> Hi all,
> Sorry for multiposting. Pressed Send button too quickly.
> I've displayed Summary Information for some JPG images which I know they are
> 72 DPI. Summary Information reports 300 DPI. Photoshop confirms actual
> resolution is 72 DPI. Doubtlessly there is a bug here.
This is horribly confusing, so please bear with me...
Windows reports whatever info is embedded in the Exif header
of a JPEG file. Devices such as digital cameras and scanners use
arbitrary DPI numbers when writing JPEGs. The numbers are
arbitrary because they aren't related to the image's actual resolution,
they're related to the intended output device.
For example, my 2 megapixel Canon A40 camera outputs JPEGs
with a DPI of "180" in the Exif header, while my 5 megapixel Sony
F717 writes "72" in the Exif header. So Canon is assuming output
to an 180 DPI inkjet or laser printer while Sony is assuming output
to a monitor. But the *actual* image resolution is completely
unrelated to *either* of these numbers.
Confused yet? It gets worse. Some graphics software such as
CorelPaint use DPI when specifying image resolution, while others
such as Photoshop use "pixels per inch", or PPI. Again we're
dealing with the same thing -- the image resolution remains the
same, just the intended output device is different. To convert one
to the other in Photoshop, go to Image/Image Size, uncheck the
"Resample Image" button and then enter your desired *print*
dimensions or resolution in the Document Size box.