Computer Graphics with Photoshop - JPEG saving options

Author

JPEG saving options

Carl Miller

2004-09-28, 7:15 am

A straight-forward answer to this might be out there, but I haven't
found it yet. (All file sizes are approximations, but representative of
actual experience.)

When saving JPEG's, the Quality drop-down menu gives you the choice of
Low, Medium, High, Maximum. Choosing Maximum gives a setting of 10.
However, you can increase the setting to 12. I've noticed that if I open
a JPEG that is, say, about 1.3mb and save it at the "default" Maximum
setting of 10, it compresses it to, say, about 900k. If I save it at the
"maximum" Maximum setting of 12, I actually end up with a larger file
than I started out with of about 3mb. Apparently it is uncompressing the
original compressed jpeg?

What's up with this, and practically speaking, should I be saving at 10
or 12?

(Take it as read that I know about tif being non-lossy, and jpeg being
lossy, etc. I'm just wondering about this Photoshop jpeg Quality thing.)

Thanks!!

--
Carl Miller
carl_miller23@hotmail.com
http://www.ezinfocenter.com/8557444

David Dyer-Bennet

2004-09-28, 7:15 am

carl_miller23@hotmail.com (Carl Miller) writes:

> A straight-forward answer to this might be out there, but I haven't
> found it yet. (All file sizes are approximations, but representative of
> actual experience.)
>
> When saving JPEG's, the Quality drop-down menu gives you the choice of
> Low, Medium, High, Maximum. Choosing Maximum gives a setting of 10.
> However, you can increase the setting to 12. I've noticed that if I open
> a JPEG that is, say, about 1.3mb and save it at the "default" Maximum
> setting of 10, it compresses it to, say, about 900k. If I save it at the
> "maximum" Maximum setting of 12, I actually end up with a larger file
> than I started out with of about 3mb. Apparently it is uncompressing the
> original compressed jpeg?

Of course it is; that's what loading it into photoshop does. And then
if you save it, it recompresses it at whatever level you select.

> What's up with this, and practically speaking, should I be saving at 10
> or 12?

I find essentially no use for either. For screen resolution, 8 is
plenty, or even 5. For printing, I wouldn't use jpeg.
--
David Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/>
RKBA: <http://noguns-nomoney.com/> <http://www.dd-b.net/carry/>
Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
Dragaera/Steven Brust: <http://dragaera.info/>

Johan W. Elzenga

2004-09-28, 7:15 am

Carl Miller <carl_miller23@hotmail.com> wrote:

> A straight-forward answer to this might be out there, but I haven't
> found it yet. (All file sizes are approximations, but representative of
> actual experience.)
>
> When saving JPEG's, the Quality drop-down menu gives you the choice of
> Low, Medium, High, Maximum. Choosing Maximum gives a setting of 10.
> However, you can increase the setting to 12. I've noticed that if I open
> a JPEG that is, say, about 1.3mb and save it at the "default" Maximum
> setting of 10, it compresses it to, say, about 900k. If I save it at the
> "maximum" Maximum setting of 12, I actually end up with a larger file
> than I started out with of about 3mb. Apparently it is uncompressing the
> original compressed jpeg?

Photoshop (or any other program) cannot edit images in compressed
format, so yes, when you open an image it will be uncompressed.

>
> What's up with this, and practically speaking, should I be saving at 10
> or 12?

All the information you've lost on the first compression, is lost
forever. On the next compression you will loose information again, so if
you insist in subsequent JPEG saves and you want to retain as much
information as you can, you'll have to use 12.

--
Johan W. Elzenga johan<<at>>johanfoto.nl
Editor / Photographer http://www.johanfoto.nl/

Eric Gill

2004-09-28, 7:15 am

carl_miller23@hotmail.com (Carl Miller) wrote in
news:00040823235949.OUI15.carl_miller23@hotmail.com:

> On September 24 2004, Eric Gill <ericvgill@yahoo.com> wrote:
> "default" >> Maximum setting of 10, it compresses it to, say, about
> 900k. If I >> save it at the "maximum" Maximum setting of 12, I
> actually end up >> with a larger file than I started out with of about
> 3mb. Apparently >> it is uncompressing the original compressed jpeg?
> whatever you need it for.
>
> Thank you mr. expert for totally NOT ansering my question. Didn't I SAY
> I KNEW about lossy vs non-lossy compressions? Don't you think that
means
> I KNOW about saving my "work copy" in a non-lossy format?

