straight. The lens doesn’t need to do much of anything (well, unless it’s trying to be a telescope). A mountain range 50km away and a building 1km away both have light rays coming in basically straight and parallel. A camera lens (or your eye) doesn’t need to do much bending at all. That’s why a camera (and your eye) can focus on a mountain and building far away with both being in focus, depsite then actually being really far apart. Doesn’t take that far to make the rays near parallel, and anything that fat or further is the same. We just call that infinite focus.
You could theoretically go past infinity too. That’s not to say objects exist more than infinitely far away, but in terms of the bending we’ve been talking about. If close means bending rays spreading out back together (converging), and infinite is the rays just coming in straight (parallel), then a lens focused beyond infinity takes in rays that are coming together and bends them apart (diverging). Not a photographer so not sure if you’d ever want to use a camera lens like that (guess if you want fisheye effect?) [but the second eye piece lens of a telescope is doing that.](http://www.bbc.co.uk/staticarchive/2648417c1b83c1609086caa1279537d9e33ebd93.jpg). You’d probably just call it a diverging or concave lens though, rather than a “beyond infinity focal length lens” because it’s quite nonsensical sounding.
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