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March 31, 2012


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John Dunn

Clearly (pun intended), this is a simplification of the topic. For a nice write-up in greater detail, take a look at:


George Storm

As you say - somewhat simplified.
Your photographer friend would have been on firmer ground if he had contested the equation: 1/u+1/v=1/f
For real (thick and/or multi-element) lenses the reference plane is not stationary and the magnification does not correspond to a constant focal length.

William Ketel

The fact is that as a number gets vary large, it's reciprocal does get very small, and if we are using fixed-point arithmetic the vary small number may indeed drop out, equivalent to becoming zero. So while a lens may not be focused on actual infinity, it can be focused on some point far enough away so that the difference is not measurable with common measurement procedures. In short terms, "close enough" to infinity.

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