losing photo "quality"

[macroMe]

I am completely lost. I am trying to sort out my photoshop dilemma and this seems my next step.
I have CS3 and am using bridge to move my photos into PS. For an example the image details are 4288x2848, 300dpi, 3.4mb.I open bridge and double click the photo that I want to edit and PS then opens with the photo open on the page. I get a prompt to change the embedded colour profile to my current work space which I do. ("convert documents colours to the working space" ProPhotoRGB).

In PS I have done a basic crop and then press save. I choose to save it to my desktop as a "copy" with the "colour profile embedded". The next prompt is "jpeg options" which is marked as Quality "10" - "maximum" and "baseline optimized". I press "ok" to save.
When I check the photo on my desktop for the details it is now 2400x3000 and 893kb.

My question is this... Why is the resulting photo so much smaller (3.4mb down to 893kb)?
What vital piece of this process am I missing? I have a PS book and can't find the answer to this dilemma.
Thanks.

[fletch]

You cropped it! If you crop you remove pixels and they photo will be smaller, both physcially in pixel dimensions and in terms of file size. (Less info smaller size)

[macroMe]

Doh!!
I feel like a right twit - hadn't even considered that as a reasoning. Sometimes it's the simplest things.
Thanks.

[vsa]

And the maximum quality in Photoshop is 12. But yea mostly the cropping does that.

[fletch]

I read somewhere that using JPEG quality level 12 in photoshop is a bad idea as it misses out some step in the compression process. May be wrong but sounds plausible. I find 10 or 11 has absolutley no detrimental effect on my photos what so ever as long as I only compress once.

[smc1377]

Originally Posted by fletch

I read somewhere that using JPEG quality level 12 in photoshop is a bad idea as it misses out some step in the compression process. May be wrong but sounds plausible. I find 10 or 11 has absolutley no detrimental effect on my photos what so ever as long as I only compress once.

You know, I use 10 or 11 as well. Not for the same reason though.

When I open up a jpeg straight from the camera and do a save as function, setting 12 somehow increases the size (in megabytes) of the image. How is that possible unless Photoshop is adding some sort of data that wasn't there in the first place? Setting 11 tends to be about the same file size as the original so that's what I use most.

[wulf]

Going up to 12 probably treats each pixel as unique whereas the image you were working on from the camera had applied some minimal compression which slightly reduced the file size while having no visible effect.

Wulf

[wulf]

As a quick follow on thought, you can highlight the differences by comparing the original file with a copy saved with a given amount of JPEG compression. Now layer them and set the top layer to grain extract mode (with the Gimp - PS probably has something similar). This highlights the differences between the layers - identical layers will be a flat grey.

You could try saving one copy as Lvl 12 on Photoshop, which I would expect to be identical to the source image; compare that with one saved at Lvl 10. In the latter case, there will probably be faint patterns visible but not enough to detract from the image unless you repeatedly resave the image, reapplying the compression each time.

Wulf

[macroMe]

I read all that doing the "uh huh, uh huh, yep, ok" then realised I had no idea what it actually meant.
What exactly does the level 10, level 11, level 12 saving process do/mean?

I tried saving it as level 12 and I got a larger size (megabites) than when I saved at 10 but still reduced from the original, a factor I attribute to the cropping. I can see the effect of the different save levels but I don't understand why there is more data with a 12 save than say a 10 save.
If I have cropped the photo and I then later wish to print it at a large size (say 12"x18") do I not want there to be as much data as possible - or am I confusing DPI with the data size?

Sorry guys but I have really confused myself with this one .

All enlightenment welcome.
Thanks.

[fletch]

Originally Posted by macroMe

I read all that doing the "uh huh, uh huh, yep, ok" then realised I had no idea what it actually meant.
What exactly does the level 10, level 11, level 12 saving process do/mean?

I tried saving it as level 12 and I got a larger size (megabites) than when I saved at 10 but still reduced from the original, a factor I attribute to the cropping. I can see the effect of the different save levels but I don't understand why there is more data with a 12 save than say a 10 save.
If I have cropped the photo and I then later wish to print it at a large size (say 12"x18") do I not want there to be as much data as possible - or am I confusing DPI with the data size?

Sorry guys but I have really confused myself with this one .

All enlightenment welcome.
Thanks.

The level you set adjusts the amount of JPEG compression. this is a very complex subject that I don't really understand. I would just use level 10 or 11 and leave it at that. It's what I do.

DPI and Data size aren't linked at all. The file size is controlled by the number of pixels and the amount of compression applied.