Aperture, shutter speed and scene luminance affect exposure*
If image lightness, aperture and shutter speed between 2 images is the same and ISO has changed then so has the exposure* changed.
* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.