Jay Turberville
Well-known member
lima said:
Eye relief is the distance from the last surface of the eyepiece's eye lens(the eye lens is the eyepiece lens closest to your eye) to your eye when viewing through the scope. A more technical explanation is that eye relief is the distance from the eye lens to the exit pupil.
When you place your eye at the proper eye relief distance, you are locating the entrance pupil of your eye at the same location as the scope's exit pupil. This gives you the full view of the scope. People who wear glasses appreciate longer eye reliefs since it allows them enough space between their eye and the eyepiece to keep their glasses on and still locate their eye at the scope's exit pupil and therefore see the entire view.
You want to do the exact same thing when digiscoping. The main problem is that the camera's entrance pupil is usually deep within the camera lens and is usually located somewhere near the iris/shutter mechanism. You can think of the camera as an eye with unusually thick glasses that change thickness when the camera lens is zoomed.
So extra long eye relief may or may not be better depending on your particular camera. But you almost always want around 20mm for the cameras that are popular for digiscoping. When eye reliefs get much shorter, there are fewer cameras that will work well. For many cameras, even longer eye relief can be much better. It just depends on the camera.
But extra long eye relief eyepieces are unusual which is probably why people have focused more on camera choices to match their scope's eye relief rather than the other way around. But with companies like Kowa coming out with eyepieces that have unusually lont eye relief, that may change.
