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Old Mar 26, 2004, 12:03 PM   #1
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Default Megapixels and file sizes?

I understand a Jpeg is a compressed file. The amount of compression used can vary by camera and by how you set it up. I have two 3 MP cameras. One makes a 700 to 800 KB file, the other averages around 1.2 MB per file. Both are set to their best settings. That means the 2nd camera has less aggressive Jpeg compression - Right? Does that mean the 2nd camera's pictures should be better - other things being equal?

How big would a file from a 3 MP camera be if it were not compressed? Does 9 MB or 3 bites per pixel sound about right? I'm guessing this because one of my cameras will save an image in TIFF format and the manual says one picture is about 9 MB.

I saw an ad for a 6 MP ultra compact camera that fits 13 of its best quality images into 16 MB of memory. Doesn't that sound like a whole lot of compression going on?

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Old Mar 26, 2004, 12:11 PM   #2
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that doesn't sound right.....13 highest quality pictures on a 16mb!
my 4Mp G2 could fit maybe 11-12 large superfines on it....and that is 4 MP......(the files are around 1.2 mb each i think....)
but it could probably fit 1-2 RAWs on it lol not many pictures....
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Old Mar 26, 2004, 12:16 PM   #3
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Default Re: Megapixels and file sizes?

Quote:
Originally Posted by Puck M
I understand a Jpeg is a compressed file. The amount of compression used can vary by camera and by how you set it up. I have two 3 MP cameras. One makes a 700 to 800 KB file, the other averages around 1.2 MB per file. Both are set to their best settings. That means the 2nd camera has less aggressive Jpeg compression - Right? Does that mean the 2nd camera's pictures should be better - other things being equal?
Yes. Take a close look at the images - JPEG compression seems to compress 8x8 pixel blocks so that should give you an idea of what to look for. If you don't see anything, try using your photo editor to compress a test image a whole bunch more - once you have seen the JPEG artifacts clearly, they become easier to see with less compression.

Quote:
Originally Posted by Puck M
How big would a file from a 3 MP camera be if it were not compressed? Does 9 MB or 3 bites per pixel sound about right? I'm guessing this because one of my cameras will save an image in TIFF format and the manual says one picture is about 9 MB.
Yes.
Quote:
Originally Posted by Puck M
I saw an ad for a 6 MP ultra compact camera that fits 13 of its best quality images into 16 MB of memory. Doesn't that sound like a whole lot of compression going on?
Sounds like something I'd rather not have.
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Old Mar 26, 2004, 1:29 PM   #4
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Default Re: Megapixels and file sizes?

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Originally Posted by Puck M
How big would a file from a 3 MP camera be if it were not compressed?
Here are some file size results I did a couple of years ago (Excel file), starting with a scanned 6x4 print.

http://mysite.freeserve.com/agtmfl01/Jpgsizes.xls

If you have an image editor that saves as jpg with variable compression levels, such as the excellent, freeware Irfanview, you can do these experiments for yourself by saving with different compression levels and inspecting the file sizes and image quality at various compression levels.

If your camera will save as as Tiff, take the same shot on a tripod as 'Tiff' and as 'fine', normal' and 'economy' jpegs, and inspect the images at high magnification (big enough to see the individual pixels) on screen.

I have found it difficult to tell the difference, pixel for pixel, between the Tiff and 'normal' jpeg compression on either of the digicams I have owned.

When in doubt, why not do the experiments yourself?

BTW, a full size, uncompressed image from a 35mm neg scanned at 2700dpi is about 27MB, so thank your lucky stars for jpeg compression. I generally find a 1.5-2.0MB jpeg is OK for most purposes.
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Old Mar 26, 2004, 4:34 PM   #5
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Default Re: Megapixels and file sizes?

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If you have an image editor that saves as jpg with variable compression levels, such as the excellent, freeware Irfanview, you can do these experiments for yourself by saving with different compression levels and inspecting the file sizes and image quality at various compression levels.
I've got that program but have not played with it enough to be able to save at variable compression levels. I can resize a file for e-mailing with it but that's about all I use it for. I am not well versed in image manipulation programs.
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Old Mar 26, 2004, 8:42 PM   #6
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The latest version of Irfanview opens a box with various options including quality or compression settings when you ďsave asĒ JPG. Older versions had something you selected in the save as dialog box but I donít remember exactly what you had to select to open the dialog box for compression settings. I usually do that in Photoshop, but Irfanview does a nice job.

You tend to see more difference in files from cameras with a lower quality compression when you work with them in an editor. Sharpening tends to enhance the compression artifacts. What you really donít want to do is save them again as a JPG. You start picking up visible artifacts quickly when you resave a lower quality JPG again as JPG.

Some cameras have SHQ JPG which is very high quality. I canít see the difference between SHQ JPG and TIFF. SHQ on a 3Mb camera would be somewhere around 2Mb average per image. Someone claimed that you could save a SHQ file many times at best quality JPG and not pick up artifacts. I tried it and after 20 saves I couldnít see any artifacts, so I guess they were right.

I can see a quality difference in a large print from an image converted to TIFF from raw compared to a best quality JPG from the same camera. Not a lot though.
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Old Mar 30, 2004, 2:56 PM   #7
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Default Re: Megapixels and file sizes?

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One makes a 700 to 800 KB file, the other averages around 1.2 MB per file. Both are set to their best settings. That means the 2nd camera has less aggressive Jpeg compression - Right? Does that mean the 2nd camera's pictures should be better - other things being equal?
ALL things being equal, then yes. However, if say had two identical images but one had low levels of noise, the other had high levels of noise. The file size would be much different. Let's say for example you had a file with virtually no noise in it that saved as 65 kilobytes at a given JPEG compression setting. If you had, for example, added 5% chromatic noise, in guassian format, the same image would save as approx. 110kilobytes at the same JPEG compression rate.

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