- aka Night blindness
- Deficiency of vitamin A causes nightblindness
- Vitamin A is necessary for the regeneration of 11-cis-retinal in the pigment cells of the eye.
- Light on the retina converts 11-cis-retinal to all-trans-retinal, a process called photoisomerization.  A series of intermediates is then formed, one of which is metarhodopsin II.