V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 131
compresses
the image data (i.e. makes image files smaller).
This is true for JPEG and NEF
52
files on a D300.
JPEG
The D300 normally stores images in JPEG format
53
(in
Windows, the three-letter file extension limit reduces this to
.JPG
, so you may also have seen this format referred to as
JPG). JPEG (pronounced
JAY-peg
) stands for Joint
Photographic Experts Group, which developed and ratified
the original standard for this file format.
JPEG files can be read by a wide variety of programs, and
JPEG is one of the file formats directly supported by HTML,
the standard language from which Web pages are created.
The wide acceptance of the JPEG format means that you can
share a JPEG-encoded file with others, regardless of what type
of computer or software they have.
To produce a JPEG file from the final digital image
information created by the imaging ASIC, the following steps
are performed (note that the words in parentheses are gross
oversimplifications to help you understand the process):
1. The image is divided into 8 x 8 pixel blocks.
2. The information in each block is run through a series
of “transforms” (calculations) to produce a set of 64
“coefficients” (results) that are then “quantized”
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Even an uncompressed NEF image can be thought of as “compressed,” as it doesn’t
have Red, Green, and Blue data points for each image pixel; that saves 16 bits per
pixel location from what would be stored if RGB data were there. Yes, that isn’t
exactly compression, but the point being made here is about file sizes, not the image
impact of compression. From a file size standpoint, a NEF file is a reduced set of data
from the final image, which can be thought of as a form of lossless compression.
53
Technically, JPEG isn’t a
file format
, but simply a data compression scheme.
However, the fact that most computers use a file extension of
.JPG
or
.JPEG
for such
files has caused users to call it a file format. I’ll bow to this common practice in this
eBook.
