Avision @V2800 - Manual (Page 113)

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When the value of highlight and shadow change, the image changes
accordingly.
Highlight: 255/Shadow: 0(Normal)
Highlight: 210/Shadow:10
Highlight: 200/Shadow:0 Highlight: 255/Shadow: 50
Adjusting the Brightness Curve
Adjusts the midtones of the image without dramatically altering then
lightest and the darkest areas.
When the curve is moved up or down, the
image turns brighter or darker.