Does anyone know how to find a description of how Exposure Information is incorporated into JPEG files by digital cameras?

I have a Canon PowerShot 70, but other cameras also include this information.

-- Michael Bate (michaelbate@mediaone.net), November 20, 1999

Answers

The exposure, zoom level, f-setting, etc. are all encoded and stored in the "EXIF" header at the beginning of the JPEG file, the data from which is usually not accessible when using ordinary JPEG viewing programs.

If you are very technically inclined, see the EXIF info here:

http://www.pima.net/standards/iso/standards/documents/N4378.pdf

It describes how EXIF is encoded for TIFF, but JPEG uses the same rules.

For a practical working application, try the program "Thumber" (shareware) or "EXIFread" (free) available at:

http://members.tripod.com/~tawba/exifread.htm

They both display EXIF information from the header of the JPEG. -bruce

-- bruce komusin (bkomusin@bigfoot.com), November 21, 1999.

THANK YOU for an excellent and detailed response.

Unfortunately, Canon has appears to be doing something different with the PowerShot Pro 70 - an excellent camera.

I have now tried three different programs, the ones you suggested and also PIE (Picture Information Extractor - available at http://www.hoju.de/ This appears to be an excellent program!). They all claim that the files do not contain exposure information. Yet the Canon-supplied TWAIN driver, when pointed at these files, does see the information.

Also, when I look at the files with a hex/ASCII viewer, I can see strings specifying the camera model and the fact that I used "Full Auto" exposure.

The exposure info displayed by the TWAIN driver reports that the file is in "CIFF JPEG" format - I have not been able to find data on this format anywhere. I searched the document you pointed to for the string "CIFF" and found nothing.

In addition, I have noted that this camera stores thumbnails in separate files, named "THM....JPG" instead of inside the JPEGs as other cameras apparently do. (This is my first digital camera).

Thanks again. If you or anyone else knows what Canon is doing here, I would appreciate any information you can give me.

I am a professional software developer and willing to write something myself if I have to. If I do, I will let people here know about it.

-- Michael Bate (michaelbate@mediaone.net), November 21, 1999.

I finally found a good answer to my question. I hope this response will be helpful to others.

I'm hazy on the details, but there is a format war going on. The promoters of CIFF (Camera Information File Format) are led by Canon. That is the format used in the PowerShot Pro 70.

A full description of CIFF can be found on http://www.ciff-forum.com. My favorite imaging program, PolyView, (www.polybytes.com) will incorporate full support for CIFF in their next release.

-- Michael Bate (michaelbate@mediaone.net), November 28, 1999.