Jpeg file size and picture quality
curious guy
picture was from a newsgroup. I did not take it.) I thought that was
too big so I experimented with resizing it with PhotoImpact 4.2. I
discovered that if I just loaded it and re-saved it without reducing
its dimensions, it went from 467,007 bytes to 37,341 bytes. I have
PhotoImpact set to use "Best" generation quality.
When I look at the pictures, I can not see any difference until I zoom
way in. Why is there such a huge difference in file sizes
(12.50654776 times)?
If there is another newsgroup where this question would be more
appropriate, please let me know.
Martin Brown
> I wanted to email someone a picture that was 467,007 bytes. (The
> picture was from a newsgroup. I did not take it.) I thought that was
> too big so I experimented with resizing it with PhotoImpact 4.2. I
> discovered that if I just loaded it and re-saved it without reducing
> its dimensions, it went from 467,007 bytes to 37,341 bytes. I have
> PhotoImpact set to use "Best" generation quality.
It is a reasonable question although you might want to look in the
archives it has been done to death several times before.
The short answer is that a JPEG encoded image uses lossy compression
designed so as not to be obvious to the human viewer on photographic
source material. And it is very good at what it does. PhotoImpacts
"Best" setting doesn't sound very good if the original image was also a
JPEG file but would be reasonable if it were a BMP.
There is a certain amount of grade inflation in the names given to JPEG
encoding settings. No software labels them "rubbish", "poor" or "barely
adequate" - the marketting men prefer "normal", "good", "very good" or
numbers.
And a few cameras like Nikon and Canon in their maximum quality setting
save with a quantisation matrix that wastes space without storing much
more information. You can generally get these files smaller without any
perceptible visual change in a first generation JPEG.
>
> When I look at the pictures, I can not see any difference until I zoom
> way in. Why is there such a huge difference in file sizes
> (12.50654776 times)?
>
> If there is another newsgroup where this question would be more
> appropriate, please let me know.
If you put the two files up somewhere it would be easy enough to see
what the actual quantisation matrices used by PhotoImpact and the
original source file are. The IJG scale of 0..100 is commonly used
Regards,
Martin Brown
Charles
Your operating system will probably allow you to adjust the size of e-mailed
photos.
The right-click (on the file) followed by the e-mail or send to options will
take your there.
As to your other question, file compression often does not show up until you
enlarge the image ... or try to print it.
curious guy
>It is a reasonable question although you might want to look in the
>archives it has been done to death several times before.
Do you know what subjects I should search for? Should I use Google
groups search or something else?
>The short answer is that a JPEG encoded image uses lossy compression
>designed so as not to be obvious to the human viewer on photographic
>source material. And it is very good at what it does. PhotoImpacts
>"Best" setting doesn't sound very good if the original image was also a
>JPEG file but would be reasonable if it were a BMP.
I converted the original 467,007 byte JPEG file to a BMP with ACDSee
2.42. It was 2,491,254 bytes as a BMP. I then used ACDSee to convert
it to a JPEG at a quality of 90%. It was 132,738 bytes after the
conversion back to JPEG.
>If you put the two files up somewhere it would be easy enough to see
>what the actual quantisation matrices used by PhotoImpact and the
>original source file are. The IJG scale of 0..100 is commonly used
Can you recommend a site for the pictures?
Bob Larter
> I wanted to email someone a picture that was 467,007 bytes. (The
> picture was from a newsgroup. I did not take it.) I thought that was
> too big so I experimented with resizing it with PhotoImpact 4.2. I
> discovered that if I just loaded it and re-saved it without reducing
> its dimensions, it went from 467,007 bytes to 37,341 bytes. I have
> PhotoImpact set to use "Best" generation quality.
>
> When I look at the pictures, I can not see any difference until I zoom
> way in. Why is there such a huge difference in file sizes
> (12.50654776 times)?
It's all about the quality setting. The file size difference between 80%
quality & 100% quality is quite dramatic.
> If there is another newsgroup where this question would be more
> appropriate, please let me know.
No, this is an appropriate group for your question.
--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
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