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Compression in JPEG files in digital cameras



 
 
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  #21  
Old August 20th 07, 09:58 PM posted to rec.photo.digital
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

David J Taylor added these comments in the current discussion du
jour ...

What I have found in practice is that using the maximum
resolution with the "normal" quality setting can beat the
lower resolution with the "fine" quality setting. Of course,
this is highly camera dependant, and you should test for
yourself.

My theory behind this is that running at the maximum
resolution, you are more limited by the MTF of the lens and
other components, so there is relatively less high-frequency
component in the image, and that the JPEG algorithm does not
need to work so hard to compress the data, and can therefore
provide a higher quality image.

So my recommendation to the OP is to stick with maximum
resolution (3000 x 2250), and to make a series of test photos
at the different quality (JPEG compression) levels, and see
where you want to draw the line between file size and image
quality. Include both sharp edges and subtly coloured (or
even varying grey) areas in your test image. I have found
that the "normal" quality setting is satisfactory on the Nikon
cameras I have owned - your camera and image quality
requirements may differ.


We've talked about this before, and agreed to disagree. The think
that I DO agree - strongly - with you about is to test, test, and
test some more. It is ultimately the opinion of the OP what
constitutes "quality" or "defects", we're all here to help,
hopefully clarify, but also hopefully not confuse him.

For the records, again I do not dispute you, I find it best on my
Rebel XT to shoot at 4 rather than 8 MP because it is too much
bother to take the pains to resample down to my usual 1400 x 1050
final size. Doing it in one fell swoop is almost guaranteed to
introduce aliasing because few resampling algorithms can take out
so much data without messing up parts of the very fine detail. In
my world of car pictures, where I see it is in slightly off-
horizontal chrome moldings on the side of the car, grille, or
windshield. We each do what we fine works best for us, yes?

Nikon do seem to have the many parameters you can adjust in
the JPEG algorithm very well chosen.

I was pretty surprised that my Rebel XT can't save to TIFF but
only mildly miffed that it didn't have a 3rd, higher-compression-
thus-lower-quality image. I did my usualy testing the first day
at 2, 4, and 8 MP and both JPEG settings, made an initial eval,
then went and shot some real cars at each, came home and looked
at them.

I will say this about more MP: I like to include absolutely as
much AROUND my cars as I can fit within the confines of the space
I have, preferring to do my final crop for best composition in
the "digital darkroom" whilst also allowing enough to crop
correctly after doing something like perspective correction. So,
Since I finish at about 1.5 MP but shoot at 4, I'm more than OK.
And, if I ever decide I'd like to re-edit to, say 3 MP, I can
also do that.

Now, if an occasion arises where I do not have enough telephoto,
I will go up to 8 to get an effective increase in "focal length"
by simply cropping out my smallist image from the BIG one.

--
HP, aka Jerry
  #22  
Old August 20th 07, 10:01 PM posted to rec.photo.digital,uk.rec.photo.misc
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

Don Stauffer in Minnesota added these comments in the current
discussion du jour ...

This is a complicated question, and not suited to quick
answers- I'd get a decent book on digital photography.
Actually, the Dummies Guide series has a reasonable book on
the subject.


For this OP, it may be quite simple. It's fine if the OP wants to
buy a book, probably a real good idea to learn the gentle art of
digital photography. However, few books can adequately illustrate
the effects of excessive compression, and a half-hour shooting a
few dozen test shots around the house and yard can be very
instructive to a new user. After all, it is THEIR definition of
"good" and "bad" that matters, not all of us would-be "experts."

Briefly, there are two different ways of reducing the size of
the file (memory size) of an image. One method is to
"downsample", or average pixels to create a new array of
numbers with fewer pixels. All the pixels in the image chip
are used, but the downsampling or averaging math reduces the
"resolution" or sharpness of the image.

Data compression schemes such as JPEG do not downsample in the
same way. Depending on the subject a jpeg will retain all or
most of the resolution. However, subtleties of color
disappear, gradients of color or brightness get flattened, and
little regions of the wrong color begin cropping up. Jpeg is
a lossy compression scheme, meaning this loss of image quality
cannot be reversed after the image file is compressed. There
ARE other compression schemes that ARE reversible.


