A question for the digital experts here

[adrian tyler]

hi chris, my files often go over 800 mb and i just write as a tiff onto a dvd.

sending you your prints off to you today, off to paris with dely mañana... all the best

ade

[paulr]

In any event, what I REALLY want to know is how the JPEG algorithm works. I am a sophisticated digital printer working in a color-managed environment, and I can see no difference either in print or on my screen at 1600%, between an 8-bit TIFF and that same file saved as a 12-quality JPEG. But the filesize difference is huge, which is strange and interesting.

Here's a pretty in-depth explanation. take a deep breath ...
http://en.wikipedia.org/wiki/JPEG

FWIW, i can't see the difference between 12-quality jpegs and uncompressed files either (on screen or in print). though i haven't done any serious comparisons. To be on the safe side, I don't use any jpeg compression on the digital files I print from. I print much smaller than you do, so I haven't felt much pressure to experimentt with it yet.

[adrian tyler]

yeah, i wouldn't risk it, after all the work one put's into getting it right, just burn a dvd...

[Jack Flesher]

I think it is you who is not quite getting this.

Then I suggest you use jpeg and not listen to me or anybody else...

As for how jpeg compresses, it is the topic of a lengthy white paper. The simple answer is the human visual system perceives brightness differences much more readily than absolute color differences and jpeg compression takes advantage of this fact. It selects a range of 'similar' colors of close to equal luminance and adjusts them all to one color of the same luminance -- and by so doing allows the total image data from an original file to be significantly truncated or compressed. How 'similar' the colors are that it initially selects and how variable the luminance is determined by the compression factor chosen. How any given human imaging system reacts to them is variable...

Best of luck,

[chris jordan]

Well I just read the in-depth article about JPEG's in Wikipedia, along with some of the links (which are even better), and learned a few things. First, as a few people here have suggested, the 12-quality JPEG in Photoshop is lossy, despite not appearing to be so. But the articles made a distinction between "true" lossless compression and "virtually indistinguishable" compression. 12-quality JPEGs are virtually indistinguishable, but not truly lossless. There is a truly lossless JPEG generator out there, but it is hard to find and the file sizes end up being almost the same as a TIFF.

My 30-saves test confirms the above: there is a visible change between the first and 30th versions when viewed at 800%, but it is not a change that appears to my eye to matter. Some individual pixels do shift slightly, but the overall colors stay visibly the same and all image details are preserved.

For printing purposes (and I just tested this too), there is no perceptible difference in prints made from Photoshop files versus 12-quality JPEGs saved from the same original files.

For emergency backup purposes, I think I'm going to do a "save-as" of all of my files to 12-quality JPEGs. That way I can save my whole image library on a single hard drive, which for me is a huge administrative gain. But I'll still print from the original Photoshop files, just because...

Thanks for all your thoughts on this stuff guys, especially Jack.

cheers,

~cj

[QT Luong]

For emergency backup purposes, I think I'm going to do a "save-as" of all of my files to 12-quality JPEGs. That way I can save my whole image library on a single hard drive, which for me is a huge administrative gain.

That's what I do to have access to all my files while travelling.

[Jack Flesher]

At the last sale, I bought Seagate 500G SATA drives for $189 from my local Fry's computer store. That's 38 cents per Gig now and they're only getting cheaper... From my perspective, at that cost, I can afford dual redundant back-up (Raid 1-1, with the second mirrored set stored offsite) onto said drives and never have to worry about catastophic file loss.

And I realize others views will vary.

Cheers,

[Greg Miller]

Greg,

Most people properly leave a JPG untagged when using the file for the internet, so it is my practice to convert to sRGB and leave the JPG untagged for that purpose. I expect you do something similar.


---Michael

Yep - I do the same.