When the eyesight is affected by Leber's Optic Neuropathy, it is the retina and optic nerve that stop working properly. Some parts of the nerve may even die and so can't send the right signals to the brain any more. The cornea, pupil and lens still work, but the light can't be converted into the right signals for the brain. The damage happens in the most sensitive part of the retina, where the detailed vision and best colour vision normally sits. That is why people affected by Leber's lose the vision in the centre and can still see 'around the edges'.
Ordinary spectacles correct problems in the cornea or in the lens.
They help the eye to focus the light onto the retina properly. When someone is affected by Leber's, the light is already being focused on the retina properly, but it isn't being converted into signals for the brain. That is why ordinary spectacles can't help the synmptoms of Leber's. Magnifiers and telescopes can help, because they make things look bigger to the eye. If you look at something through a magnifier or telescope you are using more of your retina to see it.