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Old 07-06-2004, 15:46   #1
sharpshooter
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Digital Camera question about compression

I have a Canon PowerShot S30 3.2MP camera, it does a terrific job. I know about resolution. What about compression? I can set the images to L, M, or S compression. With L I get 77 images, with S I get 275 images on the card.

I can't tell the difference in image quality, they're identical as far as I can tell. And the file sizes are the same. So what's the advantage of using the L or S compression settings?
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Old 07-06-2004, 15:50   #2
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It's all about the number of pixels. I am guessing that you are talking about a 64mb card. The larger the file, the less the compression. On my Fuji FinePix 3800 (see new model S3000), a 3mp "fine" image is about 1.2mb on the card, whereas a 3mp "normal" is only about 700kb. The larger the initial file, the larger you can print the image. My 3mp fine images make excellent 8"x10"s and occasionally larger. Try printing 5"x7" or lager versions of each file (take the same picture in each mode) and compare to find out what you really need.
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Old 07-06-2004, 16:46   #3
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Your camera might be changing the resolution, as well as increasing the compression.

Many cameras have modes such as Fine, and Standard. Standard having more compression, which means a smaller file and a slightly lowered picture quality.
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Old 07-06-2004, 23:51   #4
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I haven't tried printing the different compressed images yet, but when I zoom on Paint Shop Pro they all look identical, compressed or not. I don't understand how the printed image would look different than the one on the screen... ?
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Old 07-07-2004, 08:33   #5
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There is actually a quality difference between the compression levels, but it's not always easily seen with standard photos. If the photo had areas of all one color, such as a large white board, or blue sky, the compression would be more visible. The compression works by taking some of the pixels, and making them span several nearby pixels that are almost the same color. (Simple explanation)

Your computer monitor has a resolution of 72 dpi (dots per inch) whereas a printer can range from 300 to 2400 dpi. When you print the picture, the "spanned" pixels are spanned again several times more, make larger areas of a single color, therefore making printed images a lower quality do to large "blocky" areas of a single color (or almost single color).

See the area in the attached picture:

Tech Talk

See Also:
http://www.dpreview.com/learn/?/key=JPEG
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Last edited by Toyman; 07-07-2004 at 08:35..
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