With the cheapness of very large memory cards these days, and
also large memory banks for computers, there is little reason
to downsample in the camera, or to use excessive values of
compression (many cameras allow you to select how much
compression to use). The most common advice these days is to
shoot full resolution (the 3000 x 2250 mentioned), and select
the least amount of jpeg compression. This is frequently
called picture quality, as in super high quality, high
quality, medium, or some such combination of words.






--
HP, aka Jerry
  #23  
Old August 21st 07, 09:12 AM posted to rec.photo.digital
David J Taylor[_4_]
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Posts: 1,151
Default Compression in JPEG files in digital cameras

HEMI-Powered wrote:
[]
For the records, again I do not dispute you, I find it best on my
Rebel XT to shoot at 4 rather than 8 MP because it is too much
bother to take the pains to resample down to my usual 1400 x 1050
final size. Doing it in one fell swoop is almost guaranteed to
introduce aliasing because few resampling algorithms can take out
so much data without messing up parts of the very fine detail. In
my world of car pictures, where I see it is in slightly off-
horizontal chrome moldings on the side of the car, grille, or
windshield. We each do what we fine works best for us, yes?

[]
I was pretty surprised that my Rebel XT can't save to TIFF but
only mildly miffed that it didn't have a 3rd, higher-compression-
thus-lower-quality image. I did my usualy testing the first day
at 2, 4, and 8 MP and both JPEG settings, made an initial eval,
then went and shot some real cars at each, came home and looked
at them.


Jerry,

TIFF doesn't really help if it's only 8-bit depth - as the best quality
JPEG has so little loss. RAW would be the next step up in saved image
quality, but it has the overhead of requiring more processing which I
think both you and I try and minimise.

Have you ever gone back and revisited the resolution and quality settings?
This is something I keep meaning to do as I gain experience, but somehow I
never get round to it! I wonder if we would make the same choices a
second time, particularly on JPEG quality setting?

Yes, we agree to disagree on shooting - I go for highest resolution with
"normal" JPEG quality. I leave any resampling up to the display
software - indeed I wrote my own simple slide-show program so that I had
control over the display process.

http://www.david-taylor.myby.co.uk/s...html#SlideShow

What works best for you, is best for you, and I respect that.

Cheers,
David


  #24  
Old August 21st 07, 10:12 AM posted to rec.photo.digital,uk.rec.photo.misc
Ron Hunter
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Posts: 4,064
Default Compression in JPEG files in digital cameras

HEMI-Powered wrote:
added these comments in the current discussion du jour ...

I need some help in explaining the JPEG compression feature in
digital cameras. My camera (which likely is similar to most
others) has the feature to compress the photo JPEG files in
the storage card. It also has the choice to have different
pixel sizes (example: 3000x2250, 2000x1500, 1024x768, etc).
What is the difference of the above two features? If you
store a 3000x2250 pixel data in compressed mode, does it loose
its quality? Can it be re-instated to full uncompressed size
without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it
the same if I choose 2000x1500 pixel and no compression
instead? Thanks for info.

Resolution is measured by horizontal and vertical pixels for an
image, the area of which constitutes mega pixels. JPEG
compression is how much or little the uncompressed data is made
smaller. Since JPEG is a so-called "lossy" format, it literally
throws away pixels in order to drastically shrink the file size.
The idea of the algorithm is to choose pixels to disgard that
have a mathmatical probability of not being noticed by the human
eye.

However, when compression begins to get even moderately high,
defects begin to appear, commonly called "JPEG artifacts",
artifact meaning "what is left behind". This can easily be seen
by blobs, streaks, blurry areas, minor destruction of fine
detail, and sometimes posterization.

"Better" digital cameras will give you choices as to how much to
compress for a given mega pixel image. Lots of words used but
"basic", "normal", and "fine" are common. I don't think any
cameras tell you the actually number they use nor the Chroma
subsampling they use, but under most circumstances, one can
quickly show themselves that "basic" is pretty awful, "normal"
MAY produce artifacts 5, 10, 15% of the time, and "fine" rarely
does.

However, many - not all - of the less expensive P & S cameras
only give you marketing BS like "good", "better" and "best", but
what they're really doing is maintaining the same JPEG
compression but upping the MP. The reason that so many lower cost
but high MP cameras do that is that they also want to advertise
how many pictures you can fit on even a small memory card.

I'm not sure about the rest of your question to exampand more on
my answer. Some cameras allow TIFF, which is lossless, and even
better cameras - certainly DSLRs - can also save in RAW.

