Where does most of the focussing in the eye occur?

Answer 1

Unfortunately that's a bit broad. Let's just lay out the bits and you can draw your own conclusions.

First, there's the lense which acts primarily to bend the light into the eye. This has the advantage of pulling in light from nearly 90 degrees away from the direct line of sight in all 360 degrees (i.e. you can see things that are almost directly below, above and beside you). It pulls the light (as much as possible) into a narrow area in the back of the eye. This is called focussing as might be conceived of by a physicist. As such it is the only focussing going on in the eye.

However, the focused image is beamed onto the retina which are a map of cells which individually see and interpret light. These are densest in the center of the eye, and are sparser around the edges. Therefore, the most detailed image will be in that center while the outter edges will miss a lot of the details. In a sense that center area is focussing the image just by virtue of its location.

Because that dense area is so limited the eye can't see everything at the same time. Also there is a blind spot with no retina. In order to overcome both of those issues our eyes are continually shifting in very minute increments. The muscles which do this are working to allow a brief but complete focus of the eye on every section of the image. Again, in a sense this is focussing.

As a secondary action, the muscles between two eyes work to move in tandem. An experiment which put fish-eye classes on a bunch of small birds found that their eye muscles actually adjusted for the poor perspective and that the changes were entirely involuntary. This also is a type of focus.

Most importantly of all the brain has to interpret all of these issues. We don't see our blind spot (although it can be demonstrated). We don't notice the difference between our center vision and our edge vision (although that can be demonstrated, too). The brain does a great job of taking different images, from different angles, at different times and compiling them into a single full image. This is also a type of focus.

So, from a physics perspective, all of the focussing takes place in the lense. From a biological perspective, the sharp focus of our binocular vision is a combination of several sub-systems.