In the world of computers, you can never have too many megabytes or
megahertz.
That sounds dated in this day of gigahertz and gigabytes, but the
principle remains the same: More is better, whether you're looking at
CPU speed, system memory, hard drive space, or graphics processor speed
and VRAM.
Where it's not necessarily true is digital photography, where
there's an unhealthy fixation on megapixels.
Not that megapixels are bad! Far from it: they are a big factor in
how large you can make a photo and still have it appear to be sharp.
But in the rush to market megapixels, images can actually be worse with
a high megapixel camera than from a lower megapixel one. (If you don't
want to learn about pixels and image sensors, feel free to skip to
Beyond the Megapixel Myth.)
Pixels and Megapixels
A pixel is a single picture element, and a megapixel is a million
pixels. That's the short of it.
But sometimes things are not as simple as they appear, and this is
one place where that's true. Let's say you have one of today's popular
8 MP (megapixel) digital cameras. You know that it creates images
with 8 million pixels. Each of those pixels contains the red, green,
and blue (RGB) information for one tiny sensor.
Well, that's the theory; it's not the reality. While there are
indeed 8 million sensors (give or take a few) in your 8 MP
digicam, they are not RGB pixels. Half of them are green, a quarter of
them blue, and the remaining ones are red. (The human eye is more
sensitive to green than other colors.) Light to the camera's image
sensor goes through a Bayer filter, which has
millions of minuscule red, green, and blue filters in a checkerboard
pattern.
So how does your digicam make 8 million RGB pixels from 4 million
green, 2 million red, and 2 million blue pixels? It makes an educated
guess. It makes a very mathematical guess: In the case of a green
pixel, it averages the values of the red and blue pixels next to it.
For red, it averages the four green and four blue pixels touching the
red pixel, and likewise for blue. (The technical term for this is
interpolation or demosaicing.)
Except for the Foveon X3 sensor
(used only by Polaroid and Sigma to date), that's the way digicams
work. Every pixel is accurate in one of the three color channels and
close in the other two. It's usually close enough that you'll never see
the difference, but it's there. And it's especially challenging with
high quality black & white work like shooting a newspaper, which is
why some digicams have a high quality b&w setting - no need to
average pixels so these images can be as accurate as possible.
Fujifilm took a different spin with the Bayer filter in its
Super CCD sensor, which is used in some of the company's better
digicams. Rather than a standard grid of RGB elements, Fujifilm turned
everything at a 45° angle. This has two benefits: it makes the
sensor elements larger (octagons rather than rectangles) and thus more
sensitive, and the pixels are closer to each other than with a standard
image sensor.
Because of this, Super CCD sensors with 3 MP can produce
6 MP images nearly as good as those from true 6 MP
sensors.
Foveon, which we've already mentioned, takes a different approach.
Instead of separate sensors next to each other for the primary colors
of light, the X3 sensor has three sensor layers: a thin blue-sensitive
layer, a green-sensitive layer, and a thicker red-sensitive layer - the
whole stack being less than 5µ thick. This completely eliminates
the need to interpolate and provides the best possible sharpness at a
given resolution.
JPEG
That's how sensors work, and it's a logical next step from
interpolating pixels to compressing images. After all, an 8 MP
image with 10-16 bits of data per color channel has to deal with 48 MB
of information. Imagine how quickly those raw image files would fill up
a memory card!
The solution is image compression, and the standard for compressing
photographic images is JPEG (Joint Photographic Experts Group), which
was designed by geeks and photographers as a means of making smaller
files with good enough quality. This is a multistep process that
happens invisibly to the end user.
The first step is to convert the RGB data into YCbCr format: Y for
brightness (luminance) and Cb and Cr for color (chroma). Some cameras
will skip this step when set to "highest quality".
The second step is to downsample the color (chroma) information
while retaining the brightness (luma) information, which retains
sharpness and sacrifices some color accuracy. Again, this may not be
done at highest quality settings.
The third step is to break down the image into 8x8 pixel blocks,
which then undergoes a discrete cosine transform (DCT), just the kind
of thing CPUs do so well.
The fourth step is where the compression finally comes into play.
This is where you tell the camera or Photoshop to use high, medium, or
low quality.
