On 2013-04-11 16:18:33 -0700, Jennifer Murphy said:

I think digital photography is beyond my mental capacities (sigh).

I have several photos that were saved both as raw image files and as
jpgs. For many of them, the jpg file reports more pixels than the raw
image file. How can that be? I thought a jpg file was a compressed
version of the raw image file. So the number of pixels should be at most
the same, and I would have thought somewhat lower.

Here are a couple of examples:



Camera: Canon 5D Mark I with Canon EF 24-70mm f/2.8L lens

Filetype: CR2 (raw file)
Pixels: 2496 x 1664 Pixels (4.15 MPixels) (3:2)
Print Size: 21.1 x 14.1 cm; 8.3 x 5.5 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 12.92 MB (13,550,637 Bytes)

Filetype: JPG
Pixels: 3156 x 2678 Pixels (8.45 MPixels) (1.18)
Print Size: 26.7 x 22.7 cm; 10.5 x 8.9 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 7.07 MB (7,418,412 Bytes)

The JPG also has a quite different aspect ratio (1.18 vs 1.5).

In the above case you have two different dimension sizes; the CR2 is
2496 x 1664 p giving you 4.15MP and the jpeg is 3156 x 2678 p at 8.9MP,
an almost 50% difference due to the dimensional difference. These are
MPixels which deal only with physical dimensions.
Note the actual file (disk size) sizes are 12.92 MB for the CR2 and
7.07 MB for the JPEG. There is still more data contained in the RAW CR2.
I suspect you have an adjusted and size extrapolated JPEG there, not a
JPEG immediately converted from the CR2. The 16-bit CR2 is unadjusted
and not altered in any way.

Camera: Panasonic DMC-G1 with a 14-45 zoom lens

Filetype: RW2 (raw file)
Pixels: 1920 x 1440 Pixels (2.76 MPixels) (4:3)
Print Size: 16.3 x 12.2 cm; 6.4 x 4.8 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 13.99 MB (14,665,216 Bytes)

Filetype: JPG
Pixels: 2816 x 2112 Pixels (5.95 MPixels) (4:3)
Print Size: 23.8 x 17.9 cm; 9.4 x 7.0 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 2.61 MB (2,736,853 Bytes)

This is a similar scenario to the Canon example. the dimensional size
of the jpeg has been enlarged, while the true size of the files, the
RW2 and the compressed jpeg reflect the truth of the data contained in
each.
Also in this case the 16-bit RW2 is unadjusted

Camera: Nikon D700 with a 24-120 mm zoom lens

Filetype: PSD (raw file)
Pixels: 2910 x 2435 Pixels (7.09 MPixels) (1.20)
Print Size: 24.6 x 20.6 cm; 9.7 x 8.1 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 81.14 MB (85,081,048 Bytes)

Filetype: JPG
Pixels: 2910 x 2435 Pixels (7.09 MPixels) (1.20)
Print Size: 24.6 x 20.6 cm; 9.7 x 8.1 inches (at 300 dpi)
Colors: 16,7 Millions (24 BitsPerPixel)
Disk Size: 4.25 MB (4,455,756 Bytes)

For this pair, the number of pixels and the aspect ratio are the same,
but the size of the file shrank by almost 20:1.

This is a different situation. First PSD is not the native RAW format,
for that it should be an NEF. PSD is an Adobe file type, as sub-type of
TIFF which preserves layer and adjustments made with Photoshop and some
other editing software.
Note that the dimensions of both the PSD and the JPEG are identical,
and so both have the same total pixels, 7.09MP.

The disk or file size tells the other side of the story. The
adjustments and edits were probably made to the converted RAW NEF file
and then saved to the hard drive. It will not be saved as a NEF, but as
a 16-bit, uncompressed PSD, retaining all the data of the adjustments.
Then to save it as a compressed JPEG it will be saved as an 8-bit
compressed JPEG


--
Regards,

Savageduck