Eye test

[aka pixi]

When you have an eye test they show a red and green background side by side with a black shape/letter on it and ask which shape/letter is clearest. What does this tell the optician about your eyes?

[Androcles]

Isn't it used to check whether under or over correction is occurring with the lenses being tested - this site give a reference to it but is not clear....http://www.freepatentsonline.com/5997142.html

[rojash]

Because green has a shorter wavelength than red, the red image will be focused further back in the eye, so the eyes need to refocus slightly for each. If the prescription is wrong then one or the other of the images will be favoured (easier to focus).

[aka pixi]

hi Androcles and rojash, thanks for that :)