The Nagler 22T4 is a very easy eyepiece to use, even comes with the Instajust for eyeglass users.
Being a large 2" 82deg wide field eyepiece with a lot of glass you do get a very immersive effect while viewing. Maybe some people are trying to focus too much on something at the edge of field and inadvertently reposition their eye or that matter head - resulting in a blackout effect ( when they should possibly adjusting the telescope to take a better look)
