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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.


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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