No Subsampling vs. Subsampling. Image 4 (above) was saved on a setting of 90 (on a scale of 0-100) with Subsampling turned off. Image 5 (below) was also saved on a setting of 90, but with Subsampling turned on. The difference may not be obvious to the naked eye, but Images 6 and 7 will show that the damage is still there.

These images are crop-outs of Images 4 and 5, respectively. Although zooming in on an image always causes some damage, the difference in these images will be clear. Image 6 (above) shows clean-cut lines with only minor zoom blurring where abrupt color changes occur. In addition to the zoom blurring, Image 7 (below) shows clear JPEG artifacts along the edges of each of the white and green color blocks. These artifacts are the result of the JPEG subsampler averaging the color values of the black lines with the white or green color blocks, and coming up with color values that don’t match the original values. As your eyes move away from the black lines, you’ll notice the black lines have less and less influence on the color averaging, and eventually the green and white color blocks return to their normal values. You could print Images 4 and 5, or post them on a website, and you may never see a problem. The risk comes in using them for long-term storage. If, at some point down the road, you wanted to do some additional post-processing on a subsampled JPEG, these artifacts may well become visible. The bottom line here is, for that one original digital image you plan on keeping long term, as a replacement for a film negative, or a replacement for a TIFF, you should always use non-subsampled JPEG to ensure a high-quality original.