Digital Camera

A great photograph depends on a lot of things. A great view, a good camera, good light and of course, aperture
settings and shutter speeds. What exactly does the aperture do? It is, basically, an aperture or an opening. The
size determines how much light enters the camera through this opening and falls on the image sensor to form the
digital photograph. Did too much light get through? Your photograph could be a washout. Was it too little? You’ll
get a dark picture.

Apertures come in different sizes - all classified as ‘f’ numbers. Each number lets in double the amount of light as
the previous one. The standard is between f/1.8 and f/16. The smaller the aperture, the less light that will be let in.
So an f/16 lens will let in half the amount of light as an f/8 lens. The aperture works in conjunction with the shutter as well when it comes to the amount of light let in. The speed at which the shutter opens and shuts is also a factor that determines the amount of light. When it comes to fast-paced action, a fast shutter speed is essential to capture the motion. For a landscape or a posed photograph a slower shutter speed is fine.

A photographer who is extremely particular will adjust both the aperture as well as the shutter speed. A perfect balance between the two could very often bring about that one perfect picture. It needs a trained eye in order to judge perfectly exactly what the settings should be. What he would also take into account is the depth of field, that is, how much of the image remains in focus. In larger apertures, there is just a short range that is in focus, whereas smaller apertures have a much deeper range, going from the foreground close by to way back, deep into the background. It would all depend on the kind of shot to determine what the settings should be.

For those of us who do not have the inclination, the understanding or the patience, we can always resort to the automatic setting. It’s simple, the camera does all the work of adjusting for you and you get a good photograph. It might not be a work of art as might a photograph that a true professional photographer might have taken, but most cameras today give you a very acceptable quality.

Why do we need aperture settings at all? The simple, old cameras didn’t have any. If you choose a camera with aperture settings like a telephoto, wide-angle and maximum aperture features, you know that even in an automatic setting, you will get different kinds of pictures, not the same, flat look. It gives you the freedom to take any kind of shot, anywhere, in any light. Otherwise you might find yourself restricted to typical, posed cheesy pictures without too much character or depth.

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In many digital still cameras, the image sensor capturing our photograph in place of the standard film is a CCD (charge-coupled device). How it is manufactured and what happens in it when we press the shutter release button is of interest to any good photographer, because it may contribute in taking better photos. Moreover, basics of lingo are useful for the understanding of characteristics of a camera, especially if you are going to buy one.

A pixel element of a CCD, in its simplest form, is basically a MOS capacitor, i.e. a semiconductor structure made of three layers: Metal-Oxide-Semiconductor. The semiconductor part of the MOS is simply silicon while the oxide is silicon dioxide, acting as an insulating layer. The metal is not a real metal substance, but rather a heavily doped polysilicon layer and is named “gate“. Basically, the image is formed within the semiconductor part of the MOS system and the metal gate is used for shifting such image.

When we press the shutter release button on our camera, light starts hitting the silicon in the MOS structure, and hole-electron pairs are created via the photoelectric effect. While the holes are drained by the grounded substrate and not utilized, electrons are collected in the semiconductor part of the MOS. We are interested in these electrons, because they will build up our photograph. The region where electrons are collected is referred to as the “channel” and, of course, corresponds to the electron lowest potential energy. In order to get better images, basically all CCD manufacturers use the so called “buried channel structure“, where the electrons are collected not at the oxide-semiconductor interface, but a little bit distant, within the semiconductor (hence the name “buried”). In order to confine the photo-generated electrons in the channel, “channel stops” are created in the MOS structure, isolating each MOS element from the adjacent ones.

The number of photoelectrons collected is linearly proportional to the intensity of the photon flux and to the time this flux hits the pixel (”integration time“). Consequently, we now know that the brighter the registered scene, the greater the number of electrons. The efficiency with which incident photons are detected is known as “quantum efficiency“; a typical value is 40%. This value is pretty low because of the absorption caused by the passivation (protection) layer present on the integrated circuit and the presence of the polysilicon gate that must be passed through by the incoming photon.

A single pixel of our digital camera is made up of a structure similar to the MOS capacitor outlined above. It’s a little more complicated, though, because we don’t want electrons to be confined where they are generated forever, but want to sense how many electrons have been generated. In order to accomplish this, each CCD pixel is made up of 3 parallel gates (although variations are possible) and, perpendicularly to these, a channel stop on both sides. By keeping the central electrode (gate) at a higher potential than the other two, electrons will be attracted there and so collected. The CCD is then made up of a matrix of millions of these pixels, arranged in thousands of rows and thousands of columns.

So, upon pressing the shutter release we start the acquisition phase, and this ends after the exposure time set by the photographer (typically 1/2000s to a few seconds). At this point we must read the number of electrons collected in each pixel. The greater the number of electrons, the brighter the pixel. For this purpose, a charge transfer process must take place from each pixel to a sensing circuitry. To achieve this, a shift phase occurs, exploiting the 3 gate structure described above, after which the electrons of the first row are shifted into an array of serial registers located at the edge of the CCD matrix, the second row electrons are shifted to the first row and so on. The efficiency with which the transfer process occurs is measured by a parameter called “Charge Transfer Efficiency“, a typical value of which is 99.999% per pixel. The serial register shifts its content into a charge detection output amplifier one pixel at a time. The output amplifier converts the electrons’ charge to a voltage. The order of magnitude is an output voltage of about 1 microvolt per electron and this is a linear relationship. The slope of this curve is referred to as the “output sensitivity” or “conversion gain“. The higher the voltage, the brighter the pixel. Once all the pixels of the first row are read by the output amplifier, the shifting phase takes place again and the whole sensing process is repeated. This is so until all the pixels in the matrix are read out.

This is the end. Nothing else happens in the CCD chip. All the rest of the image processing is done off-chip. In particular the voltage read out by the CCD is first amplified and then converted into a digital value by an off-chip analog to digital converter.

Andrea Ghilardelli runs an online photo retouching service. To get your pictures beautifully retouched and for articles about photography, please visit his site: www.ilghila.com.