That was, of course, why you asked about multiple saves in JPEG.

> Now, re-read
> my post and if you can answer the question I asked, please do, but
> otherwise stop being a total lack of help. (Hint: The question is in
one
> of the sentences that ends with a question mark.)
>
> Sorry to sound pissy,

Oh, I'm sure.

> but you gave me the exact type of NON-ANSWER I've
> been getting to this question for practically six months now.
Apparently
> NO ONE knows the answer.

The answer is simply that you shouldn't be re-saving in JPEG format.

It's not my problem that you don't like the answer, and getting pissy
because you hold a misconception is stupid.

Eric Gill

2004-09-28, 7:15 am

carl_miller23@hotmail.com (Carl Miller) wrote in
news:00040823211103.OUI90.carl_miller23@hotmail.com:

> A straight-forward answer to this might be out there, but I haven't
> found it yet. (All file sizes are approximations, but representative of
> actual experience.)
>
> When saving JPEG's, the Quality drop-down menu gives you the choice of
> Low, Medium, High, Maximum. Choosing Maximum gives a setting of 10.
> However, you can increase the setting to 12. I've noticed that if I open
> a JPEG that is, say, about 1.3mb and save it at the "default" Maximum
> setting of 10, it compresses it to, say, about 900k. If I save it at the
> "maximum" Maximum setting of 12, I actually end up with a larger file
> than I started out with of about 3mb. Apparently it is uncompressing the
> original compressed jpeg?

Of course. It has to, to work on it.

> What's up with this, and practically speaking, should I be saving at 10
> or 12?

You shouldn't be re-saving a JPEG at all. Save your working copy in a
lossless format, then use save for web to optimize the JPEG output for
whatever you need it for.

> (Take it as read that I know about tif being non-lossy, and jpeg being
> lossy, etc. I'm just wondering about this Photoshop jpeg Quality thing.)
>
> Thanks!!
>

Carl Miller

2004-09-28, 7:15 am

On September 24 2004, Eric Gill <ericvgill@yahoo.com> wrote:
"default" >> Maximum setting of 10, it compresses it to, say, about
900k. If I >> save it at the "maximum" Maximum setting of 12, I
actually end up >> with a larger file than I started out with of about
3mb. Apparently >> it is uncompressing the original compressed jpeg?[color=darkred]
>
> Of course. It has to, to work on it.
>
>
> You shouldn't be re-saving a JPEG at all. Save your working copy in a
> lossless format, then use save for web to optimize the JPEG output for
>
whatever you need it for.[color=darkred]
>

Thank you mr. expert for totally NOT ansering my question. Didn't I SAY
I KNEW about lossy vs non-lossy compressions? Don't you think that means
I KNOW about saving my "work copy" in a non-lossy format? Now, re-read
my post and if you can answer the question I asked, please do, but
otherwise stop being a total lack of help. (Hint: The question is in one
of the sentences that ends with a question mark.)

Sorry to sound pissy, but you gave me the exact type of NON-ANSWER I've
been getting to this question for practically six months now. Apparently
NO ONE knows the answer.

--
Carl Miller
carl_miller23@hotmail.com
http://www.ezinfocenter.com/8557444

Scraphead

2004-09-28, 12:14 pm

"Rick" <me@privacy.net> wrote in message n
if[color=darkred]
>
> Exactly. To put it another way, when you save a file in
> JPEG format, the compression routine doesn't know or
> care about any previous compression(s).
>
> Rick
Why do they even bother with an adjustable scale then? I would think that
there is not a soul who would want to save a file with less quality!

Bill Hilton

2004-09-28, 12:15 pm

>> When you set it to "12", the result is closer to the original file

[color=darkred]
>From: "Scraphead" Watashee@{deletethis}one.net
>
>Why do they even bother with an adjustable scale then? I would think that
>there is not a soul who would want to save a file with less quality!

Every "soul" who saves jpegs for the web will soon discover that at moderate
mid-level settings well below 10 or 12 you have a much smaller file size with
acceptable quality, so people can look at your images without lengthy waits for
downloads.

Tacit

2004-09-28, 12:15 pm

>Why do they even bother with an adjustable scale then? I would think that
>there is not a soul who would want to save a file with less quality!

Yes, there are many times when you want to save a JPEG with less quality--for
example, for the Web.

The JPEG file format was created for situations where file size is critical and
file quality is not important. The Web is one such situation. There are many
times when a JPEG saved at maximum quality is just plain too big for a Web
site.

