I have many images of slides scanned at high res (4800 DPI, maximum pixels 5214x3592). Although I will be saving these as lossless TIFs, I also wish to make JPGs from them that I wish to be just less than 5 MB in file size. Aside from cropping, I know I can achieve such a reduction of JPG file size by a combination of saving to lower quality JPG compression or reducing image size. My question is, what is theoretically or practically better, achieving this mostly by reducing image total pixels or by reducing JPG compression quality. Thank you
"aside from cropping -------- lowering quality compression or reducing image size. By reducing image size are you talking about resolution in ppi? To do this you would have to have resample image checked. Also how much compression an image takes depends somewhat on the image detail.
I would just try it both ways with test images and look at image on screen to see what is sharper. If this is not clear take the image size down to 1 meg with same techniques and see which style wins.
Report back on findings.
Thank you, Curt, empircism is always good. But I was hoping this topic might have been studied systematically, that there might be a theoretical or documented benefit of one approach over the other. Anyone?
Without testing or having read anything technical to "prove" my point,
but from many .jpg files I have produced....
I would say "it depends" on the image....and the final use.
if there are few colors involved, such as a large portion having a consistent blue sky, then compression may be done more easily.
If the image overall has many distinct colors, then fewer number of pixels may make a smaller file size more easily.....
however you may also want to consider the final end use....such as print quality and size needed vs. display only on a monitor screen at 72 to 96 ppi.
Like I said, "it depends" on more info and one answer may not fit all needs.
Thank you Doug. The comments on extensive uniform blue sky vs. marked variation in color seem well taken, I'll keep this method of choosing in mind. My goal is to create a JPG family photo archive of the highest quality images that I can make for future use by non-technical descendants (thus it will supplement the TIF archive that holds the best quality versions of the same images but that may not be usable to novices). As I cannot anticipate exactly how the JPGs will be used, I just want them to be the best possible, while still being of a size that can be uploaded to, say, Costco (5 MB size limit) for making enlargements.
In general, I am often left curious as to how exactly Photoshop carries out its algorithms and how different factors influence the outcome. So often, one read "just try different techniques and see what looks the best". But I am always left wondering, what is the theory behind this and has it been systematically studied and worked out and published. In so many disciplines, such as medicine, the methods of optimization has been evaluated, systematized, and fully described. I have not yet explored what may be found in technical journals, but I'm sure much of this good stuff must be available somewhere. It would be nice to have a "How Things Work" that actually explains what Photoshop is doing under the hood.
There is a quirk in jpeg quality setting you should be aware of. Never use the setting 7. I goes into some weird funk so the qualilty is really worse than setting 6. If you look at the image size when you hit 7 you will see it make a huge dip. If you google this you will see a more technical explaination. http://www.petapixel.com/2011/08/26/a-higher-quality-setting-in-photos hop-sometimes-reduces-jpeg-quality/
Something else to consider...
Noise / grain in the image can greatly increase compressed file size. Maybe there is some image prep you'll be doing on the JPEGS and if so you might consider doIng some noise reduction As part of that.
It seems to me that by downsizing you will be irretrievably eliminating detail throughout the image, while by opting for more compression you will be eliminating detail only in portions of the image, while keeping the overall image. Obviously, there needs to be some balance. You wouldn't want to maintain a 10 gigapixel image at full size with compression set to 1 to keep the file size down. I would keep the files as large as possible at a compression setting of 8, which doesn't hugely affect the quality.
Thank you Michael. As I mentioned, I will be keeping lossless TIFs of these images as well, but want the JPGs to be less than 5 MB to be most usable to unskilled posterity. An interesting point you make that only parts of the image will be degraded by JPG compression. So far I have converted 400 images to JPGs, and have gravitated to using Adobe JPG compression 10 and downsizing to the size necessary to be saved at < 5 MB. This is proving to be about 4000 to 4800 pixels on the largest side.
The quality at Adobe JPG 10 level is pretty good, and keeping the images at 4k or more on the largest side will give you few, if any, noticeable artifacts. The artifacts occur most noticeably in areas of high contrast, so look around the edges of signs, letters, ridgelines, etc. to see if you are getting degradation. I suspect that at level 10 compression it won't be noticeable even when you are looking for it in most images.
If it were me, and my goal was a file size of approx. 5MB
and the .jpg from Ps at quality of 10 resulted in a file as much as 5.5MB (exceeds goal by 10%)
then I would not resize or do anything further to reduce quality and spend added time for very little value;
You can see file size with a 1 sec. glance and avoid added work to make slightly smaller files.
I'm guessing that some files may be less than 5MB and if so, collecting all, they will probably average 5MB anyway.
I set the goal of keeping JPGs below 5 MB for the pragmatic reason that larger files cannot be uploaded to Costco. Admittedly, this is a lame criterion, but anyway I'm sticking to < 5 MB for JPGs. It does add time to keep whittling down the pixel size to get the JPG to save at less than 5 MB (downsampling each time from a full size image), but hey I'm retired and these images are for the ages (or at least for my grandkids).
As far as Curt's suggestion to strip metadata, I specifically wanted it to be present in the JPGs because I have extensively annotated the images with IPTC Descriptions, which can be very useful to have, so I am not doing a Save for Web.
Thank you all, many excellent pointers to keep in mind.