Photoshop has now given the facility to open and edit JPEGs and other non-RAW file formats using Adobe Camera Raw. One doesn't have to shoot RAWs to do some heavy edits. So is there still any benefit that shooting in RAW makes when editing it in ACR? When a RAW file is opened in ACR why do some menu items (sliders, etc.) color up and they don't do so when a JPEG is opened in ACR?
The raw image is 12 (or maybe 14) bits, and JPG is 8 bits (less range). JPG does not handle changes of extreme range well.
The JPG image already has white balance and color profile in it, likely our bad guessed try that has to be corrected first (which is the reason we are looking at it).
Not even speaking of JPG artifacts, the Raw always is the pristine sensor image. Raw is what the camera started with, before JPG applied our bad guesses about what it might need. And then after we can actually see what we're doing, then raw then can provide actual white balance and profile menus (and more), but JPG can only handle plus and minus corrections to what is already there. Raw does Not have to Undo first.
The limitations when working with JPEG image files, as compared to working with raw data, isn't so much in the application used (unless it is a poorly written application - which could be the case for either raw or jpeg processing). The limitations are self-imposed by choosing to edit a JPEG image instead of a raw file and by the way previously converting to JPEG preserves only the information needed to display the image in that one single interpretation of all the information that was collected by the sensor and included in the raw data. Even if Adobe Camera Raw is allowing you to use the same sliders as you use when editing a raw file, the same information is not available to the app with a JPEG image.
Raw files contain the information captured by the sensor before most processing is applied to that information to make it an image that we would consider viewable. If that information were converted to a 16-bit 3-color raster image (such as a 16-bit TIFF) the file size of a 25-30MB raw file from a 24MP camera would be around 100MB. Typical color JPEG images, even using very little compression, from a 24MP camera are around 5-10MB depending on image content.
When a raw file is converted to JPEG only the information in the raw file that is needed to display the JPEG with a specific set of settings (WB, contrast, etc.) is preserved in the conversion. The data not needed is discarded. If you are saving your files in camera as JPEGs the camera's processor is making those decision for you and converting the raw data coming off the sensor into the smaller JPEG format. The information in the raw data is then discarded.
Once processing steps such as gamma correction, black and white points (which determine the darkest and lightest things in the scene that will be between pure black and pure white), white balance, etc. have been applied they are irreversible. The additional information in the raw file needed to change the values used when doing that process is thrown away in the interest of making the files smaller. When editing a JPEG image one can only choose to alter the information that is still present in the jpeg file by increasing or decreasing the amplification of the color values of each pixel. But the information that was thrown away can not be recovered.
Keep in mind that what you see on the screen when you open a raw file is not all of the information in the raw file. It is an 8-bit conversion of that data very similar to a jpeg. It is only a slice of the entire data set. When you move the sliders with a raw file you actually are drawing on the raw data to display information that wasn't being displayed before but was present in the raw file. When you move sliders with a JPEG file you can only apply a multiplier/divider to the data already displayed in the image, and that data is much coarser than the data in a raw file.
For more, please see: Why is my camera so forgiving for overblown exposure when shooting in RAW? and RAW files store 3 colors per pixel, or only one?
This answer to Why does an X megapixel sensor produce X MB of data (in image files)? discusses what information is contained in a raw file. So does this answer to Which file format is recommended for black and white photographs?
You may also find this question and its answers helpful: Does a RAW file in Photoshop contains all the data only while in Camera RAW or at all times in Photoshop?
From the comments:
So, with a RAW opened in ACR, one can adjust the gamma correction, black & white points, white balance etc. easier than a JPG opened in ACR, is it?; one can arise details hidden in shadows more easily than with a JPG? With a RAW opened in ACR, more details will emerge when the exposure slider is slid?
It's not just easier with raw. You need raw to do it at all with regard to expanding the white and black points. If the shadows in the jpeg are pure black or the highlights are pure white you can't increase the detail in those areas at all. Black and white points are totally baked into the jpeg. You can reduce pure white to a shade of grey, but everything that was pure white will be a uniform shade of grey with no details showing. Same thing with pure black. You can bring it up, but it will just be a uniform blob of the same shade of grey.
And the other parameters?
You have much less latitude with jpeg than raw before you start seeing artifacts such as banding when altering brightness, contrast, color/white balance, or gamma curves because you have much more limited information from which to work. The difference between 256 discrete brightness levels (8-bits) and 16,384 (14-bits) discrete brightness levels between black and white is immense and allows for much smoother transitions when the shape of the light response curves are altered.
It doesn't take long playing around with an image or two to see the difference. Take a photo of a reasonably high contrast scene. Intentionally select a wrong color balance. For example, tell your camera you are shooting under tungsten lights when you are outdoors. Just for fun, overexpose by a stop and a half. Save the image as RAW+JPEG in camera. Then edit the raw file, correcting the WB and bringing exposure down to reveal the detail in the blown highlights. Now try to do the same thing with the JPEG!
The data present in a raw file also makes it possible to do things to images shot under poor, limited spectrum lighting that would not be possible to do to a JPEG shot under the same conditions, even if the in-camera settings were heavily adjusted before the shot to compensate for the biases in the lighting.