The key tone in the majority of images lies at or near the mid-point between darkest or black tones and the brightest or white ones.
The visual effect of deliberately shifting the key tone is not merely to make an image lighter or darker overall, but to signal a mood or feeling to the viewer; it is to use the tonality of the image to convey meaning.
In this sense, the exposure is "incorrect" or not compliant with the standard, since you are deliberately causing under- or overexposure.
Visually, it seems that when the key tone is shifted it is best to alter the other values further to suit.
Thus, with high-key images there are generally few, if any, truly black areas, while in low-key images, white-bright areas are minimized.
Bright and high High-key overcomes shadows and signals a style full of light and air.
But making a high-key image is not just a matter of increasing exposure; it is ensuring that as many tones as possible are lighter than mid-tone.
You can do this by lighting your subject evenly from both sides, using fill-in flash or reflectors to reduce shadows, or shooting from a position that minimizes shadows.
However, if you are working in low-contrast conditions, you may indeed have to increase exposure.
With digital cameras, you can easily see the effect of setting non-standard exposures as soon as you take an image.
However, it may be easier to record the image normally and then create the high-key effect using image-manipulation software.
The easiest way to create a high-key picture is to base exposure on the shadow areas by ensuring that your meter reads light values in these areas alone.
If your camera has a spot or selective-area meter, point it at a shadow that still retains some subject detail, lock the reading in, and recompose the shot.
If you don't have this facility, you may need to move close enough to a suitable shadow so that it fills the sensor area.
If you place the darkest area with detail as your mid-tone, all the other tones in the image will be lighter and you should have very little in the image that is darker than mid-tone.