Eye relief

This is frequently called a "scope bite", or the "idiot cut", due to the obvious and long-lasting nature of such a mistake.

The eye relief given in product specifications does not always give a realistic view of what a user can expect.

Although eye-cups can usually be folded down to allow the spectacle wearer to get closer to binocular eyepieces, there are sometimes lens mountings that do not allow the theoretical eye relief to be obtained.

A better measure for those with strict needs would be one that takes account of this available eye relief, the theoretical value less any thickness of the lens' rims.

The matter should be discussed with the optician, though the only realistic way of testing the comfort is to try the optical device while wearing the usual spectacles.

In the event that a spectacle wearer cannot obtain the eye relief that they require, some cameras and microscopes allow prescription lenses to be fitted onto their eyepieces.