Effective Pixels

As we look at a digital image, we must understand that the number of pixels is not the same thing with the number of sensor pixels measurements that were used to produce that image. In conventional sensors, each pixel has one photodiode which corresponds with one pixel in the image. A conventional sensor in, for instance a 5 mega pixel camera which outputs 2,560 x 1,920 images has an equal number of “effective” pixels, 4.9 million to be precise. The effective area has some additional pixels that surrounds it, which are used for demosaicing the edge pixels, to determine “what black is”, etc. Not every time all the sensor pixels are used. For example Sony’s DSC-F505V which effectively used only 2.6 mega pixel (1,856 x 1,392) out of the 3.34 mega pixel available on the sensor. This phenomena is the consequence of trying to fit the then new 3.34 sensor into the body of the previous model. The lens couldn’t cover the whole sensor because the sensor was slightly larger. So the total number of pixels on the sensor is larger than the effective number of pixels used to create the output image. Most of the times this higher number is used to specify the resolution of the camera for marketing purposes.

Interpolated Number of Sensor Pixels

Usually, to the measurement in one pixel location corresponds each pixel in the image. For instance, a 5 megapixel image is based on 5 million pixel measurements, give and take the use of some pixels surrounding the effective area. But there are times when a camera with a 3 megapixel sensor, is able to create 6 megapixel images, for example. Shooting in JPEG mode offers you an advantage the images resulting being of better quality than those performed on your computer, because it is done before JPEG compression is applied. Enlarging JPEG images on your computer also makes the undesirable JPEG compression artifacts more visible. However the quality difference isn’t that obvious, and you are dealing with a slower 3 megapixel camera which fills up your memory cards twice as fast. The very same process takes place when you use your digital zoom. Details you didn’t capture can’t be created by interpolation.

Fujifilm’s Super CCD Sensors

Although most of the sensor pixels are square, Fujifilm’s Super CCD sensors have octagonal pixels. therefore, the distance between the centers of two octagonal pixels is smaller than the distance between two conventional square pixels, resulting in larger (better) pixels. However, the information has to be converted to a digital image with square pixels. So for a 4 x 4 area of 16 square pixels, only 8 octagonal pixel measurements were used: 2 red pixels, 2 blue pixels, and 4 green pixels (1 full, 4 half, and 4 quarter green pixels). In other words, 6 megapixel Super CCD images are based on the measurement by only 3 million effective pixels, similar to the above interpolated example, but with the advantage of larger pixels. In practice the resulting image quality is equivalent to about 4 megapixel. This leads to double the file size (leading to more storage and slower processing), while the quality improvement is equivalent to only 33% more pixels.

6 Responses to “Effective Pixels”

  1. Ivan says:

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    Have a nice day


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  4. Andrei says:

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