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I'm trying to reduce file size of a bunch of JPEG files, say 1280x720. Without changing the resolution and the visual quality much, what are the recommended approach?

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JPEG is based on DCT and entropy encoding. If your image is not entropy-optimized yet, there might be a chance to reduce the size.

If you have a Debian or any other Linux system, try using jpegoptim from the same named package, or jpegtran from libjpeg-progs. The man page of jpegtran states that it is lossless:

jpegtran works by rearranging the compressed data (DCT coefficients), without ever fully decoding the image. Therefore, its transformations are lossless: there is no image degradation at all, which would not be true if you used djpeg followed by cjpeg to accomplish the same conversion.

I also found the following two tools, which I've never tried:

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If the curent image size is larger that 1280 x 720 (which is likely if these are camera images) then the best way is to reduce the number of pixels to 1280 x 720.

If they are already at 1280 x 720, then the easiest way is to reduce the JPEG quality. Most imaging software has some way of changing this, and a lower quality will reduce the file size. You'll need to check wehther the quality is still acceptable though.

Another way may be to save the file as a GIF or BMP with compression. This will only help with simple, flat-colour images though, not with photos.

I don't know what software you are using, but I can recommend Irfanview

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Thanks, I was more hoping for a set of tweak settings when saving as new JPEG. – lang2 Aug 14 '12 at 7:27
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GIF will effectively kill the image, as it will be reduced to 256 colors. – Bobby Aug 14 '12 at 8:01
    
@Bobby correct, but if the image is suitable it will really reduce the size. Lang2 Changing the quality setting is just that, a tweak of the settings. Changing the image size requires another action before saving it. – hdhondt Aug 14 '12 at 10:50

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