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help - compression problem

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Old 02-11-2002, 12:24 AM
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Sam Sam is offline
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I really hope someone can explain that because it's something that's bugged me for ages. When you're trying to get an image down to a certain size, the only way you can check its real size is by looking it up on Windows Explorer!

In which case it doesn't matter if it's a JPEG, TIFF, PSD or whatever. PhotoShop seems to do that with all file formats.
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Old 02-11-2002, 08:17 AM
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thomasgeorge thomasgeorge is offline
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No data is lost, just compressed by the software, a crude example would be folding a handkerchief to store it in a drawer. The folded one takes up less space but is all there and when opened, assumes its original dimensions. Dont be alarmed by this behavior of the program...its purpose is to allow more efficent storage of data thus not cloging up your hard drive with stuff so quickly. In answer to you question, I am running PS 6 as well as a few others. Good luck, Tom
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Old 02-11-2002, 10:17 AM
George George is offline
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Location: Homestead, Florida
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I retested today, using Windows XP and Photoshop 6.01, and a test image of 2.3 Megs which I had downloaded to my hard drive directly out of my digital camera.

When I went to open the file in Photoshop I left clicked on the file, and it opened a small image at the bottom of the open file window and listed the file size as 2.3 Megs.

When I opened it in photoshop, the file size was 14 megs.

I made corrections with levels and then saved. I then closed the file, went to file open, left clicked on the file and it again listed the file size at the bottom of the file open window as 2.3 Megs. This is the normal compression used by Photoshop to save a jpg file. If you want more or less compression then you would select the save as feature and the use the jpg box to make the selection for compression.

If you open a jpg file that is 2 Megs when you left click it in the file open window, then in Photoshop the file should be 12 - 14 megs. If this file is then saved it would again be a 2 Meg file.

If the file is 2 Megs when opened in Photoshop, then when saved it will be compressed and be much smaller.

Look at your file by left clicking on it in the file open window and you will see the actual compressed size of the file. Then open it in Photoshop, make corrections, see what size it is when opened in Photoshop. Save and then recheck by closing the file, then going to file open, left click on the image file and see the file size at the bottom of the window, it should be very close to the same size you viewed earlier before opening in Photoshop.
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Old 02-11-2002, 10:25 AM
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cendres cendres is offline
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I think I have it figured out now.

After experimenting a bit this morning utilizing DJ's post, I opened a file and did a levels adjustment. I then did a save as and then checked "a copy". I then selected the main save button and a window popped up. I was able to access the compression settings, It was set at a mid compression setting. I adjusted it to high compression (12) and saved the file. I then went into windows explorer and looked at the file size. It actually grew the file size from the original 2.1 mg to 4.5 mg.

Just to make sure, I then took another picture, did a levels adjustment and this time rather than doing a "save as" I only did file- save. Went back to explorer, checked the file size of this new image, and yes indeed that grew as well.

What I think this tells me is this, like paint shop pro, photoshop uses the last compression setting that was previously made. You may ask why did the file size actually grow? The reason I think, is that my camera comes up with its own internal compression ratio based on the internal setting that was selected (ie fine or good). The cameras internal setting compresses the image to roughly 2 mg at the image size I selected. If I shot at TIFF right out of the camera it would be almost 15 mgs.

I think photoshops least compressed setting actually is a little less compression than my camera does, therfor, that is why the file setting increased. It kind of makes sense now after I went into Paint Shop Pro because the only area you can actually change the compression setting is when you do a file -save as. Again that works the same, whatever you last set it at, that is the level for all new saves until you go in and change it to something else. Paint Shop Pro uses a % figure to represent compression and the default setting is 15% (The lower the %, the lower the compression).

One thing I am curious about is Georges post, which said he got the exact same file size (2.3 mg) after making a levels adjustment. George if you could do this, I would be real curious as to what your settings say. Open any image, do a file -save as. Check "save as copy" then hit save. As soon as you do that, a window will open up that says JPEG options, can you let me know what number it says (1-12). I am assuming you have 6.0.

Thomas, I still think you are correct in stating photoshop uses its own compression scheme to minimize file size while keeping the best image possible, but I do think it definitely increases or decreases the amount of compression based on what it has been set at.

Well sorry again for such a long post, but I wanted to share with you what I found out. I would of course welcome any additional comments or observations. I do appreciate everyones help. - Carl
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Old 02-11-2002, 10:29 AM
George George is offline
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Location: Homestead, Florida
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To see the compressed size of a jpg image, when using Photoshop, go to file open and left click on the image you are interested in. The size of the compressed file, before being opened in Photoshop is shown at the bottom of the window.

I use this to see what size any file I am going to open in Photoshop is, before opening. In most cases since I use tif the size here and when opened in Photoshop is the same.

When I am going to sent a file over the Internet, I use the file save as feature and set the compression. I then check the file using open and left click to determine it's actual size.
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Old 02-11-2002, 10:42 AM
George George is offline
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Location: Homestead, Florida
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When I opened the file it was 2.3 megs, before opening and 14 Megs after opening in Photoshop. I made an adjustment using levels and then file > save. I then rechecked it and it was still 2.3 megs. The file should not change enough for you to see in regards to the compressed size, if all your doing is a levels adjustment.

I never use the file save as jpg, unless I am sending the file over the Internet and need to get it down to a size that can be downloaded in a reasonable amount of time.

I always change all my jpg files to tif files the first time I open them to prevent the small amount of deterioration to the image, which will happens each time a jpg file is opened and closed.
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