More DPI/ PPI And file size /quality Confusion

[jprippin]

I followed the thread that Mark started about 'DPI help ' and by the end I felt like it was soaking in.
Then I made the mistake of thinking about it more and I got lost again. I have a three fold question now.
1.) I have resized an image to match another image (3291x2592) , and set them to the same ppi (240) , saved them at the same 'quality (Max 12) and the file sizes are significantly different i..e. 3.87 and 3.05 MB... why?
2.) Then in the process I went back to one of the files to verify the 'quality' level and without changing any values I saved a copy of of same file and that new file is 4.15 MG... why?
Now I have one more...
3.) So the next logical question is when you select an 'image quality' value, what is different since the size and ppi are the same ?
Just putting this in text this is giving me a pounding headache..lol
Working in Photoshop CS3 V10.0.1

[OldBoy]

The answer to question one is that different images contain different numbers of colour pixels in different block. So a picture of a bird flying across a blue sky can be compressed more than a picture of a bird against a background of trees etc, as it requires less information to record all the blue pixels than if the pixels are different colours.

The second question is a bit strange, as I've repeated your problem with a picture on my computer with the following results. I opened a 5.4mb Jpeg file resized it and saved as a 1.57mb file. Reopened that and save again and it's now 1.58mb. Reopened the 1.58mb file and save again and it shows as 1.58mb file. It might be that the estimate for file size isn't accurate in Windows. The normal wisdom is that the more you open and resave a Jpeg file the more information that is lost. So, if you were to do this twenty times the smaller the file would become, as recompression discards more informatiom. If you did the same with a Raw or Tiff file then it should remain the same as no information is discarded.

The third question regarding the value selected by you in the slider decides the amount of compression applied to the Jpeg. So the lower the value or the more you move the slider to the left the greater the compression applied to the file. The biggest problem with this is you have no control over what information is lost. In normal operations this shouldn't make much differences to the finished image, except it will be smaller in dimensions. Hope that helps.

[jprippin]

OldBoy,
I'm impressed, you spit that out like it was simple... and it all makes perfect sense now.
Thank You very much

[OldBoy]

Originally Posted by jprippin

OldBoy,
I'm impressed, you spit that out like it was simple... and it all makes perfect sense now.
Thank You very much

I could have gone into more detail about how computers store infomatiom in bits, and how the algorithm uses this to store colour infomation, but the details would put you to sleep. If you want to read more then Wikipedia give a good guide.

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

Glad to help.

[Forseti]

Originally Posted by OldBoy

The normal wisdom is that the more you open and resave a Jpeg file the more information that is lost. So, if you were to do this twenty times the smaller the file would become, as recompression discards more informatiom.

I think you should amend your post slightly OldBoy so that it reads.....'the more times you open, edit and resave a Jpeg under the same file name the more information that is lost'. The process of simply opening a jpeg file and viewing it followed by closing it does not result in information being lost due to recompression. If this were the case most of us would have lost most of our images by now through constant viewing.

@jprippin - the process of compression of jpeg files involves the preservation of luminosity data at the expense of colour data.