Digital imaging
As probably all of you already know, the base of photography is light, which is the visible range of electromagnetic radiations. We’ll disregard the physical part of it (for obvious reasons) and I’d rather write a couple of things about the nature of light visible to humans. We can describe the color range we can sense with three colors: red, green and blue. If all three of these are present to the same extent, than we see white light, while we sense different shades of colors due the mix of these three colors in different ratios. It’s hard to define white color, as we see a sheet of paper to have the same white color at the light of a candle and under the light of the Sun, which has a roughly balanced radiation in the visible spectrum of light. This is the wonder of the human mind, but a camera cannot adapt to the environment in such a manner and that’s why we have concepts like color temperature (aka white balance). But we’ll talk about these a bit later…
Olympus E330’s cross-section view
So when taking photos we store the light itself: on film in the case of analogue cameras and to the memory card (with the help of a light sensor) in case of digital cameras. This sensor picks up light entering the camera through the lens, which is then passed along to the camera’s processor in digital format (as pixels). It follows from this that these three components (the sensor, the lens and the processing unit) affect picture quality the most. The light sensor has three important qualities: size, resolution, type. The latter one can be CCD-CMOS or CCD-NMOS. These concepts are known as CMOS and CCD for some reason. The difference between the two is in the production technology. CCD is a more complicated circuit, its production costs are higher, but in turn it has greater sensitivity, reproduces colors in better quality and has better line sharpness. CMOS us cheaper to produce, consumes less power than the CCD, and its advantage is the less noise on pictures. CMOS sensors have developed a lot lately, many major camera manufacturers have chosen this from the two – for example Canon lens reflex cameras have this kind of sensor. Based on these, it’s really hard to tell which technology is better. Cell phones almost always have a CMOS sensor, probably because of their lower power consumption and price.
Sensors among themselves
From our point of view, not the sensor type is what’s most important, but its resolution and size. The latter one changes a lot, which is not known by most of the amateur users, but this is at least as important as resolution, as these two make a pair. There has been no problem with this in analogue cameras, the Leica system used a 24 x 36 mm frames, most of the cameras used this, but the manufacturer can save a lot of money in digital cameras by putting physically small sensors in their devices – and they are saving as much as they can! This is sad because the quality of a picture is not given by its resolution, but by the number of pixels in a given area, which results from resolution and the sensor’s size. At a given resolution quality increases in direct proportion to the sensor’s size, so turning this backwards we can say that the less place a pixel (or the diode of the sensor) gets, the worse picture quality is. That’s why the ongoing megapixel war is meaningless, as from two sensors of the same size – supposing they are of the same type and have the same processor – the better one is the one that has less pixels, which is the one with the smaller megapixel number. Of course the sensors themselves are getting more and more advanced, so the quality that needed a physically large sensor a couple of years ago, is obtainable using with a smaller one today. So we can judge picture quality (noise level) and picture dynamics based on how many square microns of place does a pixel get. Below 5-6 squaremicron/pixel we get low quality, noisy pictures, while this number is around 7 and 11 in case of a high-quality camera and above 30 in case of professional cameras. As most of you might have guessed by now, mobile phones have tiny sensors (due to physical limitations), so the size of them is not published very intensely (to say the least) – in practice such information is almost impossible to find out. So increasing the number of megapixels doesn’t necessarily mean increase in picture quality, so the manufacturers resort to not so elegant solutions…
The difference in size between an APS-C from a lens reflex camera and a 1/2.5” sensor from a compact camera
I’ll write about basic photography concepts in short. Those who know what shutter speed or aperture mean can turn to the next page. These are the two things we can use to adjust the amount of light entering the camera when taking pictures; their value can be adjusted in steps of a preset value. Shutter speed is the duration of light reaching the image sensor, while aperture is the diameter of the “hole” in the lens that can be adjusted to control the amount of light reaching the image sensor. The larger the aperture is, and the more it is open, the more light reaches the sensor, this is evident. Setting these two parameters, we can alter some other things besides light quantity: depth of field depends on aperture size and decreasing shutter speed helps avoiding picture blurring due to camera movement, but I won’t go into any more details about these. The amount of light required for a correctly exposed (not too light and not too dark) picture is decided by the automatics of the camera, or at least in entry-level cameras, more advanced ones let us set shutter speed and aperture – in this situation the camera displays how many exposure values away are we from the exposure considered as correct. In case the sensor does not receive enough light, we get an underexposed picture and if it gets too much light, then the picture will be overexposed. There is a third participant in setting this, and this is sensitivity, which is measured in ISO. This can be set from the menu in digital cameras, while analogue cameras needed a different kind of film. More about ISO on the next page. By the way, the measurement is the same ASA/ISO standard we can see on the boxes of films.
Correct exposure, under- and overexposed picture
A cikk még nem ért véget, kérlek, lapozz!
