DIGITAL  CAMERA

 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 of it. Only the sensor and the lens are the topic here. The lens: 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.. Calculation:. 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 field. 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.    Aspect ratio The dimensions of sensor, or the number ixj pixels gives the aspect ratio. 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 16/9=1,77 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: 5 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 lp/mm WARNING, do not compare with the resolution of 35mm films, as the size of sensor is 5 times smaller! 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 W = 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 = 5 mm 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 focal.   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 assessments. 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
Depth of field:

Because of the geometrical characteristics of the lens and of the sensor, the depth of field of the digital imager is great. It is then difficult to faint a background. This technical disposition has some advantages, but also some inconveniences to the photographer creativity.