Aperture stop:
The stop close to lens limits the size of the bundles of rays of axis point. It determines the amount of
light reaching any given point in the image, therefore controls the brightness of image (image plane).
Field stop:
Just front of the focal plane limits the angle at which the incident bundles can get through to this plane
(image plane). It determines the extent of the object or the field. (Field stop is usually made to coincide
with image.)