Without starting another religious war, if you can get by with
JPEG and it fulfills what you want and need the camera to do for
you, you'll be just fine. It is universally readable, lots of
free or almost free editing apps as wells as commercial apps, and
you can save money on memory if that is a consideration.

But, since I know of NO camera buyer who isn't interested in the
best possible quality, I would look for cameras that offer a
choice of compression so that you can run some tests for yourself
and make up your own mind.


I vote for maximum resolution, minimal compression, and buy more, or
larger, cards. With 2GB SD cards selling for about $20 (US), these
days, the question seems moot.
  #25  
Old August 21st 07, 10:17 AM posted to rec.photo.digital
Ron Hunter
external usenet poster
 
Posts: 4,064
Default Compression in JPEG files in digital cameras

HEMI-Powered wrote:
David J Taylor added these comments in the current discussion du
jour ...

What I have found in practice is that using the maximum
resolution with the "normal" quality setting can beat the
lower resolution with the "fine" quality setting. Of course,
this is highly camera dependant, and you should test for
yourself.

My theory behind this is that running at the maximum
resolution, you are more limited by the MTF of the lens and
other components, so there is relatively less high-frequency
component in the image, and that the JPEG algorithm does not
need to work so hard to compress the data, and can therefore
provide a higher quality image.

So my recommendation to the OP is to stick with maximum
resolution (3000 x 2250), and to make a series of test photos
at the different quality (JPEG compression) levels, and see
where you want to draw the line between file size and image
quality. Include both sharp edges and subtly coloured (or
even varying grey) areas in your test image. I have found
that the "normal" quality setting is satisfactory on the Nikon
cameras I have owned - your camera and image quality
requirements may differ.


We've talked about this before, and agreed to disagree. The think
that I DO agree - strongly - with you about is to test, test, and
test some more. It is ultimately the opinion of the OP what
constitutes "quality" or "defects", we're all here to help,
hopefully clarify, but also hopefully not confuse him.

For the records, again I do not dispute you, I find it best on my
Rebel XT to shoot at 4 rather than 8 MP because it is too much
bother to take the pains to resample down to my usual 1400 x 1050
final size. Doing it in one fell swoop is almost guaranteed to
introduce aliasing because few resampling algorithms can take out
so much data without messing up parts of the very fine detail. In
my world of car pictures, where I see it is in slightly off-
horizontal chrome moldings on the side of the car, grille, or
windshield. We each do what we fine works best for us, yes?

Nikon do seem to have the many parameters you can adjust in
the JPEG algorithm very well chosen.

I was pretty surprised that my Rebel XT can't save to TIFF but
only mildly miffed that it didn't have a 3rd, higher-compression-
thus-lower-quality image. I did my usualy testing the first day
at 2, 4, and 8 MP and both JPEG settings, made an initial eval,
then went and shot some real cars at each, came home and looked
at them.

I will say this about more MP: I like to include absolutely as
much AROUND my cars as I can fit within the confines of the space
I have, preferring to do my final crop for best composition in
the "digital darkroom" whilst also allowing enough to crop
correctly after doing something like perspective correction. So,
Since I finish at about 1.5 MP but shoot at 4, I'm more than OK.
And, if I ever decide I'd like to re-edit to, say 3 MP, I can
also do that.

Now, if an occasion arises where I do not have enough telephoto,
I will go up to 8 to get an effective increase in "focal length"
by simply cropping out my smallist image from the BIG one.


Why does a person buy a DSLR, and then shoot at a resolution barely
better than the early digital cameras produced? If you want only 1.5mp,
why spend money on a DSLR? Even a $99 P&S camera will give better
images than you get that way. I can only conclude that for your needs,
small size is more important than image quality. I can only assume that
you only capture images for web use.
  #26  
Old August 21st 07, 08:09 PM posted to rec.photo.digital
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

David J Taylor added these comments in the current discussion du
jour ...

I was pretty surprised that my Rebel XT can't save to TIFF
but only mildly miffed that it didn't have a 3rd,
higher-compression- thus-lower-quality image. I did my usualy
testing the first day at 2, 4, and 8 MP and both JPEG
settings, made an initial eval, then went and shot some real
cars at each, came home and looked at them.


Jerry,

TIFF doesn't really help if it's only 8-bit depth - as the
best quality JPEG has so little loss. RAW would be the next
step up in saved image quality, but it has the overhead of
requiring more processing which I think both you and I try and
minimise.


