There are two types of cells in the retina that identify the light – stick-shaped (rod) and cone-shaped (cone). Rod cells only recognize light and dark and are very sensitive to low levels of light. Cone shaped cells recognize colors and are concentrated close to the center of your vision. There are 3 types of them that recognize 3 main colors – red, green and blue. The brain uses the information from those three types of cells to make up our recognition of color.
Daltonism can occur when one or more of the cone cells are missing or not working. Heavy daltonism occurs when all 3 types of cells are missing.
There are different degrees of daltonism. Some people have slight difficulty in telling apart light colors in weak light. Others cannot tell some colors apart in any light. The heaviest form of colorblindness, in which everything is seen in different shades of grey is unusual. Daltonism usually affects both eyes to the same degree, and the condition remains unchanged for the entire life.
Except in its heaviest form, colorblindness doesn’t affect vision sharpness. The inability to see all colors and seeing only in shades of grey is called achromatopsia.
There is no cure for congenital astigmatism. It usually doesn’t cause meaningful damage. There are special contact lenses and glasses that can, to some degree, help people with daltonism.