- Basically these cameras are constituted
of a digital sensor, of a lens, of a lot of electronics,
of a screen, of a memory card, and battery to power all
- Only the sensor and the lens are the topic here.
- The focal lengths:
- The focal lengths are necessary to the experienced photographer
(which you are because you read these lines), but neither
to him, neither to the man in the street the zoom 7,2 to
21,9 on the sensor 1/1,8" have some meaning.
- Resolution of lens:
- The classic measures of the resolution of the lenses
are difficult to apply to the digital cameras. The screen
of the sensor prevents to do these measures, or the lens
and sensor must be separated, but what will replace the
sensor?. Therefore, nowadays, not really means of technical
comparisons: everything is laying on personal appreciation,
and the digital treatment of the images by the camera itself
is counting a lot.
- The angular field:
- It is more meaningful to refer to the horizontal field
covered by the lens. As it is not given, it remains to calculate
it, or to determine it geometrically..
- Divide the half width of sensor by the focal length,
apply the trigonometrical function arc tangent, quoted atn
or atan, or atn-1, and multiply by two to get the angular
- For example, with focal lengths of 7, 2 and 21,9mm on
a 1/1,8" sensor, we get 52,6° and 18,5°.
- Geometrical setting:
- See the page angular
field and refer to it for additional information.
- This method is also able to determine accurately the
correct equivalent focal length which is not often exact
in cameras datas.
- The sensor:
In a digital camera, the sensor,
CCD or CMOS type, is receiving the light and replaces the
film. The pixels are all electronic cells that convert the
photons of the light into electric tension. The pixels are
carefully ordered in rows and columns.
The dimensions of sensor, or the
number ixj pixels gives the aspect
The common aspect ratios are 4:3 or 3:2
or 16:9 The ratio of pixels
gives one of these, respectively 4/3=1.33 or 3/2=1,5 or
For example, i x
j 3264x2448 is also 2560x1920 or 800x600, which is
aspect ratio 4:3
The diagonal size value is also
useful for calculation, and is:
for 4:3 / 3.6
for 3:2 / 18.36 for 16:9
- Dimension of pixels
In reality, it is the pitch of
pixels which is calculated, by dividing the length by the
number i of pixels, for example:
i = U / I
= 7.12 / 2560 = 0.00278mm,
or 2,78 µ (microns).
- Resolution of sensor
- It needs two pixels to get a white line, and a black
line, which pair is the resolution of the sensor, expressed
in mm. The resolving power is the inverse of the resolution,
expressed in number of line pairs by mm, lp/mm.
- So: 1000 / (2 x i) with i in microns
- The sensor 1/1,8" has a revolving power of 180
- WARNING, do not compare with the
resolution of 35mm films, as the size of sensor is 5 times
Dimensions of sensor
The size of the sensor is known
by information like 1/1,7" which no salesman is able
to explain what it is. At the best, he will assess that
a 1/1,8" is smaller and a 1/1,6" is greater. That
symbol 1/[?]" is showing the values [n]/[m]
which makes the calculation of the diagonal w of the sensor
16 x n / m
It's so simple! Therefore, the
diagonal of the sensor 1/1,8" is 16 x 1 / 1,8 = 8,9mm.
First method calculation :
Only divide the diagonal W of the
sensor by the diagonal format value and multiply by the
corresponding format value of either width and height. So
result for sensor 1/1,8" is:
width 8.9/5*4 = 7.12mm
and height 8.9/5*3 = 5.34 mm.
Note however that slight differences
may occur on some brands.
Second method calculation :
It happens most of the time, that
brands give the equivalent focal lengths to 35mm film. The
calculation consists to divide 36 by the equivalent 35mm
focal and to multiply by the true focal to get the width
of the sensor. Start with 24 instead of 36 with the same
calculation to obtain the height.
Example with the 1/1,8" sensor
which true focal length 7,2mm to 21,9mm are given as 35
to 105mm equivalent.
width = 36/105x21.9 = 7,5mm
height = 24/105x21.9 =
The differences with the first
method comes from the rounded-off numbers that brands are
giving in their commercial information datas. They will
give equivalent to 35mm even the actual one is 38mm equivalent
It's a pity
that the size of the sensor is not included in the Exif
file of the pictures.
- Circle of confusion:
- Even a great number of photographers did not understand
clearly the difference between resolution or resolving power
and the circle of confusion, (see the pages on sharpness
), with the digital camera, there are a lot of far-fetched
- Let repeat once more that the resolution is a technical
fact of the equipment whilst the circle of confusion is
based on physiological behavior of our eye. They are linked
by the relation:
- c = 1,25 r = 2,5 i