A photograph is said to be overexposed if it is too light and the bright parts (highlights) are washed out and have lost their detail. Underexposed means the photo did not get enough light and the dark parts are blending together. Exposure is regulated by changing shutter speed, aperture and ISO. Luckily modern cameras have automatic metering sytems which adjust camera settings to make a proper exposure.
Exposure Compensation Value (EV)
Sometimes the cameras metering system doesn’t pick the exposure that you want. A common example of this is when the subject is backlit (there is a bright light such as the sun behind the subject). Most DSLR cameras allow exposure adjustments, usually denoted EV, in 1/3 increments. One full increment, also referred to as a stop, will double or half the light by selecting plus or minus respectively. In simple terms, +1 EV means the picture with be twice as bright and -1EV will be half as bright. EV adjustments can be made in shutter priority (Tv or S) and aperture priority (Av or A).
Lets use a real world example. Mr. and Mrs. Waffles are our love birds. They naughty little rascals, always making a mess and biting and chewing on everything, but they are cute and we love them. If I want to take a picture of them in the window sill, my metering system usually makes them too dark, because of the sunlight outside the window behind them. One option is to use a flash, which is what the automatic setting on the camera chooses. As you can see below, the flash is not a good option, since it makes the Waffles look like scared jailbirds!
The flash created harsh shadows, so I decided to try a picture on aperture priority at f4. I chose f4 because if I opened the aperture too much the depth of field would be too thin, but if I closed the aperture too much, my shutter speed would be too slow to hand-hold the shot.
Here’s f4, 1/100th of a sec. ISO 200 no flash.
The picture has a good depth of field, a fast enough shutter speed, but its a little dark.
So I started to play with the EV. First is a picture with -1EV (minus makes the subject darker; I want it lighter, but this is an example). Since I am on aperture priority, the camera keeps the same aperture (f4) and ISO (200) but changes the shutter speed one stop to 1/200th of a sec.
Its obviously too dark, so lets go the opposite way (like we should have in the first place). +1EV changes the shutter speed to 1/50th of a sec.
This time the exposure is pretty good (the right brightness), but slowing the shutter speed to 1/50th of a sec. is to slow for these active little birds, so they are slightly blurred. To compensate for this I need to adjust aperture or ISO, click here to read further…
In the end I chose f4, 1/100th of a sec. ISO 400
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