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up vote 15 down vote favorite
1

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?

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1  
See also photo.stackexchange.com/questions/4779/… and photo.stackexchange.com/questions/2041/what-does-dpi-mean. I don't think we've got this straightforward question covered directly though. (So, +1!) – mattdm Apr 22 '11 at 18:53
1  
Well right. I am not asking what is DPI. – Andrei Apr 22 '11 at 19:53

4 Answers

up vote 14 down vote accepted

The values are arbitrary and meaningless, and only 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.

You can still ignore it, though.

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2  
I realize now that for images coming from scanner, dpi field carries real meaning, so that's why that field is there. – Andrei Apr 22 '11 at 19:55
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I think the 72 dpi number comes from the CRT days. AFAIR, this was the average dot pitch of CRT monitors back then, so it is not totally meaningless. – ysap Jul 1 '11 at 15:32
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Sure, not totally arbitrary in origin, but I'm going to stick to saying that it's meaningless in this context. – mattdm Jul 1 '11 at 15:41
For a historical perspective -- in print layout workflows a dpi number is used (or is supposed to be used) as a guide to how large the image can be on the printed page. If you set a 200 dpi value (common for newspapers) for a 2000x1000 pixel image the person placing the image in the page layout software (who is normally not a photographer) would be told that the image is 10x5 inches. The same image with a dpi set to 72 would seem to be about 28x14 inches -- but if the editor actually used it at that size the image would appear pixelated. – David Rouse Jul 1 '11 at 17:27
@David: sounds reasonable in theory, but the numbers actually put by the camera in JPEG files (refer up to the question) aren't being used that way. It'd be interesting if camera makers had decided to set this number based on a reasonable estimation of image quality for the sensor at the given ISO, or something like that. But they don't. – mattdm Jul 1 '11 at 17:44
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up vote 4 down vote

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.

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Print, or view on-screen for that matter. – mattdm Apr 22 '11 at 19:14
I don't think printing is where the DPI should (theoretically) resolve from. This is a photo, so the dimensions of the field-of-view of the camera, divided by the resolution, would be the actual dpi of the photo. – Fake Name Apr 23 '11 at 11:44
Therefore, it seems to me that if you know the angle of view of the camera's lens, and the camera had some manner of ranging facility (ultrasonic?) to measure the distance to what it is pointing at, the values could be populated with a (rough, perhaps) meaningful value. – Fake Name Apr 23 '11 at 11:45
@Fake Name - How about just using the focus-distance? As a matter of fact, you can probably write a small program that does this after the fact if your camera stores the focus-distance in the EXIF (only some do unfortunately). – Itai Apr 23 '11 at 13:02
@Fake Name - but in a typical image, each pixel records an object at a different depth. So, the conversion will be correct only at a small amount of pixels? Do you suggest that for the same lens-sensor configuration (FoV), two images of a person in different distances will produce different dpi values? I think the best that you can do following your idea is to state the Dots-per-Radian (DPR) or -Degree (DPD) which is the angular spatial resolution of your system. – ysap Jul 1 '11 at 15:39
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up vote 3 down vote

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

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I just noticed that the Olympus OM-D E-M5 has a menu option to select your own arbitrary value. Given everything else I'm not sure that's really a useful option, but there it is! – mattdm Jun 22 '12 at 21:05
up vote 1 down vote

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.

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