Only use JPEG if you *have to*--if you absolutely must have a small file, and
quality isn't important. Never save a JPEG if you do not have to have a JPEG.
("JPEG is the only kind of file I know about" does not qualify as "have to.")

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Nicholas Sherlock

2004-09-28, 7:16 pm

Carl Miller wrote:
> If I save it at the "maximum" Maximum setting of 12, I actually end up
> with a larger file than I started out with of about 3mb. Apparently
> it is uncompressing the original compressed jpeg?
>
> What's up with this, and practically speaking, should I be saving at
> 10 or 12?
>
> (Take it as read that I know about tif being non-lossy, and jpeg being
> lossy, etc. I'm just wondering about this Photoshop jpeg Quality
> thing.)

You say that you _know_ about lossy file formats, but your comments seem to
suggest you have only a very basic understanding. Every time you open a
JPEG, it's decompressed. When it's saved, it must be compressed again. No
matter what compression level you set it to, you are losing information.
When you set it to "12", the result is closer to the original file than if
you set it to "10"

Cheers,
nicholas Sherlock

Martin

2004-09-28, 7:16 pm

carl_miller23@hotmail.com (Carl Miller) wrote in message news:<00040823211103.OUI90.carl_miller23@hotmail.com>...
> A straight-forward answer to this might be out there, but I haven't
> found it yet. (All file sizes are approximations, but representative of
> actual experience.)
>
> When saving JPEG's, the Quality drop-down menu gives you the choice of
> Low, Medium, High, Maximum. Choosing Maximum gives a setting of 10.
> However, you can increase the setting to 12. I've noticed that if I open
> a JPEG that is, say, about 1.3mb and save it at the "default" Maximum
> setting of 10, it compresses it to, say, about 900k. If I save it at the
> "maximum" Maximum setting of 12, I actually end up with a larger file
> than I started out with of about 3mb. Apparently it is uncompressing the
> original compressed jpeg?
>
> What's up with this, and practically speaking, should I be saving at 10
> or 12?
>
> (Take it as read that I know about tif being non-lossy, and jpeg being
> lossy, etc. I'm just wondering about this Photoshop jpeg Quality thing.)
>
> Thanks!!

I've had the exact questions. I was hoping for a straight forward
answer, but I must agree with Carl, no one directly answered the
questions; e.g. what about Max = 10 but 12 is available. Why is 12
larger that the original (details)? If someone who really understand
P/Shop JPEG save routing could/would answer it would be appreciated.

Rick

2004-09-28, 7:16 pm

"Martin" <metpx3c@hotmail.com> wrote in message news:3a16084b.0409260857.646fd4de@posting.google.com...
> carl_miller23@hotmail.com (Carl Miller) wrote in message news:<00040823211103.OUI90.carl_miller23@hotmail.com>...
>
> I've had the exact questions. I was hoping for a straight forward
> answer, but I must agree with Carl, no one directly answered the
> questions; e.g. what about Max = 10 but 12 is available. Why is 12
> larger that the original (details)? If someone who really understand
> P/Shop JPEG save routing could/would answer it would be appreciated.

It's been explained to you in plain English. What part of this
do you not understand:

The JPEG compression routine doesn't know or care whether
an image has been previously compressed. In other words,
when you JPEG compress an image, the routine doesn't know
or care what the original file size was, or what quality settings
were used for previous compressions. At quality 12 you're
getting a less compressed image, therefore larger in size,
although it will still be compressed, and will still be lossy
relative to the original.

Rick

Scraphead

2004-09-29, 4:14 am

Aren't you supposed to be using gif's for the web? Isn't that what they are
designed for??
If not, what. They're pretty useless,otherwise.

"Bill Hilton" <bhilton665@aol.comedy> wrote in message
news:20040928104511.13148.00001683@mb-m28.aol.com...
>
>
>
> Every "soul" who saves jpegs for the web will soon discover that at
moderate
> mid-level settings well below 10 or 12 you have a much smaller file size
with
> acceptable quality, so people can look at your images without lengthy
waits for
> downloads.
>
>

Hecate

2004-09-29, 4:14 am

On Tue, 28 Sep 2004 00:51:31 -0500, David Dyer-Bennet <dd-b@dd-b.net>
wrote:

>Eric Gill <ericvgill@yahoo.com> writes:
>
>
>Even *I* will call that excessive. Slightly.
>
>In particular, if your camera produces JPEG output, then to really
>follow that rule would mean you never produced a JPEG for web
>display. Which is obviously dumb.

