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Digital
Photography: Avoid Too Much File Compression
by
Patty Hankins & Bill Lawrence
©
2003 Hankins-Lawrence
Images, LLC
Last
month, we talked about why the file size you set on
your digital camera matters, This month, we’re
going to address the related issue of what file format
you should choose, and why it matters how much the
file is compressed by your camera.
Both
last month’s article and this month’s article
are about getting the best image out of your digital
camera. If you put in the time and effort of taking
a good photograph, wouldn’t you want the captured
image to be as good as it can be? File compression
can impact on your photos. And while the number of
photos you can get on your storage media (be it compact
flash, smartmedia, or numerous others) can be very
tempting when you have it set to high compression,
to get the best out of your photos you will need to
sacrifice the small file size of highly compressed
images for the better quality of lower compression.
When
you take a photo with your digital camera, the camera
stores it as a file on your camera’s storage
medium. Most digital cameras allow the user
to choose the file type the images are stored as.
Most cameras include at least two types of JPEG files
– high-compression & low-compression, and
some allow you to save the files as TIFF or RAW files
as well. The major difference between the file types
is how much, if any, they compress the data in your
image – and this will determine the quality of
your images.
When
you take images as TIFF or RAW files, the data are
not compressed at all, or are stored using “lossless”
compression. This means that, when you open the file
in your photo editing or viewing program, all the
information that was originally recorded by the camera
is still there. So while the files are much larger
than corresponding JPEG files, the photos you can
print will be of a much higher quality.
The
data in JPEG files are compressed when you create
a file. JPEG files use a “lossy” form of
compression, which means that in exchange for a small
file size (and room for more photos on your camera’s
storage card), some of the information recorded by
the camera is lost. The amount of compression is determined
by your camera’s manufacturer. A high-compression
JPEG file is much smaller than a low-compression JPEG
file, but more information is lost from the original
image, and more artifacts from the file compression
can be seen in the photo. So, the more a file is compressed,
the more likely you are to see the annoying jagged
lines disrupting your image.
For
example - with my Nikon 775 camera – I have three
choices for the file type or image quality setting.
All three are JPEG files – a low-compression
mode (Fine mode), a medium-compression mode (Normal
mode) and a high-compression mode (Basic mode). The
files taken in Fine mode are about four times as large
as the ones created in Basic mode. The smaller files
have been compressed much more than the larger ones
– and are much more likely to show lines and
pixels that have been affected by the file compression.
You
may not be able to see the differences in the images
on the viewscreen on your camera. But they will become
visible on your computer monitor, and especially when
you print your pictures.
So
if you want to avoid the little jagged lines and other
artifacts of JPEG compression in your images –
the first thing you should do is check the settings
in your camera. Set the file quality for the highest
quality you can – either a low-compression JPEG
file or even better a non-compressed file type such
as TIFF or RAW.
Combining
no- or low-compression with using the largest resolution
your camera allows will give you the largest image
files your camera can produce, which will give you
more data to work with when you edit your images.
If all you are doing is making small images for the
web, these factors will not matter much, but if you
plan on enlarging or printing, then the camera’s
resolution and file compression settings will be important.
The end result will be better images that you can
enlarge without having to worry as much about loss
of image quality.
Examples
showing the impact of file compression on images can
be found below:
JPG
Compression in Camera
Here are two photos
taken after the recent snow in the D.C. area.
The two photos were taken with a Canon D60 digital
camera. The first was taken with the camera
in "raw" mode, which produces an uncompressed
image. The second photo of the same energetic
snow shovelers was taken using the highest jpg compression
(i.e. makes the smallest size file). To give
you an idea of size, the camera's microdrive will
hold 141 files in raw mode, and 780 files in the high
compression jpg mode. Both files have been resized
for the web, but otherwise unedited.
Raw Mode:
High Compression
JPG
At this level,
it is almost impossible to tell the two apart, but
this is an issue of viewing photos resized for the
web. The following are crops of the shoveler
on the left, with part of the sign.
The crop on the
left is taken from the raw file, the one on the right
taken from the compressed image. Take a look
around the letters in the sign and at the border of
the snow and the trees, and you will see image distortion
in the jpg image.
For a more glaring
example, take a look at the two images below, taken
by our Nikon 775 digital camera. The first photo
was taken with "fine" mode jpg, and the
second with "basic" mode jpg (highest compression).
Fine Mode JPG:
Basic Mode JPG:
Again, it is hard
to tell from the images resized for the web, but it
is visible when magnified to 100%. The following
is a comparison blown up to 300% to emphasize the
differences.
The image on the
left is the fine mode, the one on the right the basic
mode. At this size you can see individual pixels
from the image. The larger squares that you
can see in the image are due to the high JPG compression.
You can see this slightly in the fine mode image on
the left, if you look in the areas of high contrast
(where there is snow and pine cone in the same areas).
These differences can be a factor in image quality,
particularly when you are printing enlargements.
HLI
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