noun
objective lens
Technical term in optics; the lens closest to the object being observed, used in microscopes, telescopes, cameras, etc.
顕微鏡の対物レンズを交換して倍率を上げた。
I changed the microscope's objective lens to increase the magnification.
接眼レンズ is the eyepiece lens (closest to the eye), while 対物レンズ is the objective lens (closest to the object).
Compound of 対物 (taibutsu, 'facing the object') + レンズ (renzu, 'lens').