What is the significance of these different width, height and resolution parameters?

Question

An image with a pixel resolution of 640 x 480 has additional dimension and resolution parameters according to exiftool. I'm unsure what they mean.

1. Why are the X / Y Resolution parameters the same?
   Should they not reflect the pixel dimensions of the image?
2. What does Exif Image Size mean and how is it different from the pixel dimensions?
3. What is the focal plane?
   Does it have any relation to the device used to capture this image?

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$ exiftool  evil1.jpg | egrep 'Width|Height|Resolution'
X Resolution                    : 180
Y Resolution                    : 180
Resolution Unit                 : inches
Exif Image Width                : 400
Exif Image Height               : 300
Focal Plane X Resolution        : 8114.285714
Focal Plane Y Resolution        : 8114.285714
Focal Plane Resolution Unit     : inches
Image Width                     : 640
Image Height                    : 480

---

If needed, the original image can be obtained from:

 here=http://www.pythonchallenge.com/pc/return/evil1.jpg
 wget --user=huge --password=file $here

Answer 1

The X/YResolution is given in pixels/inch. Usually these are the same because the pixels are square.

The ExifImageWidth/Height are just the image dimensions stored in the metadata. These are essentially useless and in this case even worse than useless because they contradict the actual JPEG image dimensions (640x480)

The focal plane resolution is the pixel resolution of the camera's imaging sensor in pixels/inch.

Answer 2

ExifImageWidth/ExifImageHeight (400x300) are the EXIF metadata of the ORIGINAL image dimensions stored in the file header of the image (jpg, png, tiff...).

In this case they contradict the actual image dimensions (640x480).

Normally, this would not matter.

However, if you create a PDF from the jpg using Adobe Acrobat, the PDF page will become distorted because Acrobat builds pages based on EXIF info whenever available.

How to avoid this: either use Foxit PhantomPDF (which builds PDF pages based on the actual image dimensions) or selectively delete the image file's EXIF metadata.

How to do this: see http://forums.adobe.com/message/5768320#5768320