Structure and Optical Technologies Lenses of binoculars

Objective lens

The lens closest to the subject is the objective lens. Generally an objective lens combines convex and concave lenses to minimize color fringing, resulting in clearer images.

Eyepiece lens

This is the lens closest to the eye and magnifies the image formed by the objective lens.
Various techniques have been integrated into eyepiece to meet recently rising demand for binoculars with long eye relief and wide viewfield.

In general, when you focus at the center of the binoculars' field of view, the peripheral area may be out of focus. And when you focus on the periphery, sometimes the image at the center may be out of focus. This phenomenon is caused by lens aberrations. One of the ways to solve this problem is by employing field-flattener lens on the eyepiece lens.
Field-flattener lens can correct aberrations comprehensively, resulting in sharp images across the entire field of view.