Rods and Cone Cells
Your retina contains two types of photoreceptors that work in completely different ways. Rod cells are your low-light specialists, whilst cone cells handle colour vision and fine detail.
Rods contain rhodopsin pigment that breaks down in even dim light. Many rods connect to one sensory neurone (retinal convergence), allowing summation that makes them incredibly sensitive. However, this means poor visual acuity since your brain can't distinguish between individual rod signals.
Cone cells work differently - each connects to its own bipolar cell, giving excellent visual acuity. They contain iodopsin pigments (red, blue, green) that only break down in bright light. Their distribution is uneven, with most cones concentrated in the fovea where light focuses.
Interesting Fact: You have about 120 million rods but only 6 million cones - yet cones give you most of your detailed daytime vision!