The final step is to encode the compressed image.
JPEG is a lossy compression scheme, and the more you compress an
image, the more you'll notice the artifacts, particularly at edges.
Also, JPEG works with 24-bit color, 8 bits per color channel, so the
10, 12, or more bits per channel most modern sensors can measure is
truncated - yet another reason RAW images are superior to JPEGs.
Beyond the Megapixel Myth
The Megapixel Myth states that more pixels always means better
pictures, so the number of megapixels in an image is the most important
predictor of output quality.
There's some truth to this: When comparing an 8 x 10 from a 1.3 MP
camera to an 8 x 10 from a 3 MP camera, you'll really see the
difference. The lower resolution image will be a bit fuzzy everywhere,
while the 3 MP image should appear as sharp as you'd expect. And
you'd expect an 8 MP image would be even better - and that just
might not be the case.
First of all, you'd be hard pressed to see any difference in an 8 x
10 print from a 3 MP digital camera and one with higher
resolution. You'll certainly never see it in a 4 x 6 snapshot, a size
where 1.3 MP images create sharp prints. Where you will see a
difference between 3 MP cameras and those with more resolution is
when you go past the 8 x 10 size. As 11 x 14, it won't be quite as
crisp as a 4-5 MP image, and at 12 x 18, you'll definitely see some
fuzziness from a 3 MP or lower image.
I've seen 12 x 18 prints made from 6 MP digital SLRs, and they are
incredibly sharp and detailed. I've even seen 20 x 30 poster prints
made from 6 MP images, and they look great at normal viewing
distances. Sure, they're a bit fuzzy if you look close, but you usually
view a poster-sized print from several feet away, and even prints from
35mm film are less than crisp at this size.
Enough Megapixels
To make a sharp photo, you only need so many megapixels. That varies
with the size of the print.
All else being equal, a standard 4" x 6" snapshot will look just as
good from a 1 MP image as from a higher resolution camera. A 5" x
7" print from a 1.5 MP camera will look sharp, and an 8" x 10" photo
made from a 3 MP camera will be indistinguishable from a higher
resolution image short of using a magnifying glass.
There are only three reasons to shoot with more megapixels than you
need for an 8 x 10:
There's a chance you might want to print larger.
You may want to do some cropping.
You're shooting for a client who demands higher resolution
(magazines, for instance).
If you want to leave yourself some flexibility to crop, double your
pixel count. Unless you crop out a lot of the photo, you'll still have
a great 8 x 10.
David
Pogue recently demonstrated this reality by shooting an image 13
MP, reducing it to 5 MP and 8 MP in Photoshop, and having the
resulting images printed at 16" x 24". He put the three photos on
display in New York's Union Square and asked passersby if they could
tell them apart. Out of several dozen volunteers who were free to
examine the photos as closely as they wanted, only one correctly
identified them. "Everybody else either guessed wrong or gave up,
conceding that there was absolutely no difference."
Too Many Megapixels Wastes Time and Space
I normally shoot my 5 MP and 8 MP cameras at the 3 MP setting, as I
have yet to make anything larger than an 8 x 10. Not only am I using
enough pixels to get a nice sharp enlargement, but I'm saving storage
space on my memory card and hard drive, not to mention saving the time
it takes to write those images to the flash card or transfer them to my
computer.
Shooting high quality JPEGs at 3 MP, my typical file is about 1 MB
in size. If I were shooting at 6 MP, that would be 2 MB. So I
save about 40% on file size with my 5 MP camera and over 60% with
the 8 MP camera.
Tiny Pixels May Mean Worse Images
The sad truth is that the Megapixel Myth gets the manufacturers to
build cameras that offer way more pixels than you really need because
megapixels are easy to market. More is better - but not always.
As the individual sensors in your digital camera's imager become
smaller and smaller, they receive less light, which means the computer
inside your camera needs to turn up the gain. And when it does that, it
also increases the image noise in your photos.