Look at what he's saying *resaving" NOT final save.

> --

Hecate - The Real One
Hecate@newsguy.com
veni, vidi, reliqui

Bill Hilton

2004-09-29, 4:14 am

>> >From: "Scraphead" Watashee@{deletethis}one.net
[color=darkred]

[color=darkred]
>From: "Scraphead" Watashee@{deletethis}one.net
>
>Aren't you supposed to be using gif's for the web?

Try saving a photo image as a 16 color gif (or even a full color gif) and see
for yourself :) Jpegs have many more shades and tones and are much better for
photos.

>Isn't that what they are designed for??

Nope.

>If not, what.

Line art, cartoons, limited color images ...

>They're pretty useless,otherwise.

Do you know ANYTHING about web design and using the right file formats?

Orchid

2004-09-29, 4:14 am

> > "Bill Hilton" <bhilton665@aol.comedy> wrote in message
> news:20040928104511.13148.00001683@mb-m28.aol.com...
> moderate
> with
> waits for
[color=darkred]
> Scraphead <Watashee@{deletethis}one.net> wrote:

> Aren't you supposed to be using gif's for the web? Isn't that what they are
> designed for??
> If not, what. They're pretty useless,otherwise.

GIFs are designed for images which use a lot of solid colors (like clip
art) and are limited to 256 colors. JPEGs are designed for continuous
tone images like photographs. Each format is optimized to create smaller
files for a different type of image. Unfortunately, most people aren't
aware of these differences and tend to use one format or the other
exclusively often creating bigger than necessary files or less
attractive files.

If you test the Photoshop "save for web" option, you can use the 4-up
tab to compare the differences in various types of images saved as
different file types. For instance, try making a solid color square and
saving setting it to GIF in one panel and jpeg in another. The lowest
quality jpeg compression will likely take longer to download than saving
it as a GIF. Try the same with a photograph.

Orchid

Tacit

2004-09-29, 4:14 am

>Aren't you supposed to be using gif's for the web? Isn't that what they are
>designed for??

The GIF specification was designed before the Web existed.

You use both GIF and JPEG for the Web. You use GIF if you need only a few
colors, such as for line drawings or for computer graphics, or if you need
transparency. You use JPEG for photographic images with lots of color.

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Eric Gill

2004-09-29, 7:14 pm

"Artie" <spoofed@spooked.com> wrote in news:Z8D6d.115$EO3.69@newsfe6-
gui.ntli.net:

> I may need therapy but I'd love the beer

Well, there's always...beer therapy.

Scraphead

2004-09-29, 7:14 pm

"Bill Hilton" <bhilton665@aol.comedy> wrote in message
news:20040928231543.01624.00001525@mb-m28.aol.com...
>
>
>
>
> Try saving a photo image as a 16 color gif (or even a full color gif) and
see
> for yourself :) Jpegs have many more shades and tones and are much better
for
> photos.
>
>
> Nope.
>
>
> Line art, cartoons, limited color images ...
>
>
> Do you know ANYTHING about web design and using the right file formats?

If I did know everything I would be TELLING, not ASKING!
See the question marks after my QUESTIONS?
Obviously, I'm trying to learn this stuff. There was just a looong recent
thread on here where all the so-called NG know-it-alls were saying what I'm
repeating. It was generally agreed, (um, not even agreed, they were
hammering it down our throats) here that tiffs are best for line art and
such. Gifs were computer/monitor/web related and jpgs are pieces of crap
that never should have been invented. Where are they all now? lol.

And, for Christ's sake, quit being so condescending.

Tacit

2004-09-29, 7:14 pm

>How do tiffs compare, given that jpgs lose integrity as they are messed
>with?

TIFF is an ideal format for general use. In fact, TIFF is one of the preferred
formats for professional printing and pre-press use. If you want a good format
for saving just about any kind of image, use TIFF unless you have a specific
reason to use some other format.

>At any rate, I guess it's safe to say, I can't get hurt doing photo work
>in psd and saving to jpg at the final piece?

As long as you are not editing and re-saving JPEG, that's correct.

What do you plan to do with the final piece? If you plan to put it on the Web,
use JPEG. If you plan to do anything else with it--print it, place it in
another program, whatever--use TIFF.

