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

Question

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 1

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.

Answer 2

In my experience those numbers have no special meaning. The camera makers just pick one and use it in all their firmware.

Answer 3

The number is just a random filler. It has no significance since the camera does not know how big you will print.

Most cameras default to 72 which according to the EXIF standard is the default value. Some cameras let you set it yourself. Then again, it has little meaning unless you will print all your images without cropping exactly at the same size.

Answer 4

If you consider the JPEG file as a document, the divide the pixel count in the image by the DPI number and you get the Print Size of the image. This of course results meaningless as many printing systems re-size the image arbitrarily. However, there still are printing mechanisms in which the print size is given by such parameters. If you have Photoshop, go to the resize image dialog and uncheck the re-sample option. You'll observe that changing any size of the image will change the dpi number. Curiously I'm in the case where I have to print some images and the lab providing the service won't do ANY resizing for me, so I have to re-sample the images to a specific resolution and DPI number. Another use of this number is that it will allow a better approximation of how big you can print an image without distorting too much for your working parameters. So, in your case, your cameras are just setting a default print size for your images. But unless your printing service relies on this for the size, I see no other use for it and it says nothing about your sensor, so a comparison there is meaningless.

Answer 5

What physical meaning can these numbers have?

The EXIF standard doesn't actually have a single dpi field, but it does have XResolution and YResolution fields, and these are where the dpi value displayed by most software comes from. The standard doesn't say specifically whether those values apply to the image source, e.g. a scanned document, or destination, e.g. a printed copy of the image, but since the entity that writes the metadata knows where the image comes from but can't know how it will be used, the only reasonable interpretation is that those fields relate to the image source.

The standard does say that if the value for the XResolution and YResolution tags is unknown, then 72 should be used. One could reasonably argue that this was a poor choice, since 72 is a valid value whose meaning shouldn't be overloaded. It probably would've made more sense to choose a value that couldn't possibly represent a real resolution, such as 0 or -1, to mean "unknown." But that's not what the standard says, and so if your image has 72 for the resolution fields, you simply can't know whether the metadata writer knew what resolution to use or not.

The problem is compounded by the fact that some cameras set a value other than 72 when they can't really know what resolution the subject was recorded at, since that varies depending on how far away the camera was from he subject. Those camera manufacturers might have been trying to use the resolution fields to suggest a reasonable print resolution, but this only confuses the meaning of the resolution fields.

If you're trying to actually learn something about an image, you'll be better off using the FocalPlaneXResolution and FocalPlaneYResolution fields, which record the sensor resolution at which the image was recorded. And of course the ImageWidth and ImageLength fields will tell you the dimensions of the image in pixels.

Answer 6

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

Yes. It means the jpeg image is compliant with the International Standards agreed upon by camera makers and others with an interest in the production and use of digital images.

In most current digital environments the DPI reported by the camera in the image metadata has no real meaning.

The applications we use to look at a photo on our screens will almost always scale an image to fit a particular space on the screen.

The printers most of us use will almost always automatically scale an image to fit on a specified size of paper.

But there is still one notable exception: Desktop publishing. Page setting software such as InDesign and even most word processing programs still "think" in actual physical dimensions. That's why such applications usually show a scale at the top and/or sides of the page that shows the width and length of the page in inches or centimeters.

If we have two version of an image that is 720 pixels wide and the only difference between the two is the "PPI" reported in the image metadata, here's what happens when we insert both into a document that measures things in real inches/centimeters:

Remember, the actual pixels in both images are identical, the only difference is in the "DPI" value reported by the images metadata. In this case the reported "PPI" does affect how large the image appears on a "page" in the document or on an actual page when the document printed.