Is camera magnification a constant? Magnification $M$ is defined as $\frac{-d_i}{d_o}$, where $d_i$ and $d_o$ denote the image distance and object distance, respectively. For a given camera, is the value $M$ typically the same for all values of $d_o$? That is, is it true that $\frac{d_{i_1}}{d_{o_1}} = \frac{d_{i_2}}{d_{o_2}}$ for all $d_{o_1}, d_{o_2}$ where $d_{i_1}$ and $d_{i_2}$ are the corresponding image distances?
Yes for a given camera with a fixed focus lens, sensor size and sensor to lens distance the optical magnification;
$$ M = |{ \frac{ h_{obj} }{h_{img} } }|$$ 
which is object vertical length divided by its image length on the sensor (a number less than unity, hence a reduction,  unless a macro image is taken with  a macro lens wihich has a true magnification) will be a constant for most of the proper object distance range.