Confused by metadata (e.g., BitsPerSample)

[aajax]

It seems that the software provided by Canon is NOT as complete as it could be when dealing with metadata. Therefore, I've been using a number of tools that rely on a program called ExifTool for accessing/viewing metadata associated with image files related to my photographs. The specifications (per user manual) for my camera includes a line as follows:

• Image Type: JPEG, RAW (14 bit Canon original)

I have my camera set to produce raw files thinking that this produces files that contain more precision than the 8 bit limit associated with the JPEG specification and allows more latitude for the software used to develop the photos. However, the metadata associated with these raw files include an element within the group identified as EXIF:IFD0 which contains a tag named BitsPerSample with a value of "8 8 8". To be sure metadata is a complicated subject easy to misunderstand but it is my understanding the IFD0 refers to what is called the main image. As it happens these same raw files also include an element within the group identified as EXIF:IFD2 which contains a tag named BitsPerSample with a value of "16 16 16". I'm having some trouble determining the specific purpose of the EXIF:IFD2 group but based on what appear to be related values of "668" and "432" for ImageWidth and ImageHeight, respectively, I'm thinking this could pertain to a thumbnail. It would make some sense to me if the thumbnail where limited to 8 bits per sample even when the main image was 16 bits per sample. However, I can make no sense out of what's presently there.

It would be nice if Canon had some reference documentation, for those who might be interested, that elaborates on the meaning of all the metadata that their cameras record in the produced image files. Is that possible?

Anyway, can someone explain how to interpret these various elements called "BitsPerSample" that get recorded in Canon's raw files. Filetype is CR2 if that matters.

[aajax]

Sorry for late response. My computer died unexpectedly and have been busy dealing with that.

I think I realize that the camera always develops the pictures in jpg format no matter what kind of files are desired. I imagine that this is necessary for the camera to display the image on its' screen. It also provides the basis for including a thumbnail in a raw file. My understanding of the raw data is that the per pixel color data is what the sensor produces. The are different kinds of sensors that might be involved. I think mine is called "Bayer" which has 4 channels per pixel 2 of which are green and 1 each red and blue. I believe the term "Demosaicing" refers to the computational process of converting that data to RGB which is pretty much the first thing that needs to be done to create an image that can be viewed (for further editing) on a display device. Raw editors use their own format for processing purposes but it is that raw data prior to demosaicing that I think should have more than 8 bits of precision.

Prior to reply by kvbarkley I did NOT realize the extent to which Canon reuses the tif format for its' raw files. However, that is very understandable.

It now looks like I was naive enough to think that the metadata in a raw file would reflect the raw data. I suppose the idea that the 8 BitsPerSample value is correct for the jpg thumbnail could be considered a way to rectify that concept. However, I would have liked that to be more clear.

Something else I've learned about digital image processing is that it is complicated and I realize that I'll never be really confident that my understanding is correct. I do appreciate these discussions and am very grateful for all of the feedback.

Many thanks.

[ebiggs1]

"The are different kinds of sensors that might be involved. I think mine is called "Bayer" which has 4 channels per pixel 2 of which are green and 1 each red and blue."

The Bayer filter is put in front of the sensor. It is an array that emulates the human eye which is more sensitive to green. That's why it favors toward green.

"I realize that the camera always develops the pictures in jpg format no matter what ..."

The camera only makes a thumb jpg to allow you to see the image on the LCD screen.  There is no automatic conversion to jpg unless you tell the camera to 'save' the file as a jpg.

[aajax]

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@ebiggs1 wrote:
...

The camera only makes a thumb jpg to allow you to see the image on the LCD screen.  There is no automatic conversion to jpg unless you tell the camera to 'save' the file as a jpg.

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I think that is what I was trying to say.  The point I was trying to make is that in order to see anything on a display device demosiacing is needed and because the camera displays an image it had to demosaic the data.  The raw file produced by the camera contains the data that must be demosaiced using software that may support various different alogrithms for this purpose.

I'm enough of a novice that my use of terminology could be in need of refinement but, to me, the term develop also means creating an image can be seen with human eyeballs which is pretty synonomous to demosiacing.  Once that is done all manner of editing can be performed to alter what is percieved with our eyes.