How does a Digital Camera Work?

The operation of a digital camera is very similar to that of a film camera, since the share everything referring to the focus mechanism, manual and automatic exposure modes (in it's different modes) which we have seen in chapter 3. The main differences are found in the viewfinder and in the image size and sensitivity options.

We have seen that when using film it is necessary to previously get used to mentally foreseeing how the scene will be registered, since the scene's contrast and exposure level can have decisive influences in the final aspect of the photograph. Except in the case of professional cameras, where we can use instant film, the use of film is characterized by a certain degree of uncertainty. This uncertainty disappears with a digital camera. Thanks to the screen we can control the image in a greater degree than with a film camera, before and after the shot: not only do we set the frame and focus, we can see the contrast of the image, detect if a zone is overexposed or underexposed, be certain that the exposure is correct and that the color of the light is adequate. Once the exposure is made we can immediately see the image afterwards and be sure of other factors, like the flash exposure and even the expression on faces: with film there is always the doubt if the person blinked at the moment of the shot and may appear with the eyes closed in the image. The liquid crystal display screen, nonetheless, shows an image of a lower resolution than the final photo, which means we can have unwanted surprises when seeing the larger images on the computer: even though digital cameras have a great depth of field because of the reduced size of it's sensors, in some occasions a slight lack of focus can be unnoticed on the viewfinder.

A digital camera lets us choose the number of pixels of our image depending on the use we're going to give it: the quality and possibilities diminish with the size, from the maximum size, which give us the best quality -for retouching, trimming and printing on normal or photographic paper-, to the minimum size, with a quality only ideal to see the photos on the computer screen. When choosing the size and type of file (JPEG or TIFF) also influences the memory we have. When we are lacking memory we choose a medium or small size, a file mode with more compression and probably eliminate the pictures we don't want. When we run out of memory capacity and don't want to eliminate any of the images stored, there are some cameras with the possibility to resize files. If all our images were recorded using a large size, we can always free some memory by reducing the number of pixels.

An exclusive characteristic of digital cameras it the so called digital zoom , which allows us to close in on part of the image with electronic means. With this system we select a part of the digital sensor to obtain an effect similar to using a lens with a greater focal length than the maximum our camera has. In reality, the image quality is reduced, because the amount of information is lower when using only the central part of the digital sensor. The camera converts this smaller zone of the image into a photograph the size we have selected by filling in the missing pixels with a mechanism called interpolation . This effect can also be done afterwards with an image editing program.

The sensitivity of a camera is variable in two aspects: the ISO value we need and the color sensitivity.

The ISO sensitivity can be chosen in a digital camera only by selecting it on the menu. The same as with film, the lower the ISO sensitivity we use, the greater the image quality is. In a digital image, high sensitivity translates to a lower contrast and greater presence of noise, or confusion in the color of continuous pixels. The increment of sensitivity is made by amplifying the signal originated in the sensor, which comes with a deterioration of the image in comparison to the minimum sensitivity.

The other variable factor regarding sensitivity is that of it's response to different colors of light. We have seen when working with film we have to choose one which is balanced to day light or tungsten depending on the type of light we have. In the case that we have to use a day light film with tungsten light or vice versa, we are forced to convert the color of the light with a color conversion filter. This causes an important reduction in the brightness and is equal to a reduction in the film's sensitivity. If the available light is another king, for example, fluorescent light (of which there are many kinds), low consuming or incandescent lamps, correct filtering is an approximation which we can only get to know completely after testing.

With a digital camera these problems disappear. The sensitivity of video cameras and digital cameras is variable , since we can electronically correct the signal according to the available light (6.4.2).