If you say parenthetically "not in mirrors" you might want to explain briefly why not. If that explanation is irrelevant, then it may be better to leave out "not in mirrors". Even if you choose to leave the phrase in you can still drop the parentheses without loss. ...occurs in lenses, but not in mirrors, ...
You could also drop the dashes with no loss: can be corrected to a great extent by ...
On the whole, it is a good and lucid piece of technical writing provided you are writing for a reasonably technical audience.
- For Teachers