DPI in .jpg Metadata

J Peters · Jan 29, 2019

Hi, I have long thought that an image file does not have an inherent DPI (or PPI), believing any such figure to me meaningless until you come to print the image or view it on a monitor. As such, when I print, I don't give DPI a moment's thought - I just choose a paper size and the printer software does all the scaling for me.

However someone at the club I go to is concerned about the DPI figure and they always set it explicitly when they print. Moreover they have just bought a new camera (Micro 4/3rds sensor as a companion camera to their DSLR), and they noticed a value of 180 DPI is written into the metadata of the out-of-camera .jpg files. This has led them to believe the files are not as good quality as their previous camera of similar design, which embedded a value of 300 DPI. I started to explain that this figure is meaningless and it's just the sensor resolution which matters, but I don't have enough confidence in my own knowledge on this to be able to explain it any further to someone else without checking my facts. So I have two questions:

Why is a DPI figure written into the metadata of a camera's .jpg file? Does it have any purpose at all?

Why does some printing software ask you to specify a DPI setting when this would normally be mathematically implied anyway from the other two variables: resolution and paper/print size? Is there a way to tell such software "Please just map the PPI of the file to the highest possible DPI of the printer" - which I'm guessing is what happens by default if you just right-click and print a file in Windows for example?

Sorry that's 3 questions.

markkuk · Jan 29, 2019

J Peters said:
Why is a DPI figure written into the metadata of a camera's .jpg file? Does it have any purpose at all?

The EXIF standard for image metadata was originally specified in 1990's when the most common way to generate digital image files was by scanning analog photos. That's why the standard contains fields that only make sense for scanned images, including the DPI number.

Tom Axford · Jan 29, 2019

J Peters said:
Hi, I have long thought that an image file does not have an inherent DPI (or PPI), believing any such figure to me meaningless until you come to print the image or view it on a monitor. As such, when I print, I don't give DPI a moment's thought - I just choose a paper size and the printer software does all the scaling for me.

What you mean is ppi (pixels per inch), not dpi (dots per inch) which is quoted for printers but is a totally different thing and irrelevant when printing an image.

J Peters said:
However someone at the club I go to is concerned about the DPI figure and they always set it explicitly when they print. Moreover they have just bought a new camera (Micro 4/3rds sensor as a companion camera to their DSLR), and they noticed a value of 180 DPI is written into the metadata of the out-of-camera .jpg files. This has led them to believe the files are not as good quality as their previous camera of similar design, which embedded a value of 300 DPI. I started to explain that this figure is meaningless and it's just the sensor resolution which matters, but I don't have enough confidence in my own knowledge on this to be able to explain it any further to someone else without checking my facts. So I have two questions:

Why is a DPI figure written into the metadata of a camera's .jpg file? Does it have any purpose at all?

The ppi figure is there because, together with the pixel size of the image, it specifies the printed size of the image. As you say, the ppi figure is irrelevant for most other image processing operations, although software often uses it to allow you to specify the image size in inches instead of pixels, when resizing an image, etc. It just gives a bit more flexibility to the user.

All modern printers allow you to override this ppi value and specify whatever print size you wish, but you can instead just ask to print the image at 100% and then the ppi figure is used. For example, an image of 3000 x 1500 pixels at 300ppi gives a 10" x 5" print when printed at 100%.

J Peters said:
Why does some printing software ask you to specify a DPI setting when this would normally be mathematically implied anyway from the other two variables: resolution and paper/print size? Is there a way to tell such software "Please just map the PPI of the file to the highest possible DPI of the printer" - which I'm guessing is what happens by default if you just right-click and print a file in Windows for example?

J Peters said:
Sorry that's 3 questions.

57even · Jan 30, 2019

Every printer has a different DPI rating because it depends on the print head design.

However, this is dots per inch (dots of ink that is) not pixels. CMYK printers need quite a few dots to simulate all the colours in an 8-bit colour space (17 million) with just four colours plus white. This means an array of several dots is required for each pixel.

Hence printer DPI ranges from 1200 on large format printers to 2400 or more on small format (A3+) photoprinters.

