This gives a formula for a modified lens strength, based on distance moved.
Seems to me that “corrected” and “perceived” are not equivalent.
- perceived means moving the original lens to distance ‘x’ will behave the same as the putting the calculated lens at the original distance
- corrected means that if you move D to ‘x’ you must also adjust it to the calculated value to maintain the same effect.
(If the perceived lens is stronger, then you would have to replace it by a weaker lens at the new offset to give the same effect.)
The rules given here are that for a plus lens, moving away from the eye means it is perceived as being stronger, and vice versa. A minus lens does the opposite. (Note that they are using the opposite sign for distance : positive value is closer to the eye. Hence the opposite sign in the equation.) These are consistent with the woolly hand-wavey description I had planned to add to the page before I realised the answers I was getting were backwards.
Our wiki page gives a bigger value in the plus case. So I guess it is using the perceived story, rather than corrected, version.
- I convinced myself, so boldly made the edit. But perhaps someone could double-check.