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I am reviewing some old image processing notes and have the following problem:

I have a camera with a projection plane of $2cm\ by\ 2cm$ and a $15mm$ lens. If you want an $1.5m$ object to take up half of the resulting image, how far away should it be?

It seems like the standard $x = \frac{fX}{Z}$ equation should be useful here, but I am not sure if this information gives focal length at all. How should this be solved?

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up vote 2 down vote accepted

"15mm lens" the 15mm would be the focal length

ps. make sure you use consistent units

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Is that always the case? The size of the lens is the focal length? – Porridge Dec 18 '11 at 14:39
@Porridge in astronomy you would be more interested in the diameter but otherwise if you are only given one number it's the focal length – Martin Beckett Dec 18 '11 at 15:52

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