jpg bit depth

[MikeFromMesa]

I thought that all jpgs were, by definition, 8 bit files. I am not referring to jpg-2000 images, but normal jpg images with the jpg suffix. But I found out today that I was wrong and you can save a jpg as a 16 bit file in some editors. I was curious as to whether or not Affinity Photo could open a 16 bit jpg and, sure enough, it can. But when that jpg is saved back to disk AP converts it to 8 bits. Hence my questions:

 

1) Does AP automatically convert a 16 bit jpg into an 8 bit format when it is opened? If so, why?

 

2) If not, why can I not save it back as a 16 bit jpg? Is there some option to allow me to do that? If so, where? And, perhaps most curious, 

 

3) Since 16 bit jpgs can be created, why are they not more commonly used? Most of the complaints that I have seen about processing with jpgs is that they are only 8 bits, but if 16 bit jpgs can be used, that concern seems to disappear.

 

I know I am missing something, but what exactly?

[Alfred]

18 minutes ago, MikeFromMesa said:

I thought that all jpgs were, by definition, 8 bit files. I am not referring to jpg-2000 images, but normal jpg images with the jpg suffix. But I found out today that I was wrong and you can save a jpg as a 16 bit file in some editors.

 

Are you sure you aren't referring to bits per pixel rather than bits per channel, Mike? JPEGs are usually 8 bits per channel, for a total of 24 bits (RGB) or 32 bits (CMYK) but a JPEG saved as RGB565 has a total of only 16 bits (6 for the green channel, and 5 each for the red and blue channels).

[Frank Jonen]

Mike probably means JPEG XR.

[MikeFromMesa]

Let me try to explain.

 

I have always believed that jpgs are 8 bit files, period. If you want 16 bits you need to move to some other format like tiff, or psd or, of course, raw.

 

However I noticed some posts on the internet yesterday that referenced 16 bit jpgs. Here is one that specifically says that jpgs can be saved as 16 bits so, out of curiosity, I decided to see if the poster had any idea what he or she was saying. AP is my editor of choice but I do also have PhotoLine on my system so I loaded an existing jpg image into it and checked the bit depth in the information screen. It showed the image as 8 bits:

 

But the entry is a drop-down box and can be set to 16 or 32 bits. I am a really curious person so I set it to 16 bits and saved the image, again as a jpg (nor XR, not 2000). I then closed the current image and opened the new one and, sure enough, it showed 16 bits. I have attached the screen shot.

 

I then loaded the file into AP to see if it would load. It did and I again saved it and then reloaded it in PhotoLine and it showed as an 8 bit image.

 

Now I do not claim to know what all of this means. Perhaps the document size in PhotoLine does not imply a 16 bit image, but rather a 16 bit envelope around an 8 bit image. I do not know, and I do not know how to determine that with any certainty. I could load the image into a hex editor to see the binary, but since I do not know the format of a jpg that would be a useless exercise. Perhaps what I should have done is post this question on the PhotoLine forum.