# How does the focal length of the lens change the distance from the camera to an object?

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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