V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 148
higher levels of sharpening if you’re using higher ISO values
and lower JPEG quality levels.)
Note: If you rotate a JPEG file and resave it, it is possible to lose
information! That’s because each 8 x 8 block must be
rotated in place to preserve its compression information. If
an image editing program simply grabs rows of pixels and
converts them to columns, when you resave the file, a JPEG
recompression may again be applied to the entire file. If the
8x8 blocks are rotated in place and each block individually
placed in the new orientation, JPEG compression is
preserved and no new recompression takes place.
Fortunately, almost all current programs that allow rotation,
including Adobe’s and Nikon’s, preserve the data during
rotation (and can do so automatically during downloads to
the computer if you’ve set
Image rotation
to
On
). But
make sure you know what the programs you use do if you
are using older software! Pre-2003 software programs tend
to recompress rather than rotate correctly.
Note: There used to be a persistent myth circulating that said that
any time you resave a JPEG file that you’ll force a full
recompression of the file, adding artifacts. Well written
software doesn’t do this. For example, beginning with
version 6.0 of Photoshop, the only recompression that is
done on JPEG images that are resaved is on portions (8x8
pixel blocks) of the image that have changes made to them.
In other words, if you bring a JPEG file into Photoshop 6 (or
later version) and make no changes, resaving it results in no
degradation of the image. We’re now four major revisions of
Photoshop past version 6, so the myth doesn’t get repeated
often, but don’t fall for it if you hear it.
This is probably a good point to introduce the interaction
effect of certain camera features, since I’ve already mentioned
a few. The D300 does have a few image quality issues that
you need to be aware of, and we’ve come to one of them.
•
JPEG compression has a tendency to increase the visibility
of moiré
. For example, if you’re shooting a tight fabric
pattern—which might generate moiré—you probably