Well the brain can theoretically perceive near infinite resolution so it's just a matter of your eyes ability to register and send detail.
Your eye initially sees low-res blobs of colour. It sees more detail the longer it concentrates on an area. However detail is seen quicker in the frontal region of your field of vision, which is generally regarded to be about 30o on most people.
Spinning round in a circle and you'll just see blurs. Stop and stare and you'll gradually gain a more detailed view. Most people reach their 80% of their maximum resolution after about 1 second and reach their best after two or three.
The instant resolution of the eye is only about 1002<->2002. Most people can see a maximum of 50002. There have been freaks who can see an estimated 50,0002.
I remember a story about a hustler who tricked people to hold cards behind his head. He would be able to tell the number and suit from the reflection in the crowds eyes.
disclaimer: this is all from fuzzy memory, apologies