The Megapixel Myth

Megapixels can be often used as a sales gimmick. Many  cameras with lower MPs can make better images than other cameras with higher MP.Other factors are more important than megapixels. Items like color correction ,sharpening algorithms, light calibration and noise reduction are quite important.

An LCD  monitor showing a picture at a resolution of 2560x1600 is displaying a four megapixel image. Cameras producing images of four megapixels rarely create an image as clean or crisp as the desktop background of a PC monitor of this resolution. why ?

Camera manufacturers promote the number of megapixels a camera has in order to make you believe it has something to do with camera quality. They use it because even a tiny linear resolution increase (the number of pixels in one direction x or y axis) results in a huge total pixel increase. An almost invisible 40% increase in the number of pixels in any one direction results in a doubling of the total number of pixels in the image. This allows camera makers to brag about camera improvements, even though the improvements are negligible.

Doubling the number of megapixels, if all other factors remain the same, has only a very subtle effect .You need to double the linear resolution in order to make a significant improvement this is the equivalent of quadrupling the megapixels.

Light Sensor And Pixel Quality Are More Important

Light lands on a sensor, the sensor (like film in old cameras) is exposed to light for however long you have the exposure time set.

Generally, the more pixels packed onto a sensor, the higher the resolution of the images it can produce but image quality mostly comes down to the size of the sensor and the pixels.

Notice this webcam sensor,electronics and lense housing are almost the same size as just the sensor of the DSLR above
The sensor itself is less than 5mm wide.

You can fit a much bigger sensor inside of a DSLR (top of page) than you can inside of a cellphone or webcam. This means you can fit more pixels on the sensor and you can fit much larger ones too. Sensors inside of DSLRs are huge compared to the ones found in compacts and mobile phones (detailed size chart)

Sensor sizes are referenced against 35mm film as a standard — cameras with sensors equivalent in size to 35mm film are called full-frame

Umm Didn't What You Just Say Indicate Higher MP Is Better ?

Kind of , but not really ....the core of it is that the pixels in mobile phones and compact cameras are small , low quality and crammed into small inefficient sensors.

To increase megapixels they only need to increase the number of pixels, they may even make them smaller to fit a sensor size.

Larger pixels are  more sensitive to light within a wider range.

To get the high megapixel counts found in some smaller cameras and phones manufacturers pack as many small pixels as tightly as they can onto tiny sensors.

When you pack pixels in so tightly it creates a lot of heat, which is one of the ways noise is generated (the rainbow colored random dots you might have seen in digital photos). Noise gets worse as you increase the ISO, amplifying the sensor's sensitivity to light.

The pixels in standard point-and-shoots aren't the same kind of high-quality pixels found on DSLRs and generally average cameras will offer lower quality pixels than the higher-end cameras of the same class , which results in poorer color accuracy and usually lower dynamic range too. This is the reason your 5MP camera phone or 5MP point and shoot is not as good as the 5MP DSLR. 

So, on a given sensor size, a lower megapixel count with bigger quality pixels will produce a cleaner image.

To produce a good image with a camera manufacturers must balance MP count, pixel size, pixel quality and sensor size. The sensor size and cost are the limiting factors.

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