This isn't a problem with digital SLRs, because they use large
imagers (typically around 22 x 15 mm), but the smaller digicams tend to
use much smaller sensors (around 9 x 6.5 mm), and we're already seeing
the results of that. Too many tiny sensors means more noise, which
looks like grain in your photos. I've seen 4 x 6 snapshots shot on
small, high MP cameras that have absolutely unacceptable levels of
noise. And this only gets worse on low light situations, where the gain
gets turned up even higher.
There's nothing to be gained from having twice as many low quality
pixels. As Joe Marney says,
"My contention on this issue is that the average photographer does not
need more megapixels, they need better megapixels."
Getting Great Quality Photos
There are many factors in taking a technically good photo:
sufficient light, a sharp lens, no camera shake, enough pixels,
negligible image noise. My mobile phone takes 1.3 MP images, but they
all look horrid on anything but a phone because the lens is terrible.
My 8 MP digicam takes grainy images in low lights, and often with
camera shake despite shake reduction technology.
When possible, shoot in good light. When not, use flash if you can -
and be aware of the distance limitations of your flash. Only shoot in
dim light without flash if you have no choice in the matter, as you are
likely to see camera shake, have lots of noise, and have to deal with
to odd color spectrum of most indoor (and many outdoor at night) light
sources.
Almost any camera will get a decent shot on a bright, sunny day -
even those single-use cameras. It's low light that's the bane of cheap
cameras, because they tend to use less costly lenses that transmit less
light along with less costly image sensors that tend to have more
noise. Add a slow lens, a cheap sensor, and excessive megapixels, and
you have the recipe for photographic disaster.
The best digital photos will almost always come from a DSLR because
these cameras have large image sensors, which means less noise, and are
invariably coupled with much better lenses than the garden variety
digicam, both in terms of sharpness and lens speed (how much light they
transmit) - and also because photographers who choose SLRs tend to be
more involved in the process of creating photographs.
Not that SLRs are for everyone: I shot film SLRs from 1973 until
about five years ago, and had a digital Canon Rebel for a year or two.
I loved the photos but got tired of the bulk.
My current digicams are a Konica-Minolta
Dimage X50, a very compact 5 MP camera with a 3x zoom lens, a 2"
display, and an honest-to-God optical viewfinder. I love how easy it is
to carry and use. The other is a Kodak EasyShare P880,
an 8 MP camera styled like an SLR but with a built-in 24-140mm
equivalent zoom. It's bigger than the Dimage but a lot smaller than a
DSLR, and the zoom range is wonderful.
Both cameras spend most of their time set at 3 MP, and they both
have pretty pedestrian built-in flashes. I'd love to pick up the Kodak
P20 Zoom Flash for the EasyShare someday, as that would provide a
25-30' flash range, over twice that of the built-in flash, but it adds
bulk and costs about $120. Still, for all those times when the built-in
flash didn't have the oomph, it would be a nice addition.
It would be nice to someday add a DSLR to my stable, as those big
sensors with low noise and high sensitivity coupled with fast lenses
make for great low light shooting, but between the Dimage for snapshots
and the EasyShare when you want more zoom range (gotta love that 24mm
wide-angle setting), I'm good for now.
The key to great digital photography isn't more megapixels, although
that number is a factor. The key is knowing your needs, knowing your
equipment (how far the flash reaches, how close it can focus, how well
it performs in low light), knowing how to use the most important
features, and having the camera handy.
Further Reading
The Megapixel
Myth, Ken Rockwell, 2007. "There is little visible difference
between cameras with seemingly different ratings. For instance, a 3 MP
camera pretty much looks the same as a 6 MP camera, even when blown up
to 12 x 18!"
Breaking the Myth of Megapixels, David Pogue, New York Times,
2007.02.08. "'The more megapixels a camera has, the better the
pictures.' It's a big fat lie."
The
Megapixel Myth, Joe Marney, The Tech Lounge, 2004.04.24. "The
problem lies not in the number of pixels recorded, but in the quality
of those pixels."
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Fujifilm took a different spin with the Bayer filter in its
Super CCD sensor, which is used in some of the company's better
digicams. Rather than a standard grid of RGB elements, Fujifilm turned
everything at a 45° angle. This has two benefits: it makes the
sensor elements larger (octagons rather than rectangles) and thus more
sensitive, and the pixels are closer to each other than with a standard
image sensor.
