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.
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.
The circle of confusion of the
c' = 2,5 X/x
Usually, c' is close to the value
of circle of confusion of the eye at 250mm which is c =
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
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
G = Æ
[3872 x 2592 / 210000] = 6,9
The factors 8 or 10 will suit perfectly
but factors 4 or 6 will be insufficient.