Saving JPG files in PS

[Hudson]

Hi all. Can anyone explain why Photoshop is reducing the size (not dimentions) from 8MB to 3.5MB when I save as a JPG?
I open a JPG (size 7.4MB), add a 10 adjustment layers with masks and save the PSD file.
I then select: File > Save As > select JPEG (*.JPG;*.JPEG;*.JPE) for "Save as type:" None of the 7 check boxes are ticked.
I click "Save" and the options are: Quality: 12, Baseline ("Standard"), then I click OK to save and the file size becomes 4.4MB.
The file saved is actually 4.46MB. Using the Export As method saves the image at 4.2MB.

The client asked me why the images were only 1/2 the size of the originals and I have no idea.
I did a long winded work around by saving as a TIFF file and then using the Faststone Image Viewer "Save As" method to save as a JPG at 100% quality and the image size is back to 7.1MB.

Is there an easy explanation as to why PS reduces the file size by almost 1/2? (v2024 - 25.4)
Thanks in advance for any help.

 

[thebestcpu]

Hi @Hudson

There are several reasons why the JPEG images can be smaller after processing, even with the same dimensions.
Yet, I will assume you took your saved flattened version of the image to TIFF and then saved it to JEPG from within FastStone, so we are only talking about differences in how JPEG was compressed in different applications and not talking about why a modified image compressed smaller.

Assuming I am correct in that assumption, there can be several things going on

- The internals of JPEG compression have many options on how to compress. I don't know of two applications that use the same settings for a given compression number.
Some applications don't necessarily use the least compression settings for their highest-quality setting.
- Besides the above, how much compression depends on the image content. Depending on the image content, some internal JPEG compression settings will compress more highly.
- A third factor can be that when saving in JPEG (or almost any other format), there are options for whether to save metadata. Sometimes, those are not options in applications, yet they may be included in one and not another. That could also explain the difference. Given the file sizes we are discussing, I doubt this is an issue in your case.

That file size change can (and in your case has) make a customer/client, etc., worry about losing quality when the file size is smaller. I suspect Fastston 100% quality JPEG has the obsolete smallest amount of compression, and Photoshop does not take that approach. The most likely reason is that in Adobe's judgment, their best compression setting, not set at the least compression setting for the JEPG standard, still yields a quality level that the end user cannot tell the difference and, therefore, has a smaller file and no perceived difference in quality..

The easiest way to test this is to compare the JPEG file from Faststone and Photoshop and see if you can tell any difference when looking at the target image size on your monitor (or a side-by-side print).

If we needed to get more technical, it would require the specific files extracting out the setting used in the JEPG file directly and comparing. The extraction program runs on window machines and I no longer have access as I urn on a Mac without the virtualizing capability of the Windows OS

Hope this helps some
John Wheeler

 

[Hudson]

Thank you John. I can't see any differences in the large and small jpg files. It would be great if PS let the user select the amount of compression required. Thanks so much for your detailed reply - much appreciated.

 

[thebestcpu]

Thank you John. I can't see any differences in the large and small jpg files. It would be great if PS let the user select the amount of compression required. Thanks so much for your detailed reply - much appreciated.

You're welcome @Hudson
John Wheeler

 

[thebestcpu]

It would be great if PS let the user select the amount of compression required.

I thought I would add a PDF file of the JPEG compression options/algorithm. Start at page 9, and you will see many settings a software designer could choose from to determine what approach to use in JPEG compression.
I am pretty glad that software designers decided not to expose those details to the average customer and instead give compression options such as 1 through 10. Each program and its software designer got to choose what JPEG internal parameters to set, and they don't line up between software companies (in some respects, not even within one piece of software - Photoshop has Save/Save As and also Save for Web .
I suspect that when the standard was made and a large number of companies had non-aligned inputs led to having access to the lower level parameter options as the JEPG standard was created..

So, bottom line, in my opinion, No, it would not be great if PS allowed us to set the JEPG compression parameters the way we wanted

Your desires and questions are good ones, I'm just trying to let you know what software designers had to deal with in choosing JPEG compression options for their customers.
John Wheeler

 

[nurgle]

Thank you John. I can't see any differences in the large and small jpg files. It would be great if PS let the user select the amount of compression required. Thanks so much for your detailed reply - much appreciated.

The amount of Compression depends on the Percentage when you save.the small the Quality number, the higher the compression. when using the highests number, Quality 12 in photoshop 2024, it should keep all the detail. (some earlier Photoshops onl;u went up to a max 'Quality 10'.

Jpeg is a Loss-ee format, compressing some pixles. I prefer to keep all my files in Photoshop native format. it has been compatible between PC & Mac for some time.

Regards, Sandy