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And then there was light. And the light showneth upon the CCD, and it made
a photo. And the photo was good. The consumers rejoiced. And 50 million
digital cameras were sold in 2003.
Film is so last week. It’s all about the CCD now, and the megapixel.
What are these new devices that are working to replace film? Well, a CCD
(Charged Couple Device) is a piece of silicon with “photo detectors”
(commonly known as pixels) laid out on the silicon in a grid. The “photo
detector” is a light sensitive diode that measures the amount of light
striking it, giving it a “charge.” This “charge”
is converted to an electrical signal, which is sent to a to a recording
device (memory chip) inside the camera. A key piece of information, “photo
detectors” vary in size, and their size relates to their overall sensitivity
to light. Larger “photo detectors” are more sensitive to light
and are more effective in conveying that information. Smaller “photo
detectors” need amplification before the information they convey can
be useful. The total number of “photo detectors” on that piece
of silicon gives you your mega pixel count.
Here’s where it gets interesting. The camera manufacturers would like
consumers to believe that the larger number of megapixels means the better
the picture. They want you to believe that their 5 megapixel camera, for
instance, is SO much better than your 4 megapixel camera. Nope. Not always
the case. Unfortunately it’s not that simple.
I read an interesting article recently about the Mars Rover. The article
stated that the pictures coming from the mars rover were IMAX quality. Which
is absolutely amazing because the images are actually only 1 megapixel each!
Huh? 1 megapixel?! So why do they look so good? There are a few reasons
for this.
1. Optics quality
2. CCD size
3. CCD quality
Optics. The cameras on the Rover are worth about a million bucks
apiece (not exactly consumer products). Much of their cost has to do with
lens design. The lenses on these cameras are for all practical purposes,
flawless. That means all wavelengths of light are perfectly bent through
the lens system to meet again with absolute precision on the CCD. With a
normal lens we might purchase here on earth from a camera shop, however,
the light is never absolutely perfectly aimed into the sensor. They're flawed.
And these flaws result in different wavelengths of light bleeding into spaces
where it shouldn't. It’s the cost of mass production and, and low
cost design “tolerances.”
CCD Size. The latest sub-$1000 digital camera has 8 mega pixels on
a CCD sensor that measures .35 inches by .26 inches. Compared with the 1
megapixel Mars Rover camera whose sensor is .5 inches square, that’s
roughly 4 times the area. The Rover’s camera thus has 2000 pixels
per inch on its sensor. The 8 megapixel camera has 9300 per inch (and more
is not better in this regard). The pixels on the 8 megapixel camera are
almost 5 times smaller. Smaller pixels are not as sensitive to light, so
the information must be amplified, which of course compromises quality.
Combine that with the bleeding of light mentioned above, and you have further
decreased the image quality.
CCD Quality. Simply stated, the CCD on the Mars Rover’s cameras
are the world’s highest performing chips in terms of light sensitivity
and chip quality. These are not mass produced chips like the ones used in
our digital cameras. They are many magnitudes of order better. The more
light-sensitive the chip, the less amplifying is done by software to translate
the digital signals into a picture.
Here’s a quick comparison. The current PowerShot S45 and the PowerShot
S50 are both very nice cameras from Canon. Both cameras have a 7.2 x 5.3mm
CCD sensor. The S50 has crammed 5 megapixels onto that sensor, but the S45
only 4 megapixels. They share the same 3x zoom lens. So which one is going
to take a better picture? Technically, the increased pixel count of the
S50 is not going to yield a better picture; unless it has a lens that is
upgraded to better match the specifications of a 5 megapixel sensor. If
the sensor were increased in size to accommodate new pixels, this would
also yield a better picture.
So when buying a digital camera, be somewhat skeptical of otherwise identical
cameras that differ only in megapixel ratings. In many cases the manufacturer
wants you to pay more money for more megapixels even though there will be
no apparent increase in image quality.
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