sensor - Does the dpi number reported by camera in JPG have any meaning?

I have two cameras, a 6mpix Panasonic FZ8, and 14mpix Canon A2200.

When I compare shots from two cameras, one of differences I notice is
dpi number that camera reports in JPG info:

• FZ8 reports dpi=72


• A2200 reports dpi=180

What physical meaning can these numbers have? I am at loss for guesses.

I am fairly familiar with notion of dpi in scanning and printing. I can calculate density of pixels on the sensor of the camera. But then, the linear density of pixels on the sensor will be hundreds times larger than number above. So what, if anything, does it mean?

Answer

The values written in JPEG files are arbitrary and essentially meaningless. They don't relate to anything about the camera, its sensor, or the resulting images. They certainly don't relate to image quality or acceptable resolution for printing. Really, they mostly serve to confuse people.

The EXIF standard seems to imply that if the tag is missing, 72 is the (still-meaningless) default. However, it is apparently mandatory for the TIFF standard, from which the JPEG/EXIF format basically inherits everything. So maybe it has to have some value to properly comply with the standard.

Others have noted that some desktop publishing or word processing software reads this value and will use it for default scaling on the page. So, I guess in that sense, there is "meaning", but I'd argue that this is really misapplication, because the original value doesn't have meaning. Garbage in, garbage out, as the saying goes.

Now, maybe in an alternate universe the standard could relate to a standard print size. Or, the camera could estimate real detail in the image and give a recommended maximum print size. But none of that is the case. In practice, these values are meaningless and you should ignore them, even if some software makes assumptions based on them when opening files.