Question

What is the difference between principal focus & focus?​

Answer 1

Answer:

Principal focus is a point on focal plane and is located on the principal axis.

Means both are same.

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Answer 2

Answer:

When the light rays are not parallel to the principal axis and focused using a lens then the focus happens at a point not lying on the principal axis then it is called as focus. Generally we refer principal focus as focus itself. “Focus” and “Principal focus” can be used as synonyms but are not identical to each other.