Effective
Number of Pixels
A
distinction should be made between the number of pixels
in a digital image and the number of sensor
pixel 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 megapixel camera which outputs 2,560 x 1,920
images has an equal number of "effective" pixels, 4.9
million to be precise. Additional pixels surrounding the effective
area are used for demosaicing
the edge pixels, to determine "what black is", etc.
Sometimes not even all sensor pixels are used. A classical example
was Sony's DSC-F505V which effectively used only 2.6 megapixel
(1,856 x 1,392) out of the 3.34 megapixel available on the sensor.
This was because Sony fitted the then new 3.34 sensor into the
body of the previous model. As the sensor was slightly larger, the
lens was not able to cover the whole sensor.
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So the total
number of pixels on the sensor is larger than the effective
number of pixels used to create the output image. Often this
higher number is preferred to specify the resolution of the
camera for marketing purposes.
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Interpolated
Number of Sensor Pixels
Normally,
each pixel in the image is based on the measurement in one pixel
location. 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. Sometimes a camera with, for
instance, a 3 megapixel sensor, is able to create 6 megapixel
images. Here, the camera calculates, or interpolates,
6 million pixels of information based on the measurement of 3
million effective pixels on sensor. When shooting in JPEG
mode, this in-camera enlargement is 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 is marginal and you are
basically dealing with a slower 3 megapixel camera which fills up
your memory cards twice as fast—not a good trade-off. It is
similar to what happens when you use a digital
zoom. Interpolation cannot create detail you did not capture.
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Fujifilm's
Super CCD Sensors
Normally
sensor pixels are square. Fujifilm's Super CCD sensors have
octagonal pixels, as shown in this diagram. Therefore, the
distance "d2" between the centers of two octagonal
pixels is smaller than the distance "d1" between two
conventional square pixels, resulting in larger (better) pixels.
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However,
the information has to be converted to a digital image with
square pixels. From the diagram you can see that, 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.
The drawback is that you have to deal with double the file size
(leading to more storage and slower processing), while enjoying
a quality improvement equivalent to only 33% more pixels.
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