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In the small world of digital pocket cameras, increasingly the trend is to buy one with higher number of megapixels. Camera manufacturers seem to be trying to convince us that more megapixels mean better pictures. However, in reality they are not building better cameras that support higher mexapixels, they are often simply extracting more megapixels from their existing cameras.

The Digital Light Sensor

In order to understand the effect of the megapixel count on a digital camera we must understand how a digital camera records a picture. Before digital cameras, still pictures were recorded on a 35mm black film (negative). This 35mm film is called full frame. With the advent of digital photography, the film was replaced by a digital sensor, usually a CCD1 sensor. The catch however was that the sensors were much smaller than the traditional 35mm film because larger digital sensors are seriously expensive to manufacture.

In low-light, if their sensor size is identical, a camera with low megapixel count will always give a clearer image and capture more detail than a camera with a higher megapixel count.

Now, lenses on any camera capture light and project a circle of light onto the film or sensor sitting behind them. The circle of light produced by the lens must be large enough to fully cover the sensor area. Obviously since the old 35mm film was large, the cameras required larger lenses. Digital cameras these days use smaller sensors can make do with smaller lenses. However, a digital camera having a larger sensor will need a larger lens. This is why digital SLR cameras which have digital sensors close to the original 35mm size usually have larger lens attachments than digital pocket cameras.

The megapixel count tells you how many pixels are available on the surface of a digital cameras sensor. There are two ways of increasing this number. The more expensive way is to make the sensor larger; however the cheaper way is to reduce the size of each pixel, and this is what most camera manufacturers do.

Effect of Pixel Size

The images below show what happens to the pixel size on the sensor when the megapixel count is increased without increasing the sensor size. The yellow boundary indicates the digital sensor size.

Sensor Comparison

So what happens when pixel size becomes smaller? Well the amount of light available to each pixel becomes less and the sensors ability to detect light actually reduces making the picture more prone to noise! Pictures become grainy and as a result visual information is lost. So a camera with lower megapixel count will perform better in the same light conditions than a camera with higher megapixel count given both have the same physical sensor size. The effect of noise becomes more obvious when you try to shoot in low-light conditions without the flash. Because light available is scanty, less light enters the lens and some of the pixels on the many megapixel sensor are not triggered at all. If their sensor size is identical, a camera with low megapixel count will always give a clearer image and capture more detail than a camera with higher megapixel count in such low-light cases.

Sensor Size

Sensor sizes are expressed as a ratio e.g. 1/3.6" or 1/1.7" and a 1" sensor has a diameter of 25.4mm, so a 1/1.7" sensor has a diameter of 14.9mm.

How Many Megapixels Do You Need?

The answer depends on how big you want your prints to be. The megapixel determines how big your pictures can be printed before the print quality starts to degrade. So if you intend to make poster size prints you will need over 12 megapixels. But most pocket camera users make simple 6x4" or 7x5" card prints and don’t really need or use all the surplus megapixels.

Also note that setting the megapixel count on a camera to a lower value using the camera settings (e.g. shooting only 6 megapixel images on a 10 megapixel camera) does not improve the image quality because physical pixel density on the sensor surface does not change 2.

The table below gives the approximate minimum megapixels needed to get a high-quality (300dpi) print at the specific size.

Megapixels Common Print Size
2MP 6x4"
3MP 7x5"
4MP 8x6"
6MP 10x7"
8MP 14x10"
12MP 24x16"

From the table above it should be clear that most of us hardly need more than 6.0 megapixel from our pocket cameras. So don't get carried away by the megapixel count when buying a digital camera. If you really need bigger prints, then you should look at professional SLR cameras or semi-professional cameras with large sensors to meet your megapixel needs. A camera will a small sensor boasting a lot of megapixels will probably disappoint you in the long run.

Further Reading

  • 1. Most digital cameras use either a CCD image sensor or a CMOS sensor. Both types of sensor accomplish the same task of capturing light and converting it into electrical signals.
  • 2. The only exception to this rule are FujiFilms EXR sensor based cameras, where the camera cleverly combines information from adjacent pixels to give better image quality when set to shoot an image at lower megapixel value.