Lost Bytes with roatation in Windows Photo and Fax viewer

[Scott092707]

I wished to rotate .jpg photos so that in a slideshow people don't have to
crane their necks constantly to view the pictures. I have found that after
rotation, the picture size is reduced by 44 bytes (one example;don't know if
it's consistant with each picture), and that the bytes don't come back if
rotated back to the original orientation. Further 90-degree roation adds or
removes an additional 5 bytes, but no more. Naturally, I want full
resolution when I go to print the photos, in the original orientation, so I
want to know:
1) what causes the loss of the initial 44 bytes;
2) what causes the additional loss/gain of 5 bytes;
and
3) whether those lost 44/5 bytes are unnecessary and I shouldn't worry about
it,
or I should never rotate pictures, or what?

In case the info is necessary, I am using Windows XP-Pro Vers. 2002 SP 2

Actual data: orig. photo 1.823 KB, rotated photo 1.779/1.774 KB

-Scott

 

[Yves Alarie]

At 1,823 KB file size you have more bytes than you need a loss of 44 bytes
is nothing.
But really, this is not the way it works for printing, the file size
obviously reflects how large a print you can get but the real numbers you
want are the pixel dimensions.
The "rule of thumb" is if you want a great print you divide the pixel
dimensions by 300.
At 1,823 KB file, I am guessing that the pixel dimensions will be something
like 2048 x 1360. So you can get a print of 2048/300 = 6.8 inches by
1360/300 = 4.5 inches. Now you can reduce 300 to around 200 and still get a
very good print.
Take a look at you file size in a particular photo folder. For a particular
camera, the pixel dimensions will always be the same for all photos in the
folder, but the file size will vary according to how much information you
have in each image. The extremes would be taking a photo of a plain black
and white object (photo # 1) and a landscape photo with lots of details and
colors (photo #2). If you look at the size of the files, photo #1 would be
very small while photo #2 would be very large. However, if you look at the
pixel dimensions, they would be exactly the same for both photos.
To find the pixel dimensions of your photos, just place your mouse pointer
over a thumbnail or file name. A box will open and Dimensions will be the
first item listed. Another way to get it is to use the Details view for your
folder. Open the folder in Details view and right click on the Name column
header, a list will open, click on More and then check Dimension to add this
info as a column.
The bottom line is: pixel dimensions control print quality, not file size.

 

[Scott092707]

Thank you, Yves.
You have relieved me as to potential loss of quality in the print.
And taught me an interesting rule-of-thumb.

I am still curious, tho' :
a)why the pixels initially disappear,
b)why they do not subsequently do so,
and c) why the size bounces back and forth by 5 bytes
depending on the orientation.

(I'm ALWAYS curious...)

 

[Yves Alarie]

If you open a photo file to view it and then close it without making any
change, the size of the file will not change.

However, if you make a change such as rotation, and you close the file the
compression software on your computer is more efficient than the compression
software in your camera and when "saving" the change the file size will be
slightly smaller but there will be no loss in quality (the term used is
lossless rotation).

If you make other changes such as adding text, removing red eye, etc. and
you save the file there will "some" loss in quality since the file will need
to be compressed again to save it. However this loss in quality is not
something you can detect. So no need to worry about it. It is best to make
all your changes and then "save as" and give another name so you still keep
the original file and best to do all your editing so you will do only one
"save as", but don't worry about two or three editing of the same file.

You can read a lot more details about JPG and how this compression work at
this site:

http://en.wikipedia.org/wiki/JPEG#Color_space_transformation