Why is the image size different in portrait and landscape mode?

Question

Today i noticed that my Canon EOS 5D mark II seems to create different image sizes when holding the camera in landscape or portrait mode. The Portrait Image size is always 3753x5634 pixels while the landscape size is 5616x3744 pixels (using RAW images). Wikipedia says that the sensor size of the eos 5dmkii is 5616x3744, so that would mean you would get extra 18 pixels on the longer edge and extra 9 on the shorter edge when holding the camera in portrait mode.

I also found out there are some rare cases (i just found one photo out of >100) where the resolution of an landscape image is also 5634x3753.

Is there any reason why the resolution of the image changes?

edit: from the comments i checked with exiftool. I found the different image sizes shown in darktable, where it is very constitant. With exiftool all images have the same size written in the "Image Size" tag.

But there is also a Sensor Width and Height Field:

Sensor Width                    : 5792
Sensor Height                   : 3804
Sensor Left Border              : 168
Sensor Top Border               : 56
Sensor Right Border             : 5783
Sensor Bottom Border            : 3799

which is also the same for landscape and portrait. Maybe darktable is calculating with these values somehow and got something wrong in portrait. I think i found a bug in darktable ;)

Answer 1

The EOS 5d mkII generates 5616x3744 JPEGs, while its sensor actually has 5634x3753 active pixels - hence the difference.

So I'd suspect the difference must be in the workflow. In case of landscape photos, you're viewing EXIF information originating from camera (perhaps the preview JPEG in RAW); portrait photos have to be rotated and/or demosaiced by your RAW tool that doesn't have to follow size convention of the camera, and you're viewing EXIF of the result. That single larger landscape image was also demosaiced in computer (perhaps some adjustments were applied?).