Is camera magnification a constant?
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.