Both ordinary JPEG and TIFF are both 8-bit. I only mentioned
TIFF, either the uncompressed kind or LZW, as an alternative to
the OP or any lurkers who want a simpler lossless file system
than RAW,

Have you ever gone back and revisited the resolution and
quality settings? This is something I keep meaning to do as I
gain experience, but somehow I never get round to it! I
wonder if we would make the same choices a second time,
particularly on JPEG quality setting?


Yes, David, as recently as this spring as I was getting ready for
the summer car show season. It isn't that I dispute you at all,
you're much more skilled and knowledgeable than me, it is that MY
experience shows that the amount of work necessary to get an 8 MP
image down to my 1.5 MP finished size without introducing
aliasing and other undesirable non-compression artifacts just
isn't worth my time and effort.

I never revisited the lower quality, i.e., higher compression,
option on my Rebel because I could easily see JPEG compression
artifacts at a large enough percentage that I didn't want to risk
blowing otherwise good pictures. As you and others have so
correctly pointed out, memory is so cheap, that it really doesn't
matter if the image is 500KB or one meg.

Yes, we agree to disagree on shooting - I go for highest
resolution with "normal" JPEG quality. I leave any resampling
up to the display software - indeed I wrote my own simple
slide-show program so that I had control over the display
process.

http://www.david-taylor.myby.co.uk/s...ging.html#Slid
eShow

What works best for you, is best for you, and I respect that.

Thanks for the link, David. And, I respect YOUR preferences.
People in this NG, like most Usenet NGs, get along a whole lot
better if they respect each other's rights and freedoms to do
things in alternate ways, and I think you and I have previously
agreed that there NEVER is one or even two "right" ways to do
anything. And, as you so eloquently say, "what works best for
you, is best for you", where "you" means anyone anywhere anytime
doing anything.

Have a great day!

--
HP, aka Jerry
  #27  
Old August 21st 07, 08:14 PM posted to rec.photo.digital
HEMI-Powered[_2_]
external usenet poster
 
Posts: 447
Default Compression in JPEG files in digital cameras

Ron Hunter added these comments in the current discussion du
jour ...

Why does a person buy a DSLR, and then shoot at a resolution
barely better than the early digital cameras produced? If you
want only 1.5mp,
why spend money on a DSLR? Even a $99 P&S camera will give
better
images than you get that way. I can only conclude that for
your needs, small size is more important than image quality.
I can only assume that you only capture images for web use.

There are many reasons why a DSLR is preferable besides just the
mega pixels it can produce. And, I've stated enough times why I
prefer 4 to 8 generally, so I shan't repeat myself here. Suffice to
say that there are times I go to 8 and as I just replied to David
Taylor, I DO perform controlled tests on a number of key set-up
parameters to determine what seems to work best for the particular
photographic situations I find myself in as well as my current
level of skill in PSP 9. David said it best: what works best for
you, is best for you.

As to the conclusions you reached, be wary of jumping to incorrect
conclusions based on no facts and only unfounded assumptions. If
you don't know me or the reasons I do what I do, as evidenced by
periodic posts I make on the subject of mega pixels, then you might
leap to a wrong view of my intelligence either when purchasing a
camera or using it. I have found in life that it usually is not
wise to take extreme views and verbally spank others who happen not
to agree with me.

--
HP, aka Jerry
  #28  
Old August 21st 07, 08:37 PM posted to rec.photo.digital
Thomas T. Veldhouse
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Posts: 962
Default Compression in JPEG files in digital cameras

HEMI-Powered wrote:

Both ordinary JPEG and TIFF are both 8-bit. I only mentioned
TIFF, either the uncompressed kind or LZW, as an alternative to
the OP or any lurkers who want a simpler lossless file system
than RAW,


TIFF is not fixed to a given bit depth. I often create 16-bit TIFF images.
The compression type is not limitted in TIFF either. Photoshop will create
16-bit images and use no compression, LZW compression or ZIP compression.
TIFF can be used with ANY compression algorithm if it is built that way ...
there are just no clients that support it. Also, keep in mind that DNG,
adobe's digital negative, a RAW format, also is based on the TIFF format.

--
Thomas T. Veldhouse

We have more to fear from the bungling of the incompetent than from the
machinations of the wicked.

