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jpeg compression (save, save as) question
Old 05-01-2004, 09:12 PM   #1 (permalink)
val
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I'm sure this has been addressed before but...

I want to work on jpeg files without losing original image quality or file information. Assuming I'm using Photoshop, image quality "12" for best quality, not changing size or resolution, and "save as" to save a file to a different folder, or under a different name, do I lose image quality or file information?

The files look good and the file size remains the same or larger -- I just want to be sure. Thanks,

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Re: jpeg compression (save, save as) question
Old 05-01-2004, 09:42 PM   #2 (permalink)
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Well logic would have it that if the filesize is changing at all, then the file is changing as well. Thus it probably is losing small quantities of detail somewhere. Ideally you should save as a photoshop (.psd) file when resaving. If you simply want to move to another directory then use explorer.
 
 
Re: jpeg compression (save, save as) question
Old 05-01-2004, 10:00 PM   #3 (permalink)
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Caveat 1 - I don't use Photoshop - although the JPEG algorithms are not unique to Adobe, so the following SHOULD apply

Caveat 2 - I'm certainly no graphics programming guru

The JPEG algorithms will always seek to "compress" your file, although the "compression" may actually be an expansion and increase the file size.

In short, virtually every "save" or "save as" in the JPEG format will cause SOME alteration of the image. Enough repeated saves, even at low compression levels (ie.e., high quality settings) will cause noticable changes.

Bottom line - don't store "master copies" as JPEGs. Work in TIFF or bitmap formats (I don't know about GIFs) and only save the final version as a JPEG.

Disclaimer: see my tag line.
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you just ruined my day
Old 05-02-2004, 12:07 AM   #4 (permalink)
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great, now I get to redo a days worth of edits the right (tiff) way. Thanks a lot... [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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Re: jpeg compression (save, save as) question
Old 05-03-2004, 09:43 PM   #5 (permalink)
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NEVER USE GIF!!! Unless you are preparing a shot specifically for the web you should never use the GIF format for anything. And even if you are preparing a shot for the web, be careful of when you use it. The biggest problem with the .gif format is that it only supports 256 colors total. The .jpg format on the other hand supports 256 shades each of red, green and blue giving you over 16 million colors.

Other than that, your advice is good!

Cheers!
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Re: jpeg compression (save, save as) question
Old 05-03-2004, 10:21 PM   #6 (permalink)
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Hey Val. This is a very tricky area to be sure. Tricky enough that most of the advice you get will be the same, but some details might be different in the workflow of the individuals. Here's my take.

-= If you save as a .jpg, there WILL be degradation. =-

However, as many people will tell you, a level 12 for compression has so little degradation that it's almost not worrying about. Almost. You have to decide what you want to be doing with you images. If they're just samps of the family you might want a print of at some point, .jpg will be just fine. If they're files you want to send to a magazine for publication, it should be good for most images. The magazine will let you know what they prefer. If you have any intent on making prints for sale or stock agencies then you'll want to use .jpg ONLY to provide web ready images for your web site.

Jpeg is fine if you work like this;

- save images from camera to computer
- load images into photoshop
- edit images
- finished editing
- save images

But if you work like this;

- load images into photoshop
- edit images
- NOT finsihed editing images
- saving partly edited images to work on later

You will want to use;

A) .tif if you haven't used layers
B) .psd if you have used layers.

Now for more trickery!

Even though .tif will save layers, I don't like doing that because looking at a file 6 months from now, I need to load it up to see if it used layers or not. By saving as a .psd it gives me a quick visual clue that layers were probably saved here!

Also, if you have a selection made when you go to save, the .tif file will save with what's called an Alpha Channel. It's basically a mask that let's one layer dictate what get's seen from the layers on either side of it (if that makes sense). This forces photoshop to try and save a copy of the file and can be quite frustrating at times.

One drawback to these two options that .jpg laughs at is the files sizes are BIG! Especially .psd's when using layers. Working in 16 bit will DOUBLE the file sizes. The full size, 16 bit, 20 x 30 inch, 300 ppi .tif file used for this article I wrote on upsizing is 316 Megs!!!

I prefer to play safe and never use .jpg except for web images. I only use .gif for thumbnails providing it can offer me the Exact Colors option when saving or when it offers me smaller file sizes than .jpg.

Cheers!

PS - The reason your .jpgs sometimes end up larger than the original is that the files coming straight out of the camera might be in a compressed form similar to Photoshop's 9, 10 or 11 levels. When you load the file into Photoshop it gets decompressed to it's full size which could be 3 or 4 megs depending on the image. Photoshop will then re-package everything when saving it again but at a level 12 it can't compress as much so the final file ends up bigger!
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Re: jpeg compression (save, save as) question
Old 05-26-2004, 05:46 PM   #7 (permalink)
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Hi val, you may have already heard this but I am saying it again. The best thing to do is to upload your images into your computer and then immediately make a CD of all the images. After that create a 'Working Images' (or some other name that is meaningful to you) and once you have worked with an image(s) then do a Save As and save the image as a .Tiff file. If you have ReSized the image to print specify 300 DPI in the resizing window. I have learned this through trial and error and it you stick with this system it will never fail you. Good Luck.
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