High Key

You will enhance the beauty of an object/person/scene or anything else by exposing it such that it produces tones that fall mostly between white and gray, with very few dark-gray or black tones and still very bright or absent shadows. This exposure will give you some areas of pure white (e.g. large parts of the background) and very smooth surfaces at the relatively brighter parts of your subject, while maintaining some contrast and outlined edges at dark parts of it. You will use this pureness and smoothness to produce an image that carries stronger emotions than a normal exposed copy of it would possess. In the digital age this means that most values in the histogram will be above 128 with the exception of a limited number of tones below 128. This means that in high key also a lot of the tones of the subject will fall above 128, so it does not work (or will be very difficult to achieve) for darker subjects
The exposure must mainly be achieved IN CAMERA. post processing a regularly exposed shot to create the effect is not what we're trying to achieve here. The ways you can achieve the this exposure includes: Exposure compensation setting of your camera, the AEL button by pointing the camera at a darker object and usage of M mode. But of course you are not limited by those and may discover other ways of achieving this effect.
In case you are in a studio different lighting techniques can take care of the pure white background that is so characteristic of high key images. Outside the studio with different backgrounds other (over) exposure techniques will be needed to create the desired effect and some PP will be allowed to create an overall pleasing image.
Just Google "High Key" and you'll find plenty of articles and examples on the net. Also referenced below is this article which gives a good description of the effect, however it mainly concerns shooting in a studio or controlled environment and this assignment is not limited to that. So if you encounter situations that are less controlled and you still want to achieve a high key effect you need to be innovative

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