Photoshop bit size questions

I have a JPG photo that says it is 24 bit, then I open a new Photoshop file (different specs) and specify it to be 16 bit; then I bring in the 24-bit picture into the Photoshop file and save it as a JPG… The JPG info says that the file is still 24-bit. I did the same with a 8 bit Photoshop file, and the file info still say 24-bit. Don’t understand.

Second puzzle: I only see in Photoshop what the bit size is when I create the New Document and specify the bit size… Didn’t see anywhere else in the Image Size or the File Properties a feature to indicate what bit size is the document.

How are you “bringing in” the 24-bit picture into an already open photoshop doc?
To see bit size, Image>Mode down at the bottom of the list something should be checked.

Ok, I just realized this has nothing to do with the picture. My camera makes 24 bit photos according to the Properties > Details’ specs (right click on PC) of the photo. But Photoshop is doing its own thing that I don’t understand:
I make a brand new Photoshop file – 8 bit with no picture brought in.
I save the file as JPG, close it, and look at the properties of the JPG file… it says – 24 bit. Why?
I open that JPG on Photoshop again, go to Image > Mode > it says 8 bit. (the original specs of the created document)

I’m not quite certain where you’re seeing the 24-bit description (not using Windows), but when Photoshop refers to an 8-bit image, it’s referring to the bits per channel, which in an RGB image adds up to 24 bits.

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When I open this file on Photoshop – it is 8 bit.

You’re an unknowing victim of mixed concepts here. Here’s how I’d explain it:

Concept #1: When an image is open in Photoshop, it doesn’t have a format. The file is read (it’s storage format is decoded) into memory and the result of that read is shown in the Photoshop editing environment. If you choose Images > Mode, that menu will indicate via a check-mark which editing space is in use; 8-, 16-, or 32-bit.

Concept #2: When you save/export a file from Photoshop, the contents of the editing window are encoded to a storage format. That format will retain a particular bit depth as a property. A JPEG file, of RGB color mode will always report its Bit Depth property as a value of 24. This is because an RGB-mode image is made up of 3 channels, each of which is the mathematical equivalent of a grayscale image, with 256 gray levels possible, or (3) 8-bit images (see Just-B’s post above). Using the same mathematical rule, a grayscale JPEG’s, bit depth will always be 8 (1 channel x8), and a CMYK JPEG’s bit depth is always 32 (4 channels x8).

I see (to some extent)… Starts to kind of get too deep for me once numbers get involved. I’m not a numbers brain, but thanks to all of you for trying to clarify it.

It’s an example of technology getting ahead of the vocabulary to clearly describe it.

HotButton did a great job explaining it, but here’s another attempt from a different angle.

An RGB image, like your camera shoots, is composed of three separate monochromatic channels: red, green and blue. Each of of these channels is composed of pixels — each of which contains 8 bits of information. This is why, it’s referred to as an 8-bit image. Overlap these channels on a monitor, and the channels merge to form a full-color image.

A grayscale image (what some people call black and white) is also an 8-bit image, but a grayscale image only has one channel.

A CMYK image is also an 8-bit image, but it’s composed of 4 separate channels; cyan, magenta, yellow and blue. When those channels are printed out on top of each other on a printed page, it creates a full-color image.

So none of that should be especially confusing, but here’s where the confusion comes into play. Even though an RGB image is composed of three separate 8-bit channels, the RGB file itself can be referred to as 24-bit since, well, add up the 8-bit channels and it totals up to 24 bits.

This whole terminology problem could be fixed if people would just refer to these images as, maybe, 8x3-bit or 8x1-bit or whatever, but that’s not the convention.

It gets more complicated, though. There really are images composed of pixels with more than 8 bits. These images, like those with 16-bits per pixel, contain much more color information. If your camera shoots raw files, it’s shooting them at a higher bit depth to capture more information, which gives you the ability to pull more tonal values out of the image before saving or exporting it to a regular 8-bit-per-channel, 24-bit RGB file.

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