物镜是使用在显微镜、望远镜、照相机或其他的光学仪器前端，第一个接收到被观测物体光线的透镜或面镜。物镜也称为接物镜或接物玻璃。

显微镜物镜的典型设计是等焦距的，这意味着当你将样品由一个物镜换至另一个物镜时，样品的位置仍然会在新物镜的焦点 上。显微镜的物镜有两个参数，即放大率和焦比。前者典型的范围从5 X 至 100 X；后者从0.14至0.7，相当于焦距从40mm至2mm。对于更高倍数的应用，必须使用油浸物镜。这种物镜经过特别的设计，使用时必须浸没在折射率匹配的油脂（一种折射率相符合的材料）内。

摄影用的变焦镜有些也是等焦距，所以也能变更放大率而无须重新调整焦距。

In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.