I was wondering if it is right to say that underexposing or overexposing by playing with shutter speed or aperture or ISO is the same as dialing in -+EV values in manual exposure?
If you are using an automatic exposure mode (e.g. Program mode, Aperture or Shutter-priority) then dialing in +/- exposure compensation tells the metering system to adjust the exposure up or down. Otherwise it will always attempt to produce the same exposure for the same scene.
If you are in Manual mode, then you accomplish the same thing by simply adjusting ISO, shutter speed or aperture. The camera isn't automating the exposure, so you don't have to tell it to adjust - you have complete control, and the camera will ignore any EC setting.
So if you have the camera set to some combination of ISO, aperture and shutter speed, then dialling in EC will not have any effect on those settings in M mode - they will still apply and you'll get the same exposure (there is at least one exception - on some Nikons, if you have Auto ISO enabled then dialling in EC will adjust ISO, which will affect your exposure)
Dialling in EC may affect the exposure indicator in the viewfinder (on a Nikon anyway).
An example - let's say you're taking your sunset photos.
Camera is set to ISO 200, f/5.6 and 1/125th second. Looking through the viewfinder, the exposure meter indicator is zeroed out. You take a picture. It's well exposed, but you want it a bit darker to get better color.
In manual mode, you change the aperture to f/8. The meter now displays that you are underexposing. You take the picture anyway, and it looks good.
Still in manual mode, you change the exposure compensation down a stop. Now the meter is zeroed again. However this won't affect the exposure, just the display in the viewfinder. Take another picture, it will still be f/8, 1/125th, ISO 200, same as last time, so same exposure.
Because the indicator in your viewfinder is zeroed out, and you have the amount of underexposure you want, as it gets darker, you can use the indicator (keep the meter zeroed) to adjust your aperture.
So EC can be used indirectly to help you maintain a constant amount of compensation, but it does not directly affect the exposure in Manual mode.
No, it is not the same thing, although as operator/photographer you may do exactly the same thing with your hands in most cases.
Exposure compensation is a way to compensate for mis-measurements of the build-in meter in order to achieve the expsure value that you want. It is typcially done to avoid under- or over-exposing. On the contraray, you do that to expose right.
Under- and overexposing are not really tasks, things you do. It is rather an evaluation of the result. An underexposed image turns out (partly) darker than you want it. You will compensate for that by either using the exposure compensation or adjusting the manual settings. That is up to you.
let me get it in simple words for you gentlemen ....
When you mean underexpose/overexpose you are exposing the sensor by physically controlling the light by adjusting aperture , shutter speed and ISO(in film days)
Using exposure compensation we are controlling the light digitally.(cannot be done in film days)