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Bill Hilton

2004-09-29, 7:14 pm

>From: "Scraphead" Watashee@{deletethis}one.net

> There was just a looong recent thread on here where all the so-called
> NG know-it-alls were saying what I'm repeating.

Not exactly.

>It was generally agreed, (um, not even agreed, they were
>hammering it down our throats) here that tiffs are best for line art and
>such. Gifs were computer/monitor/web related and jpgs are pieces of crap
>that never should have been invented. Where are they all now? lol.

Yes, they are probably LOL ... you misunderstood what was written. The
"experts" as you call them said not to save your working files as jpegs because
every time you save the files get compressed again and eventually you lose
information, whereas tiffs are lossless. This is not the same as saying there
is no use for jpegs at all ... they are the best solution for creating web
images of photos.

Bill

Scraphead

2004-09-29, 11:14 pm

"Bill Hilton" <bhilton665@aol.comedy> wrote in message
news:20040929171732.29586.00001322@mb-m02.aol.com...
[color=darkred]
> Yes, they are probably LOL ... you misunderstood what was written. The
> "experts" as you call them said not to save your working files as jpegs
because
> every time you save the files get compressed again and eventually you lose
> information, whereas tiffs are lossless. This is not the same as saying
there
> is no use for jpegs at all ... they are the best solution for creating web
> images of photos.
>
> Bill
Got it, sounds like a couple of options, anyway.

Scraphead

2004-09-29, 11:14 pm

"Tacit" <tacitr@aol.com> wrote in message
news:20040929163307.03318.00001412@mb-m17.aol.com...
> What do you plan to do with the final piece? If you plan to put it on the
Web,
> use JPEG. If you plan to do anything else with it--print it, place it in
> another program, whatever--use TIFF.
>
> --
> Art, literature, shareware, polyamory, kink, and more:
> http://www.xeromag.com/franklin.html
Actually, I do a pretty good business designing ads.
I do a lot of them in corel draw and all of the photos I import are
psd files. The publishers that I deal with all request hi-res pdf file,
which Corel can produce.
To most PS diehards I'm sure that sounds quite bohemian, but when a mag
prints at 240 dpi and I supply an 800 pdf file, it seems to work. The ads
look great printed so what the hey!
Here's one. It's a "jpg", but gives you an idea

Scraphead

2004-09-29, 11:14 pm

"tacit" <tacite@ahole.com> wrote in message
news:BD8083D2.12BFB6%tacite@ahole.com...
> in article 10lm67lb3kr7c1b@corp.supernews.com, Scraphead at
> Watashee@{deletethis}one.net wrote on 09/29/2004 1:22 PM:
>
>
>
>
>
> Or what will you do?

Thank You!

Carl Miller

2004-09-30, 4:14 am

On September 29 2004, tacitr@aol.com (Tacit) wrote:
> The GIF specification was designed before the Web existed.

This is true, technically, but keep in mind who GIF was designed by
(Compuserve) and why (in order to transmit images).

--
Carl Miller
carl_miller23@hotmail.com
http://www.ezinfocenter.com/8557444

Scraphead

2004-10-02, 12:14 pm

"David Dyer-Bennet" <dd-b@dd-b.net> wrote in message
news:m2oejreyak.fsf@gw.dd-b.net...
> The point is *sequences* of jpeg saves. Other than in emergencies,
> *I'll* save to jpeg exactly *once*, and never touch that file again (I
> might *replace* it with a new version, reworked from the original or a
> losslessly-saved intermediate). If I started with a jpeg file from
> the camera, that's a "resave", though.
Once out of the camera, I convert it to a psd file for working on. Am I
losing anything there? I've been doing that since last year when this had
been brought up, for the millionth time

<http://dragaera.info/>

Scraphead

2004-10-02, 12:14 pm

Martin

2004-10-03, 7:14 pm

Artie

2004-10-03, 7:14 pm

FWIW I don't know exactly either

Urban myths are:
resaving JPEG to JPEG at low levels cummulatively reduces (?) image size and
hence quality
JPEG compression algorithms are independent steps
for example
(a)image save at JPEG level 5 then image reopened and saved at JPEG level 12
=> this will not return the image to its original appearance because JPEG is
lossy BUT saving and resaving at JPEG level 12 will not cummulatively reduce
(?) image quality

(b) image saved a JPEG level 5 then image reopned and saved at JPEG level 5
=> a second application of the JPEG compression algorithms => a further loss
in image quality

I suppose (a) and (b) may be tested empirically (I can be bothered at the
mo')

