The screen:
The LCD screens are commonly defined by the size of their diagonal in inches or in cm, or by the total number of pixels.
It is rather easy to verify the format and the size by a direct measurement.
Remind that the digital image format are 2/2, 4/3, 16/9 which we name a/b in a general way.
Let be:
i, j number of pixels of the sensor.
x, y width and height of the screen in pixels.
N total number of pixels of the screen.
Y, Y dimensions of the screen.
Some calculations:
The circle of confusion of the screen is:
c' = 2,5 X/x
Usually, c' is close to the value of circle of confusion of the eye at 250mm which is c = 0,25mm
Convert the diagonal from inch to mm is
Dmm = Dinch x 25,4
The number of pixels x, y of the screen is not always given. It can be calculated by:
x = a Æ [N/(a b)]  and  y = b Æ [N/(a b)]
 Æ is for square root of expression in brackets
Check the sharpness:
The screen of the digital camera is not only used to aim and to compose the image, it is also useful to check the sharpness of the image.
Often, a pic that seemed sharp on the screen is blurred when printed or seen on a large computer screen. Why that? Because the reduced image of the camera screen  is much less demanding in sharpness than an enlargement.
To check the sharpness, it is necessary to have a zoom function on the screen. But what is the minimum magnification to be sure of the image sharpness?


The magnification factor necessary to check the image sharpness is calculated by:
G = i / x = Æ [ i x j / N]
Let's have a digital camera of 10Mpx giving a 3872 x 2592 pixels image checked on a screen 2,5" of 210000 pixels.
The screen which format is 3/2 measures 53x35mm with a diagonal of 63,5mm. This screen is 560x375 pixels.
The necessary magnification factor is:
G = Æ [3872 x 2592 / 210000] = 6,9
The factors 8 or 10 will suit perfectly but factors 4 or 6 will be insufficient.