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Originally Posted by [1ponders]
What confuses me is that the objective, as Geoff says, focuses what passes though the objective to a single point (am I reading that the right way?).
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Yes, you read that as I intended. A small or a large objective of the same focal length will still focus all light it collects to a point at the focal length in the same way, just the bigger one will focus
more light to the same point.
In the picture all the rays from a distant source come to focus at a single point. This is the position of the focal plane .
If the viewed object is x degrees off centre, it will be the same angle off centre at the focal plane. Now for our eyepiece with the much shorter FL, the angular displacement at the focal plane is much greater. This is how magnification works.
It doesnt matter what size the objective is, it can be a pinhole and the true field of view is the same.
You could model it in mechanical terms with a lever with its pivot at the position of the objective, the length of the lever is the focal length.
The end of the lever is pivoted to a short lever of length of your eyepieces focal length. A small movement in the "objective lever" would create a much greater anglular movement in the "eyepiece" lever.