I was confused with this when I first started too.
Exposure compensation basically just tells the camera to Over or Under Expose X EV (Where X is a number e.g. +1 or -2) from the correct exposure.
So for example.
I am about to take a portrait indoors - The camera gives me the Correct Exposure at +-0 at the following settings chosen by the camera in P mode.
Shutter Speed = 1/40 | Aperture = F/2.8 | ISO = 400
I take the picture and it is correctly exposed.
Exposure compensation, activated on a Canon by holding down the AV/+- button on the rear of the camera and then turning the control dial next to the shutter release. Moving it up and down will make the exposure metering slider go up and down between -2EV and +2EV, where - is under exposed and + is over exposed.
So if you set it to -1EV the camera would adjust its settings to something like.
Shutter Speed = 1/60th | Aperture = F/3.5 | ISO = 400
Now all you have done is make the photographer "darker" by the camera making shutter speed faster which allows the light to hit the sensor for less time, and closing the aperture down a little so less light can get through the lens.
So increasing exposure compensation is just the change of the shutter speed, aperture size or both.
So in the same situation of an indoor portrait, setting the camera to +1EV in P mode would come with settings like this.
Shutter Speed = 1/15th | Aperture = F/2.8 | ISO = 400
This means that it will be overexposed slightly. The camera would also change to these sort of settings if you lowered the ISO down to 200 at +-0 EV compensation (e.g. a normal exposure)
It is all pretty hard to wrap your head around at first, at the start I thought I could adjust the exposure compensation in Manual Mode...then I realised that what you were actually doing in full manual mode lol!
Hopefully my post hasn't confused you further, I'm sure others may be able to explain it better than myself but hopefully what I've typed will help you until then
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