On an eye test, why does the optician ask if the red or green light is clearer ?

Okay, so I do have bad eyesight, and she kept changing the lenses, and asking me if i could read the red light (with letters on) or the green light (with letters on), and which was clearer.


I was just wondering what i the need for this ?


Nearly ever time I could only ever read the green, what could this mean as well ?On an eye test, why does the optician ask if the red or green light is clearer ?
the green is for minus power and the red is for plus power when they ask you which is better green or red they can tell if they have over corrected you. whichever one you say is an indicator as to which power to add or subtract from. if they are equal then they got it right.On an eye test, why does the optician ask if the red or green light is clearer ?
Different frequencies refract different amounts.





All lenses are prisms. What that means is that the ideal focus distance for a lens is actually a compromise. One color will focus above the retina, and the colour at the other end of the spectrum will focus below it.





They use green instead of blue because it's the colour we're more sensitive to. But if the green image is sharp and the red image is blurry, it means they don't have the focus adjusted properly. What they want is for both of them to be equally blurry, which means they've adjusted the trade off properly.
I asked about this too. depending which one you choose they will decide whether they've put lenses that are too strong in front of your eyes. I can't remember which colour is which though.