Resolution
The reproduction quality of any image is ultimately tied to the number of
pixels that comprise that image. Pixels, shorthand for “picture elements,”
are present in the millions in all of Canon’s cameras, even their amateur
snapshot cameras, as it takes a million pixels to equal 1 megapixel. Thus,
a 16 megapixel sensor contains 16 million picture elements, and each
element, electrically charged to receive and process information from
the light that strikes it, becomes 1/16,000,000th of the fi nal image. The
number of pixels that make up an image is the resolution of the image.
Logic would dictate that, the more pixels in an image, the more
information (and the higher degree of enlargement) in that image. That’s
true, but only to a degree.
Image quality is also dependent on the size of the chip that receives the
image. A 10 megapixel CMOS sensor in a Rebel, for example, is smaller in
physical size than a 10 megapixel sensor in a 1D Mark III. Larger sensors
mean larger pixels, which equates to better light gathering capability and
cleaner images, so even though the Rebel has the same number of pixels,
images from the larger sensor will appear to be sharper, with less noise
and more latitude for enlargement.
After chip size and resolution, the degree of image compression factors
into the quality of the fi nal reproduction. When you look at the resolution
settings in the menu of a Canon prosumer camera or Digital Rebel, you
will see Large (L), Medium (M), and Small (S) jpeg, plus RAW by itself and
a RAW combination that will take a RAW fi le plus a Large, Medium, or
Small jpeg at the same time. The 1D Mark III even has a Small RAW (sRAW)
choice.
The Large jpeg setting will use all of the pixels on the sensor to create the
image, while the Medium setting uses fewer pixels and the Small jpeg
choice creates an even lower resolution image. You also have a choice
of how that jpeg fi le is compressed. Compression doesn’t aff ect the
number of pixels used to make up the image, but how that information is
remembered and stored.
Here’s one way to visualize what jpeg compression does. Let’s say we’ve
chosen to shoot Large resolution jpegs. When an image is compressed into
a jpeg, the processor looks at pixels and their neighbors. If a neighboring
pixel is only slightly diff erent than the inspected pixel, the neighbor
is remembered by the processor as the same color.
If the degree of
compression is low, pixels that may be only slightly diff erent than their
neighbors are remembered by the processor as being diff erent and the
result is a Large fi le. If the degree of compression is high, as many pixels as
possible are lumped together as the same color, tossing out some detailed
information in the name of saving space. The result of high compression is
a fi le that takes up less room on your hard drive or your memory card but
may not have the same smooth colors and gradations as a less compressed
fi le. The resolution (or number of pixels) of the fi le is still the same whether
you choose a Large jpeg with low compression or with high compression.
You see, the jpeg fi le format, as convenient as it is, is what’s known
as a “lossy” format. The subtle color variation that was there before
compression between some of the pixels is gone forever. The same thing
happens when the image is recompressed in a program like Photoshop.
After a certain number of compressions/decompressions, the image
develops “artifacts,” areas of color that look chunky because the pixels
are not transitioning color or tone correctly.
Artifacts are most likely to
form around areas of sharp focus against a plain background, such as this
chain against a bright sky. Once they form there’s nothing that can be
done about them, but they’re much less likely to be visible in Large fi les
with low compression (FIGS 1.19 and 1.20).
Does this mean that jpegs are a bad format? Not at all. Jpegs are a fi ne
format, suitable for almost everything, provided you understand their
operation and limitations. Working with jpegs can be a viable part of your
workfl ow, actually saving you lots of time.
Personally, I believe you should always shoot at a level higher than you’ll
need. If you are just going to shoot snapshots, with maybe an occasional
8 10, I’d recommend you shoot Large (Normal). The number of images
you can fi t onto a card is double that of Large (High) fi les yet will yield
high quality prints. With an 8 megapixel camera, that works out to over
500 shots on a 1 gigabyte card.
Pros should always shoot Large (High) or RAW to get the most out the fi les.
On the other hand, should you want the versatility of RAW but need a
quick reference for an image catalog, you might want to shoot RAW
Small (Normal). It will be easy to use the Small jpegs in Canon’s Image
Browser or other programs such as iView Media or iPhoto. Use the
catalog of jpegs for reference, then process the images you like from the
RAW fi les. If you’re worried about losing important data to artifacts (you
shouldn’t be) process those images as Tiff s, a lossless format.
The important thing to understand is how compression aff ects your
picture quality. As always, I suggest you test your equipment to
determine what makes you happy. Large or not so large, Canon’s CMOS
sensor technology allows for exceptionally clean data.
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