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--------------------- Optics Primer - Determining the Angle of View

--------------------------------------------------------- By Bill Claff

Let's return to our 50mm lens focused at infinity and show the off axis rays:

We're only showing 7 equally spaced rays for illustration purposes.
One of the incoming rays crosses the optical axis at the center of the aperture; this is called the chief ray.
Here is the location of the chief ray in our example:

The chief ray is important for a number of reasons.
The angle of the chief ray before it reaches the lens determines the angle of view.

In our example the angle of the chief ray (times 2) matches perfectly the specified angle of view of 46.14°

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