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When I took pictures using my digicam, it saves those on higher size (over 1 MB) JPEG files. What I usually do is that I open them on some image editing application such as Paint (windows) or Preview (Mac) and then save it to another file using a different filename.

I end up having the same quality like the original (although I'm aware its not the same quality anymore) file but only to have it on a smaller size version.

I naturally suck in the graphics field and didn't have any inclination to learn the details back then, until now that I'm interested on starting a blog.

Could anyone enlighten me on this?

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My guess would be that computers are just better at compression (due to more resources and time availability) whereas Cameras just compress it a tiny bit to get it done. I'm very curious to see an answer to this question as well! –  Johannes Oct 2 '12 at 15:34
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Should be migrated to photography –  DA01 Oct 2 '12 at 16:12
    
I was basically pondering that one, if I should post it on photography. :) –  Panoy Oct 3 '12 at 2:25
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Just an opinion, but you should never use Paint for anything other than doodling... it will ruin your photographs. At least in older versions of Windows, stopped using it after XP... :) –  Olle Sjögren Oct 9 '12 at 13:55
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2 Answers

up vote 2 down vote accepted

Various possible reasons:

  • you are re-saving the images at a lower resolution. fewer pixels = less data = smaller file size

  • your camera is likely saving JPGs with minimal compression. Your desktop software is using a higher level of JPG compression.

  • Even if your software is using the SAME compression as your camera, each time you re-save a JPG, you lose data due to the lossy compression. So it will still be smaller.

  • your desktop software is stripping out a lot of meta data that the camera attaches to the image.

  • your camera is perhaps NOT saving as JPG, but in RAW format, which is a lot larger than a JPG file.

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+1, but not sure "... using the SAME compression ... each time you re-save a JPG, you lose data due to the lossy compression" is quite true? –  e100 Oct 2 '12 at 16:44
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yes, that's true. JPG is a lossy compression method. If you take a source JPG, open it, save it as a JPG, and repeat, you will start noticing a degradation in the image. –  DA01 Oct 2 '12 at 16:54
    
I seem to notice that too. It's the reason why I posted this question. I get the JPG file in my camera's memory card and then paste a copy on my PC, then save it to a new file, presto, you have a smaller JPG file size with little notable difference on the quality of the image. BTW, my cam is saving it in JPG not in RAW format. It ends in JPG, so it must be JPG right? –  Panoy Oct 8 '12 at 14:46
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The answer is obviously compression.

An typical camera image is 3 long strings of numbers (between 0 and 255), one string for Red, one string for Green, and one string for Blue. The number is the brightness value for an individual pixel. The length of each string is the pixel width of the image mulitplied by the pixel height of the image. A byte is a unit of storage which is capable of storing a number between 0 and 255. So the size of an RGB image is approximately Width x Height x 3 bytes in size.

To store this, one can merely transcribe this (by hand on paper, computer file, etc).

In practice, there are long runs of the same number in each string. So instead of transcribing a long sequence of one hundred 2s (100 bytes), you can write down a single byte as a flag, the number 100, and the number 2 (3 bytes). This is very basic compression. The "flag" is needed for when you want to take the file from storage and re-render the image.

JPEG is a more complicated form of compression. The example I gave above is a lossless form of compression because the transcription process preserves the original pixel numbers. JPEG can give you (generally) much smaller files because it THROWS AWAY pixel numbers. So at higher quality settings, the transcribed file size is larger but the image quality is better. To get a smaller file size (with the same size rendered image), JPEG degrades quality.

For a camera, there is a tug of war between picture quality and the cost of embedding file storage in the device. So the manufacturer wants to have a default setting which allows their marketing team to say "Stores 6 Gabillion images!*" without reviewers saying "Image quality sucks. DO NOT BUY". For the marketing (and Just Want it to Work consumers), JPEG compression is a requirement. For the reviewers, a high quality, low compression, higher file size setting as default is a requirement.

Note that for anything to be rendered on a screen, printer or other device, it is no longer JPEG. Compression and file type, GENERALLY speaking, only exists when it is stored unopened and unviewed. This is often confusing to people, partly because the same units of measurement are used for memory size and mass storage capacity.

TLDNR; your file sizes are larger coming off the camera because when you open them, you are resaving them with a lower quality, higher compression.

When I do camera work for business (still-shot, commercial style photography), I shoot them in Camera RAW format, which is the best possible quality from the camera because it is nearly identical to what the sensors in the camera report (ignoring any engineering {magic and funny business}). The file sizes are larger.

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