|
The limitation expressed above refers to the exit pupil size which is eyepiece focal length divided by the f-ratio of the scope.
The reasoning is that a fully dilated pupil would not allow in any more light and your eye becomes the limiting factor in vignetting available light. The 7mm figure relates to a young person and decreases with advancing years.
There is nothing to stop you using longer fl eyepieces but as exit pupil size increases the field background as viewed with the eye becomes brighter and brighter, especially where light pollution is a factor.
Width of field is independent of exit pupil size. What sets field size is the size of the field stop in the eyepiece, with the maximum being the internal barrel diameter of the eyepiece eg for 1.25" format eyepieces you can get up to about 27mm field stop. You can get that maximum field in a 32mm, 50degree plossl, or you can get the same field in a 24mm 68degree panoptic and enjoy a darker sky background.
|