When you use your camera in some automatic modes like Program—or one of the semi-manual modes like Aperture Priority or Shutter Speed Priority—you don’t give up total control over everything: you can still control the exposure using exposure compensation. Here’s how that works.
What Your Camera’s Light Meter Sees
When your camera calculates what exposure settings to use, it makes one big assumption: that when you average everything out, what’s in front of it is pretty much gray. In other words, all the lights and darks sort of balance out to a middle gray.
This is the photo your camera is trying to take.
And, it’s a pretty good approximation. Here are some of my photos with the luminosity levels averaged out.
It’s not a perfect match, but your camera, just by assuming it’s trying to take a photo of a boring gray wall, is going to be in the right ballpark for a lot of scenes.
But not for every scene. Here are some more photos of mine averaged out.
Read the remaining 17 paragraphs
Source: How-To Geek
