Most modern box lenses do exactly this, servos in the lens move the elements around to achieve the desired zoom and focus. Older box lenses use a convoluted se

Most modern box lenses do exactly this, servos in the lens move the elements around to achieve the desired zoom and focus. Older box lenses use a convoluted set of gears to achieve the same thing. Some handheld ENG lenses for shoulder carried cameras get close to parfocal by making the lens elements travel in a complex way, but its not as perfect as the box lenses get.