Confusingly, DPI is often used interchangeably with PPI, which is pixels per inch. You have to set this to determine the print size. The default values used in Photoshop etc are fairly meaningless.

In terms of camera quality all that matters in megapixels and sensor size. DPI and PPI are irrelevant.

rurikw · Jan 30, 2019

J Peters said:
Hi, I have long thought that an image file does not have an inherent DPI (or PPI), believing any such figure to me meaningless until you come to print the image or view it on a monitor. As such, when I print, I don't give DPI a moment's thought - I just choose a paper size and the printer software does all the scaling for me.

That is common sense.

J Peters said:
However someone at the club I go to is concerned about the DPI figure and they always set it explicitly when they print. Moreover they have just bought a new camera (Micro 4/3rds sensor as a companion camera to their DSLR), and they noticed a value of 180 DPI is written into the metadata of the out-of-camera .jpg files. This has led them to believe the files are not as good quality as their previous camera of similar design, which embedded a value of 300 DPI. I started to explain that this figure is meaningless and it's just the sensor resolution which matters,

You are absolutely right.

J Peters said:
but I don't have enough confidence in my own knowledge on this to be able to explain it any further to someone else without checking my facts. So I have two questions:

Why is a DPI figure written into the metadata of a camera's .jpg file? Does it have any purpose at all?

The default PPI from my different cameras are all over the place: 72-180-240. No idea why they are given. Would prefer to set the value in Photoshop where I often think in mm, maybe with a vague idea of possible print sizes in mind. Sometimes I even convert from PPI to ppmm (10ppmm=254ppi). For no purpose at all, just fooling around. When architectural drawings were still made by hand the thinnest available ink pen was 0.13mm. As a black line on white paper it didn't seem to be anywhere near the visibility limit so I thought the eye can probably resolve smaller than 0.05mm. Was a bit surprised to learn that 0.08 (300ppi) was considered sufficient.

J Peters said:
Why does some printing software ask you to specify a DPI setting when this would normally be mathematically implied anyway from the other two variables: resolution and paper/print size? Is there a way to tell such software "Please just map the PPI of the file to the highest possible DPI of the printer" - which I'm guessing is what happens by default if you just right-click and print a file in Windows for example?

My Epsons don't ask but I once read a recommendation that to get the best out of the 3880 printer you should resample the print file to 360ppi as the printer does 1440dpi (4x360). Even followed the advice a few times but haven't done a comparison. Doubt I would see any difference.

J Peters said:
Sorry that's 3 questions.

Dan Marchant · Jan 30, 2019

J Peters said:
However someone at the club I go to is concerned about the DPI figure and they always set it explicitly when they print. Moreover they have just bought a new camera (Micro 4/3rds sensor as a companion camera to their DSLR), and they noticed a value of 180 DPI is written into the metadata of the out-of-camera .jpg files. This has led them to believe the files are not as good quality as their previous camera of similar design, which embedded a value of 300 DPI.

Tell him to download one of the many free EXIF editing tools and change the number to 0 - does the image look different? Now change to 300 - does it look different?

Mystery member · Jan 31, 2019

J Peters said:
Hi, I have long thought that an image file does not have an inherent DPI (or PPI), believing any such figure to me meaningless until you come to print the image or view it on a monitor. As such, when I print, I don't give DPI a moment's thought - I just choose a paper size and the printer software does all the scaling for me.

Don't give it any more thought, as you are on the right track.

The scaling firmware of printers and display screens is very good. Although I generally trust the screen scaling, I must admit that I crop/resample my 6000x4000 images to 3840x2160 when displaying them on my 4k screens.

Doug Pardee · Jan 31, 2019

You are correct. The value in the file means nothing, unless the file was created by a scanner (in which case it allows you to determine the size of the original document).

Technically, the EXIF specification requires digital cameras to set that field to 72. Well, "those fields," since there are separate ones for X and Y direction.

In practice, many camera manufacturers put something else in there. Since those fields have no effect whatsoever on the image or its rendering, that causes no problem.

You are right to ignore it.

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DPI in .jpg Metadata

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