Cameras have a light meter that measures the amount of light coming into the camera. When you’re shooting in aperture mode you decide how wide the lens opens and by using the light meter, the camera automatically determines how long the exposure needs to be. Similarly in shutter speed mode you choose how long the exposure time is and the camera automatically determines how wide the lens needs to be open.
Light meters are very sophisticated today, and most of the time they’ll get you a good exposure. Essentially, the meter will average the brightness of all the objects it sees, so that the bright tones will be captured and the dark tones will be captured and all the other tones will be in the middle.
But the light meter can’t know what you are taking a photo of. When we come across a photo situation that’s supposed to be very bright or very dark, the meter will look at the scene and try to average the exposure to give to a medium grey tone. Blech...
When this happens, we have to wrestle control away from the camera and show it who’s boss!
Try this experiment:
Find a place where the lighting is even and consistent. Could be inside your house at night, outside on a cloudy day or in the shade. Get a plain white piece of paper, a T-shirt, whatever, and a plain black one. Set your camera to aperture priority. Shoot a photo of the black and the white objects side-by-side.
Now, without changing the camera settings, fill the frame completely with the white object. Shoot a photo and then do the same with the black object. You’ll have to get close, focus is not important.
Flip through the three photos and notice how the two close-up photos of the black object and the white object look pretty much the same!
This demonstrates how the camera will try to average a scene to get a good exposure. This is great in most situations! But when we are shooting something that is supposed to be bright - like a sand pit or a close-up of a white object - we have to tell the camera ‘hey, that’s supposed to be bright!’ Same goes for objects that should be dark, dial the exposure down a notch or two.