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JPG Quality Level 12 Compression Loss?

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Thread started 02 Jul 2007 (Monday) 19:33   
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sapearl
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I realize that saving a RAW, PSDF, Tiff, DNG, etc. file as a jpg will result in loss of information through the compression nature of the jpg process. But will there always be significant (noticable?) "loss" even if you save at Quality level 12 in PhotoShop?

I am assuming that at Quality Level 12 in CS2 it will be minimal since in CS2 it shows "12" as MAXIMIUM qualityl. My starting point is always a RAW file, and I can see the file size shrink when I do a before and after comparison. So obviously there is loss going on.. I just wonder if it's only something that a mathematician will notice as compared to a person looking at an 11x14 print. - Stu

Post #1, Jul 02, 2007 19:33:51


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Damo77
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There is some detail loss at Jpeg Level 12, but it's very negligible, and almost impossible to see with the naked eye.
As I've said in other threads, Jpeg is absolutely fine for a final save - eg to send to a client, or a lab, or whatever. But for your working files - the ones that you'll be opening and re-saving several times - you're better off choosing a non-compressed format.

Post #2, Jul 02, 2007 19:55:55


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sapearl
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I completely agree with you Damo - my starting point is always the RAW file, and after making general tweaks there I'll save it as a PSD for final adjustments when I'm doing my attempts at "fine art" prints ;) . But for most of my wedding JPGs that end up as 4x6, 5x7, 8x10 prints, (starting as a 12-14MB RAW) I'll save at Quality level 10 prior to sending to the lab. Thanks Damo.

Damo77 wrote in post #3476863external link
..... But for your working files - the ones that you'll be opening and re-saving several times - you're better off choosing a non-compressed format.

Post #3, Jul 02, 2007 20:18:49


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mabas9395
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Not to take this off on a tangent, but I've noticed that when I start with a jpg (my first digital only shot jpg's) and save it as a jpg with quality level 12, the resulting file is larger than what I started with. Why would the file be larger? Is this helping me or hurting me (any more than a re-save of a jpg would)?

Post #4, Jul 03, 2007 10:55:34 as a reply to sapearl's post 14 hours earlier.


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kevin_c
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mabas9395 wrote in post #3479880external link
Not to take this off on a tangent, but I've noticed that when I start with a jpg (my first digital only shot jpg's) and save it as a jpg with quality level 12, the resulting file is larger than what I started with. Why would the file be larger? Is this helping me or hurting me (any more than a re-save of a jpg would)?

Because you are editing the uncompressed file, which is actually quite large, when you go to save it the program will re-compress the file to the new compression level, which could be higher or lower.

Post #5, Jul 03, 2007 14:32:43


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René ­ Damkot
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The 'out of camera' jpg is probabely a higher compression jpg. (Say: comparable to PS jpg quality 8 or so)
If you use 'Save' it gets saved at the same compression level. If you 'Save as' you set the compression level. If you set 12, the rsulting file will be larger, and compressed less. (Have better IQ)

Post #6, Jul 03, 2007 14:50:55


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sapearl
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Which is what I'd been doing on a number of files.... using SAVE AS and selecting Quality level 12.

I guess what I'm still a little confused about is, if it was already compressed DOWN to a file of some smaller size, and then I saved it at 12, and the resultant is larger file, what is being added to it? Are more "filler" one's and zero's being pumped into it to make it larger?:lol: Obviously something I'm missing here.....

René Damkot wrote in post #3480993external link
..........If you use 'Save' it gets saved at the same compression level. If you 'Save as' you set the compression level. If you set 12, the rsulting file will be larger, and compressed less. (Have better IQ)

Post #7, Jul 03, 2007 15:16:12


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René ­ Damkot
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If the jpg is opened, it is 'decompressed'. That's why a jpg out of an 8Mp camera of, say, 2Mb opens to a file that is roughly 23Mb according to Photoshops file info.
If you save it as a tiff, all individual pixels are saved, resulting in a large file (n pixels x 8bpc x 3 channels for an RGB file). If you save a s a jpg, the file is compressed again.

Post #8, Jul 03, 2007 16:32:43


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chauncey
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Sapearl-this is from a neophyte, but why don't you save your "best" as a "smart object"?

Post #9, Jul 03, 2007 17:11:19


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prime80
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OK, I'm confused here...If the initial image is a jpg out of a camera that is saved at PS equivelant of "8", then hasn't the lossy compression already thrown away the information required to make it a "12"? I understand that we can save-as a "12", but that can't return any image info that has already been discarded by the original jpeg compression. Aren't we just saving an "8" as a "12" and doing the least degradation of the "8" image on the resave? We're still stuck with the original "8" quality, aren't we?

Post #10, Jul 03, 2007 17:45:24


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Damo77
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Bingo! Yes, you can never get that detail back. That is why Raw photography is preferable in some cases.

In my experience, the highest quality jpeg setting in my wife's 5D is about equivalent to Photoshop's quality level 10. Perhaps somebody out there can confirm/contradict this.

Re: file sizes after compression - you have to remember that every photo is different. Large areas of low detail will compress a lot. Eg a photo of sky with one bird in it will compress a long way, whereas a complex photo of trees won't compress as far.

Likewise, if you apply sharpening to an image, the compressed size will be larger. This is why a lot of people recommend not too much sharpening on photos that you post on web sites - the bigger the file, the slower the viewing speed.

Post #11, Jul 03, 2007 18:22:51


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sapearl
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Ah, Rene - I know I can always count on you for a simple but thorough and comprehensible explanation.:D In addition to your impressive photographic talents, you must also write for a living.;)

René Damkot wrote in post #3481450external link
If the jpg is opened, it is 'decompressed'. That's why a jpg out of an 8Mp camera of, say, 2Mb opens to a file that is roughly 23Mb according to Photoshops file info.
If you save it as a tiff, all individual pixels are saved, resulting in a large file (n pixels x 8bpc x 3 channels for an RGB file). If you save a s a jpg, the file is compressed again.

Post #12, Jul 03, 2007 18:40:07


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sapearl
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Hello there Chauncey - well, I must confess my ignorance here as I don't know what a smart object is at it relates to Photoshop. BTW, is CRNA some flavor of RN? My wife is one of those :D .

chauncey wrote in post #3481671external link
Sapearl-this is from a neophyte, but why don't you save your "best" as a "smart object"?

Post #13, Jul 03, 2007 18:43:34


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tim
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A smart object looks at the original source and works with that, not the current view. Say you take a 3000x2000 pixel image into photoshop, resize it to 3x2 pixels, then back to 3000x2000 pixels, you'll have almost a solid color with NO detail. The image will be 6 HUGE pixels. If you made it a smart object to start with, then do the same thing, when you take it back to 3000x2000 it will look fine. The easy way to explain it is to say photoshop remembers the original image, and checks back with it as needed. The disadvantage is it's a little slower and your files are larger. It'd be a handy technique for someone who designs wedding albums in PS and does lots of resizing.

Post #14, Jul 04, 2007 04:26:31


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tim
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Also, to make a layer a smart object you right click, or I think there's a menu command "paste as smart object" somewhere.

Post #15, Jul 04, 2007 04:27:14


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