My own preference on working images is to take JPEG to PSD and keep
everytthing in PSD form flattening out into JPEG at level 12 setting until I
am pleased with result. Mike will probably confirm that I am easily pleased
with results - possibly too easily pleased LOL

Saving to JPEG at anyother setting - I try to do this from the PSD file
rather than JPEG to JPEG (I can't remember why tho :-)

Artie

"Martin" <metpx3c@hotmail.com> wrote in message
news:3a16084b.0409260857.646fd4de@posting.google.com...
> carl_miller23@hotmail.com (Carl Miller) wrote in message
> news:<00040823211103.OUI90.carl_miller23@hotmail.com>...
>
> I've had the exact questions. I was hoping for a straight forward
> answer, but I must agree with Carl, no one directly answered the
> questions; e.g. what about Max = 10 but 12 is available. Why is 12
> larger that the original (details)? If someone who really understand
> P/Shop JPEG save routing could/would answer it would be appreciated.

Rick

2004-10-03, 7:14 pm

Tacit

2004-10-03, 7:14 pm

> Why is 12
>larger that the original (details)?

Very, very simple.

You have an image. Let us say for the sake of example that the image, when
UNCOMPRESSED, is one megabyte.

You save it as a JPEG. You use quality, say, 4. It saves to about 200
kilobytes.

You open the JPEG, It uncompresses. The image is 1 megabyte uncompressed.

You save it as JPEG with quality 12. It compresses again. You saved it with a
higher quality so this time it compresses to 500 kilobytes.

Make sense?

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Tacit

2004-10-03, 7:14 pm

>Why is 12
>larger that the original (details)? If someone who really understand
>P/Shop JPEG save routing could/would answer it would be appreciated.

It's very, very simple, and has already been explained.

Let us say you have an image that is 1 megabyte in size uncompresssed. Let's
say you save it as a JPEG with quality 4. So it saves to a file that's, for
instance, about 150 K in size.

Now you open that 150 K JPEG. It uncompresses. Now it is 1 megabyte again.

Now you save it again. You save as JPEG with quality 12. So it compresses
again, this time to 500 kilobytes.

Make sense?

--
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

David Dyer-Bennet

2004-10-03, 11:14 pm

metpx3c@hotmail.com (Martin) writes:

> I've had the exact questions. I was hoping for a straight forward
> answer, but I must agree with Carl, no one directly answered the
> questions; e.g. what about Max = 10 but 12 is available. Why is 12
> larger that the original (details)? If someone who really understand
> P/Shop JPEG save routing could/would answer it would be appreciated.

That second question *was* specifically answered, I thought, but I'll
go through it again.

When you open an image in Photoshop, it reads whatever format the
image file is originally in and decodes it into computer memory where
it's stored as an uncompressed bitmap. (Yes, if any of the Photoshop
programmers hang out here I know there are additional complexities,
not nearly well enough to explain them, so I'm not trying.) This is
the *full-sized* bitmap -- if it's a 1600x1200 digital original in
24-bit color, that's a 5,760,000 byte image, even though the jpeg
might have been 512KB.

So, there's this bitmap image in memory. Big. You edit it some,
change various bits, whatever. (Or just leave it alone, not change
any bits).

Now you tell Photoshop to save the image. It pops up the dialog for
you to select the quality level, and then starts coding the image into
a jpeg using the setting you gave it and writing the result to disk.

If the new quality level you give is the same as was used in the
original jpeg, the resulting file will be about the same size (if you
haven't changed the bitmap extensively). If you pick a higher quality
level, the resulting file will be larger -- it won't compress it as
much. If you pick a lower quality level, the resulting file will be
smaller -- it will compress more.

(The photoshop quality levels are a layer above the actual parameters
to the jpeg compression algorithm, by the way; that's why every
program has a different way of expressing degree of jpeg compression,
and they're not easy to compare to each other.)

Have I managed to make it make sense? Sorry if I still haven't.

As to your *first* question, about the "level 12" quality, I think I
know a tiny bit about why that's so. I believe that the range was
1-10 in earlier versions. They decided to add some options at the
top, and rather than changing the meaning of the previous options,
they just added more numbers (11 and 12) to describe the new options.
I *don't* really know exactly how those differ from 10, though.
--
David Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/>
RKBA: <http://noguns-nomoney.com/> <http://www.dd-b.net/carry/>
Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
Dragaera/Steven Brust: <http://dragaera.info/>

Tacit

2004-10-06, 7:14 am