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Understanding of Megapixel :)

In digital imaging, a pixel (or picture element) is a single point in a raster images.
A megapixel (MP or Mpx) is 1.000.000 pixels, and is a term used not only for the number of pixels in an image, but also to express the number of image sensor elements of digital cameras or the number of display elements of digital displays.

It is commonly used to describe the resolution of digital cameras.

Camera nowadays keeps upgrading. I'll be talking more on Dslr :).
Every companies are fighting for market. Each have their own marketing strategies and marketing schemes. Mostly on heavy "increase of Megapixel". A high megapixel count may sound good on the specs, but in reality, it doesn't affect the quality of your pictures. It's the size of the censor, the lens and how those pixels are processed ;thats affects the quality.

Have u ever think why Nikon mid-range/semi-pro Dslr mostly with 12.1mp? Rather than other brands (not to mention :|) some boost up til around 15 - 18m. I did a research on comparing 2 Dslr which on the same level and prices (closest competitor).

Eos 550D (18.7mp) vs D90 (12.1mp)

The Canon Rebel T2i features a 18MP sensor compared to the Nikon D90 with a 12.3MP sensor: The common assumption is that more is better and that the Canon Rebel T2i offers an advantage with its higher resolution sensor.

When we first compared the 15.1MP resolution Rebel T1i versus the 12.3MP D90, we stated that we would have been more impressed with Canon's new product direction if they had opted to recycle the better quality output 10MP sensor from the Canon EOS 40D and incorporate it into the Rebel T1i.

Now Canon has gone one step further in the megapixel race by upping the resolution in the new T2i / 550D to 18MP, while keeping the sensor the same size. In theory, this means that the image quality will suffer as each individual pixel has to be reduced further in size in order to be able to wedge the extra pixels onto the sensor.

Canon's workaround this problem has been to adopt a gapless microlens design for the Rebel T2i's sensor, which means that the quality of each pixel (photodiode) has been looked at by Canon and improved. By using a more advanced micro-manufacturing process, the circuitry around each diode has been reduced in size so each individual diode can be larger, giving better high ISO performance and a wider dynamic range. Although the sensor records the light it is the microlenses above each pixel that funnel the light into the pixel ‘well’.

So although the Canon Rebel T2i / 550D features a higher resolution sensor compared to the T1i / 500D, the pixel pitch (size of the individual pixel) on the T2i is larger than on the T1i. (Improved to 4.7µm vs. 4.2µm on the T1i)

The above diagram shows how the Rebel T2i / 550D (like the higher end Canon EOS 7D) uses a gapless microlens design compared to the T1i to gather the light more efficiently. Likewise, the distance between the sensor and the microlenses has been reduced. The combination of these improvements in the T2i / 550D is lower noise levels at all ISO settings and higher pixel sensitivity compared to the earlier Canon T1i / 500D.

All in all though, in order to fit 18 megapixels on a sensor that is slightly smaller in size compared to the DX format sensor found in the Nikon D90, in comparison the pixels on the Rebel T2i sensor still have to be smaller and packed closer together. (Pixel pitch on the D90 is larger at 5.5µm vs. 4.7µm for the T2i)

Pixel density (or how closely together the pixels are located) is calculated by dividing the number of pixels on a sensor by the imaging area of the sensor. Even with the gapless microlens design improvements, the pixel density on the Rebel T2i / 550D is still higher at 5.5MP / cm² versus 4.5MP / cm² on the Rebel T1i / 500D. The Nikon D90 features a notably lower pixel density of only 3.3MP / cm².

There are a number of disadvantages associated with higher pixel density sensors. One important consideration is that you will want to invest in a higher end quality lens that can support the sensor's increased resolution capability. The simple rule is that if you invest in a higher-resolution camera you need a lens that offers sufficient resolving power, which generally means spending more.

Other issues related to high pixel density include; lower dynamic range, reduced high ISO performance, diffraction issues, and increased possibility of camera shake. Although the extra resolution from a 18MP sensor provides for gain in fine detail compared to a 12.3MP sensor, the disadvantages typically outweigh the gains. By adding even more pixels to the Rebel T2i / 550D without increasing the size of the sensor, Canon is pushing beyond the resolution threshold of APS-C sensor technology, and in our opinion has once again opted to sacrifice on quality and practicality in favour of a marketing based decision.

taken from Digital Review site:

http://www.digitalreview.ca/content/Canon-Rebel-T2i-EOS-550D-Compared-to-Nikon-D90.shtml

Not to say that Nikon owns Canon in pictures quality..but it's just based on my deep research on comparing these 2 Dslr which opened my eyes on different perspective view concerning MEGAPIXEL. :)