Image Size vs JPEG Quality

[rboucher]

Hello. I would like to know the best (retain highest quality) way to reduce JPEG file size. I've typically used one or both of two methods (JPEG quality and downsampling) somewhat indiscriminately in the past but would like to make a more informed consistent choice when reducing for email etc.

e.g. a 3872x2592 image saved as a JPEG at the highest quality (12) produces a 6.9MB file. I can reduce it to 1.1MB (no magic to this number; I just picked it as a reasonably smaller file) by either saving 100% of pixels at an 8-Quality (which of course compromises quality through compression) or by using Elements' Image/Resize/Image Size to resample the image and saving at 12-Quality JPEG (for this example, 1.1MB was achived with Bicubic resampling at 41% of the width and height pixel dimensions to 1588x1063), which also compromises quality by explicitly removing pixels.

I'm not hung up on using only one or the other of these; if there is some consensus around some other best way I'm happy to hear it.

Thanks in advance.
Robert.

[TonyW]

Hi there and welcome to the forum. Interesting question and I don't have a definitive answer as in part it all depends on what you're doing it for. You mention emailing and usually that's for someone who'll be viewing it on the screen and in that case I'd use the combination. No sense in sending more pixels than will fit on the viewers screen because their viewing software will resample it anyway to fit the screen. And no sense in sending a jpeg compressed at less than about a 5 because they won't see a difference. If it's emailing a jpeg for printing then I'd try and keep the pixels and compress as much as needed.

A good way to see the effects of compression is to use Save for Web (and ignore the memory message with big files - it still works). Zoom in close (200% or more) and watch carefully as you move the compression slider. You start seeing compression artifacts creeping in at about 50 but they aren't really noticeable until about 30 and viewing at 100% they are hardly noticeable at all.

Tony

[Chuck S.]

My approach to e-mail size is to use Save for Web as Tony suggested, first changing the pixel dimensions to 1024 by 768 (for a point-and-shoot photo) or 1024 by 683 (for a DSLR photo). Then I adjust the quality slider and watch the file size decrease. I usually don't e-mail anything larger than 300 kb. This limit is usually achieved at quality in excess of 50 on the 100 scale in Save for Web.