The iris contracts automatically in the presence of bright light. This makes a flashlight appear bright in darkness, but outside in daylight you can't even see it's light.
The iris is changing your sensitivity to the intenseness of light. But physically all is happening is the iris is getting smaller or larger. I can see how this allows less light to go in if it gets smaller, because it's less surface area. However, I would imagine this would change the field of view though if I didn't know any better. Why does it change the intenseness of light instead?