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Appeared on: Monday, October 31, 2005
The secrets of quality in photography


1. Page 1

Quality related to the photographs taken by a digital camera is a resultant of different parameters such as the lens, the image sensor and the digital signal processing algorithms, and not just a matter of pixels.

Manufacturers often exploit consumers’ ignorance when implying that a high resolution provided by a camera ensures better quality in the photographs taken. This is by no means true, as image quality relies on numerous other factors than the one mentioned above. One of the greatest problems met with image sensors is that each and every photosensor element found on their surface is capable of collecting information on just one color. The colors left, are calculated only through complex mathematical methods (interpolation) so that the final image is produced with the highest possible color-level. Some manufacturers have been making use of similar techniques to increase image resolution, but when it comes to quality, results remain doubtful.

Artificial color information

In reality, a 6 Megapixels image sensor camera, does not produce images at a 6 million pixel resolution, as its image sensor is limited to two colors only: white and black. To produce a colored picture, tiny colored filters corresponding to the three basic colors, that is the red, the green and the blue (RGB), are placed over the light gates. The typical ratio normally includes 25% of red color, 50% of green and 25% of blue, meaning that the image sensor of such a camera, includes 1.5 million light gates for the red color, the same number for the blue, whereas the remaining 3 million are used for the green light, since human vision is more sensitive to this shade.

Yet, if we wished to produce an image correctly, the way it happens with analog photos in other words, we would have to make use of 6 million light gates for each main color, equivalent to a total resolution amounting up to 18 million pixels (18 Megapixel). Such a thing of course is non-existent since, in reality, we are capable of registering only 1/3 of the information required. The remaining 2/3 are being artificially produced with the help of highly complex algorithms and are afterwards added to the existing information, to produce thus an as close to the picture representation as possible.
Therefore, the quality of the final result is dependent to a great extend on the digital processor as well as the algorithms used.

Compressed pixels

The resolution of digital cameras is being continuously increased, and a 4 to 5 Megapixel scale is considered a selection to be suggested when it comes to consumers’ digital compact cameras, while as far as the professional digital SLR (Single Lens Reflex) solutions are concerned, resolution may reach up to 16.7 Megapixels (ie Canon EOS-1Ds Mark II). The dimensions of the image sensors vary and are normally smaller than the ones of the analog film ( 24mm height X 36mm width). The digital SLR cameras with a full frame image sensor like the Kodak DCS Pro 14n (CMOS sensor 13.89 Megapixels) are an exception to the rule, yet they are designed for professional use. With digital compact cameras, weight and size are the factors, which greatly affect the ability to carry and use them easily. These are the two major criteria influencing selection among the cameras belonging to the specific category. To accomplish such an aim, manufacturers have utilized small size CCD image sensors, while to achieve greater resolution they have reduced the size of light gates and the in-between distances. As a result we are now provided with an ample supply of tiny light gates, compressed in restricted space. However, such a process has a negative effect on the dynamic width, as the small size of the light gates is not sufficient enough when attempting to capture all fluctuations connected with hues, starting from the darker and ending up to the brighter sections of a photo. Furthermore, short in-between distances contribute to the appearance of “noise” in photographs, a fact that is dependent on the effectiveness of the algorithms used to remove it.


2. Page 2

Is there a solution to the problem?

An amateur photographer does not have the same needs with a professional, yet one must be cautious with the selection of the digital camera to be purchased. The image sensor, the lens and the digital signal processing algorithms are the factors determining the quality of the result, while efforts are being made to improve color scale with the development of new technologies related to image sensors, like the following.

Foveon X3

The Foveon company have presented their new image sensors’ technology called the “Foveon X3 Direct Image Sensor”, which is none other than an imitation of the process used to register information on an analog film. It provides two CMOS image sensors with a 10.2 and 4.5 MP resolution, and with three pixel levels incorporated in silicon. This way they have benefited from the ability attributed to the red, the green and the blue light to penetrated silicon into different depths (the way it happens almost with the different film chemical coating levels) and register full color in every pixel. This technology may be used with both consumer as well as professional digital cameras and thus promotes the perspectives of development in digital photography.

 

 

Fujifilm Super CCD HR/SR

Fujifilm initially presented image the Super CCD HR image sensors, where photosensor elements are diagonally arranged and pixels are changed from square to octagonal ones, while the digital processor is capacitated to double the number of pixels registered in the storing tool. Later, they improved the aforesaid design by adding two light gates per pixel, receiving information about the same spot in the picture. The main light gate is more sensitive so that it is able to register information about the light reflected by the object, while the secondary light gate is less sensitive (meaning that it is able to register a darker image), so that it can collect more information from the brighter areas of the photo. The above do nothing but improve detail in a photograph, especially in both dark and bright areas and cater for the creation of qualitative printings, without the purchase of a more expensive digital camera being required. Despite all these, algorithms are made use of, to facilitate the calculation of the remaining information and the company itself maintains that an image sensor with 3 million pairs of light gates in 3 million pixels may yield 6 Megapixel image files, yet it is not a “real” 6 MP CCD sensor.

Sony Super HAD CCD

The efforts to incorporate more pixels in smaller sized CCD image sensors has resulted in reducing the size of light gates and therefore their sensitivity. To deal with the problem, Sony has added tiny lenses (on-chip microlens) over each photosensor element, focusing on the incoming light, increasing thus sensitivity. In addition, it has developed the Super HAD (Hole Accumulation Diode) CCD technology, which improves the shape of these tiny lenses, so that the “dead” area in-between them becomes minimized and light absorbance is increased. Finally, it uses an extra green light filter named Emerald and occupies 25% of the image sensor (from the total 50% of the green light). This way, a closer to nature color quality is achieved with photographs.



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