Save jpeg at quality 10 vs 12...

[Darren]

I am new to photoshop and am curious to see what quality setting others use when saving photos as jpeg.

The size difference between saving at 10 and 12 is almost 3 times the size, I see no quality loss when saving at quality 10.

I have some photos that are original size of around 5mb, after editing the photos saving at quality 10 gives a file size of around 5mb while quality 12 a file size of around 12mb-15mb.

 

[Tom Mann]

Save the same image twice. Once at quality=10, and once at 12. Bring both into PS on separate layers. Set the blend mode of the upper layer to subtract or difference. Next, put a "levels" adjustment layer on top of that. Bring in the RH slider to lighten up the difference between the lower two layers.

You'll be able to see the difference using the above technique, but it will be quite small, and completely negligible for most uses. That being said, if I want to save an image in a form that's readable by everyone in the world, will likely continue to be readable for decades by future software, and don't want to save another copy as a tif or bmp, I'll save it as a quality 12 JPG.

T

 

[iDad]

I try to save everything tif(unless the image not important to me) JPEG loses quality even just changing from one hard drive to another even at quality 12 it will still lose.just saying....

 

[Tom Mann]

JPGs degrade only if you first open the file, and then re-save it (either at the same location or somewhere else on your system). If you copy a jpg from one drive to another using the operating system there is absolutely zero degradation of the file unless you have a major problem with your system, and if that's going on, losing a bit of quality of a few images will be the last of your worries. ;-)

WRT quality=12 JPGs vs TIFs, I know my recommendation goes against the usual recommendation to use TIFs, but do the little exercise I described in my earlier post and you'll be amazed at how good quality=12 jpgs actually are. JPGs have the advantage that they are probably the world's most used image format, so they will likely to be around for a long time and can be read by many more programs than TIFs.

The one area in which JPGs are definitely poorer is that even at quality =12, they don't tolerate bit rot at all, whereas an uncompressed file format degrades much more gracefully.

T

 

[ALB68]

Just as a matter of information, I just happened to have a large tiff that had been saved with LZW compression. I resaved it as a 12 level jpg and PS says it is 37.5mb on disc opposed to 75.2mb for the tiff. I saved it again as a level 10 and it still shows 37.5mb on the disc (not pixel dimensions which is the same in all as it was not resampled). Since I have retained the same pixel dimensions in all, regardless of the format, wouldn't the print quality remain intact?

 

[Tom Mann]

Here is a comparison of the original of a high resolution image to one version saved as a q=12 JPG, and another version saved as a q=8 JPG. I used the subtraction technique I described in my previous post.

To accentuate any possible JPG related problems, I started with a high resolution image (5.6 kpix wide) and did all operations at that size, then, as the final step, for display on this forum without triggering the forum compressor algorithm, I cropped out 1:1 600 pixel squares from each of the versions.

To demonstrate how sensitive the technique is to differences between images, I wind up the series by using the same technique to show the difference between the original and a copy moved exactly 1 pixel to the right.

First, to give a sense of scale, here's the entire original (down-rez'ed to 600 px wide).

 

[Tom Mann]

Next, for reference, here is a 600x600 1:1 crop from that image.

 

[Tom Mann]

Here is the difference between the original and a JPG resaved at q=8.

 

[Tom Mann]

Here is the difference between the original and a version resaved as a q=12 JPG.

As you can see there is essentially no visible difference, even though I'm using the same technique that I used on the other images in this series to wildly exaggerate any possible differences.

 

[Tom Mann]

Finally, to show how incredibly sensitive this comparison technique is, here is the difference between two identical copies of the original, with one shifted by one pixel to the right.

 

[Tom Mann]

The reason that absolutely no differences can be seen between the original and the q=12 resaved JPG copy is because any differences are less than 1 quantization level at 8 bits per channel.

In addition to the universality advantages of JPGs that I mentioned in an earlier post in this thread, there is a huge savings in file size compared to TIFFs. Below I compare the file sizes for the images in this demo. I also generated (using PS CS6) two types of TIFFs and give their file sizes for comparison:

original: 9.8 Meg

resaved as a q=8 JPG: 3.8 Meg

resaved as a q=12 JPG: 17.8 Meg

resaved as a zipped TIF: 99.6 Meg

resaved as a uncompressed TIF: 118.1 Meg.


T

 

[Tom Mann]

ALB: "...I saved it again as a level 10 and it still shows 37.5mb on the disc (not pixel dimensions which is the same in all as it was not resampled). Since I have retained the same pixel dimensions in all, regardless of the format, wouldn't the print quality remain intact?..."

I have never seen the file size stay constant as you go from one quality level to another. That is extremely odd. I suspect you did something wrong. Attached is the progression of file sizes for the image I used, when I saved it at JPG quality factors ranging from 8 to 12.

T

 

[ALB68]

Wow Tom! Where exactly did you manage to acquire all of this knowledge? Great demo.

 

[Darren]

Thanks for the replies and thanks Tom for the in depth explanations and examples.

Well, I guess after editing all of my photos (photoshop elements) i will re save them at quality setting 12 after all. Although my internal hard drive is only 1tb, external storage is pretty cheap. I will invest in some external storage to keep my photos on.

 

[Tom Mann]

Don't forget that the question is never, "... if my hard drive fails", but is "...when my hard drive fails", so do yourself a favor and get yourself two identical external drives, keep one at a different location and have a rotating backup system.

Do two backups, and then go through the procedure of restoring a few files to prove to yourself that your system is actually working. I can't tell you the number of people who mindlessly followed a backup plan, but never tested it. Unfortunately, when their main drive failed and it came time to use the backup, they found some major problem with it.

Also, while it's not an issue for backups, don't forget my comment about the susceptibility of JPGs to bit rot (ie, one bit gets flipped) when it comes to long-term archival storage. With a JPG, everything from that point on in the file is hosed. With an uncompressed TIF, only the color of one pixel is changed. That being said, a rotating system of two external drives constantly being updated gets around that problem.

T

 

[NemesisWorks]

well another thing : if you save your jpg on 12 then it is possible that some old phones even browsers wouldn't show your pictures but don't worry about it too much.

 

[Tom Mann]

Hi E-N, long time no see.

About compatibility of q=12 JPGs with older software, I've never personally run into any obvious problem specific to the q=12 setting, but then again, my experience is limited, and I haven't been looking for problems. Maybe it's something subtle like some software decoding a q=12 file the same as it decodes a q=10 version (ie, it throws away the improvement), but I've never seen any software that works on other quality JPGs completely fail on a q=12 version.

If you've run into specific examples of q=12 incompatibility, could you describe them? I would be very interested in hearing about this.

Tom M

PS - BTW, are you sure you are not thinking about the well known compatibility problems with the 12 bit per channel medical image variant of JPGs. Most ordinary decoders will go bonkers when presented with that type of image.