  #29  
Old August 22nd 07, 03:15 AM posted to rec.photo.digital
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

Thomas T. Veldhouse added these comments in the current
discussion du jour ...

Both ordinary JPEG and TIFF are both 8-bit. I only mentioned
TIFF, either the uncompressed kind or LZW, as an alternative
to the OP or any lurkers who want a simpler lossless file
system than RAW,


TIFF is not fixed to a given bit depth. I often create 16-bit
TIFF images. The compression type is not limitted in TIFF
either. Photoshop will create 16-bit images and use no
compression, LZW compression or ZIP compression. TIFF can be
used with ANY compression algorithm if it is built that way
... there are just no clients that support it. Also, keep in
mind that DNG, adobe's digital negative, a RAW format, also is
based on the TIFF format.

OK, but again, Thomas, I interpreted the OP as being a fairly new
digital owner and thus a novice on file types, so I just
mentioned TIFF as an alternative if their camera supports it.
TIFF is universally recognized, although news readers cannot
decode it in line, and about its only drawback other than large
size is that if you want to save EXIF, you cannot use LZW
compression, or at least AFAIK.

I don't want to start yet another religious war about bit-length.
It seems to me, though, that 16-bit color - no matter where it
comes from or in what format(s) it is saved to - is a subject for
VERY advanced people who have software that can correctly
manipulated it. And, the person behind the camera and behind the
keyboard also has to be quite a bit more knowledgeable to gain
any real advantage over 8-bit.

But, I have a question for you Thomas: everytime 16-bit color
comes up, part of the raging debate is that most/all cameras and
apps, including PS CS2, really only have 11 or 12 bits of real
information, the others being basically just noise that is
ignorned by the software. Has that improved in ANY format,
whether it be JPEG, TIFF, or RAW? i.e., is something closer to
16-bit or true 16-bit now available for them with deep pockets?
If so, could you just give me 25 words or less as a heads-up on
today's status so I can go looking? Yes, I know Google is my best
friend, but on things like this, it is like the old saying "I
don't even know enough to ask an intelligent question", and it
goes to the extreme frustration I have had for well over a year
trying to find a RAW for Dummies kind of book that isn't keyed to
PS CS2 or Elements that will at least get me started up the
learning curve.

In any event, I find these threads fascinating but always feel
bad for the poor OP who has some simple or easy question like
he does the size of the image or compression cause image
problems? I kinda doubt that level of knowledge is ready for all
of the sophisticated answers the more experienced folks have been
bantering about. Yes, /I/ learn something, but a rank novice just
gets totally snowed under and may feel so intimidated as to not
even come back for a 2nd round of questions.

--
HP, aka Jerry
  #30  
Old August 22nd 07, 08:21 AM posted to rec.photo.digital
David J Taylor[_4_]
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Posts: 1,151
Default Compression in JPEG files in digital cameras

HEMI-Powered wrote:
[]
Both ordinary JPEG and TIFF are both 8-bit. I only mentioned
TIFF, either the uncompressed kind or LZW, as an alternative to
the OP or any lurkers who want a simpler lossless file system
than RAW,


OK, I appreciate that.

Have you ever gone back and revisited the resolution and
quality settings? This is something I keep meaning to do as I
gain experience, but somehow I never get round to it! I
wonder if we would make the same choices a second time,
particularly on JPEG quality setting?


Yes, David, as recently as this spring as I was getting ready for
the summer car show season. It isn't that I dispute you at all,
you're much more skilled and knowledgeable than me, it is that MY
experience shows that the amount of work necessary to get an 8 MP
image down to my 1.5 MP finished size without introducing
aliasing and other undesirable non-compression artifacts just
isn't worth my time and effort.

I never revisited the lower quality, i.e., higher compression,
option on my Rebel because I could easily see JPEG compression
artifacts at a large enough percentage that I didn't want to risk
blowing otherwise good pictures. As you and others have so
correctly pointed out, memory is so cheap, that it really doesn't
matter if the image is 500KB or one meg.


Yes, I would expect that when using less than the native resolution of the
camera, the images will be slightly sharper (meaning that there are fewer
pixels covering a black-white transition), and therefore more likely to
show JPEG artefacts at a particular compression (quality) level. So when
using lower resolution, stick with the highest JPEG quality. Makes sense
to me, anyhow!

Cheers,
David


 




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