How does PPI relate to DPI in photo metadata?

Question

I have been asked to provide pictures in JPEG format at a resolution of 300DPI. My computer give me a picture sizes in kilobytes and Photoscape the photo editing package that I use, converts kilobytes to PPI when a picture is loaded, the picture size can then be adjusted in PPI which is then changed back to kilobytes when the picture is returned to the computer.

I think I have read, that the numbers for DPI will, or should be the same in PPI. So, if it’s as simple as 300DPI = 300PPI.

Can I assume with any certainty that a picture changed to 300PPI by Photoscape and then converted back to kilobytes will actually be read as 300DPI by the recipient?

Answers that an old technophobe can grasp please. Many thanks for any help you can provide.

Answer 1

First, the "size" of your image is absolutely NOT measured in kilobytes. That is only the file size, which largely depends on the degree of compression you choose. Due to the JPG Quality factor selected, larger JPG files are better quality, and smaller JPG files are lower quality (lower quality is not normally the best choice), but the image size in pixels remains unchanged then.

This first part only addresses that confusion, but the answer is in the next part below.

Any JPG file is standard 24 bit color, which means the size of the image data is 3 bytes per pixel. But the image size is the number of pixels, for example, lets say image size is 4000 x 3000 pixels, which is 12 megapixels, so the uncompressed data size (data size when opened into computer memory) is 12 x 3 = 36 million bytes. However data (in the JPG file) is always compressed in some degree (controlled by the JPG Quality option), so the actual compressed file size might typically be maybe from 1/4 that to 1/12 that size. But this example image size remains the 4000x3000 pixels, regardless of compressed file size.

The 300 dpi is about printing it on paper, and 300 dpi means 300 pixels per inch of print paper. Therefore, to set print resolution (of this example 4000x3000 pixels image size) to be 300 dpi means it will print 4000/300 x 3000/300 = 13.3 x 10 inches in size on paper. This specific print size is simply because the image size is 4000x3000 pixels, and that's how far 300 pixels per inch will stretch when printed on paper.

Review:
uncompressed data size: 36 million bytes
JPG compressed data file size: Perhaps 9 to 3 million bytes.
print size at 300 dpi: 13.3 x 12 inches
image size: 4000 x 3000 pixels.
Each of these concerns have different units.

Whoever asked you for 300 dp should have specified the printed size in inches that they seek. Otherwise, 300 has no meaning or use for them. Dpi has nothing to do with image size, which is in pixels. Dpi does affect print size, but which needs to know the image size in pixels to have any meaning.

The asker probably implies they want a large image, probably the original camera image size in pixels. Meaning, they should have requested image size in pixels, or at least the printed size in inches if at 300 dpi.

The terms dpi and ppi used in printing are interchangeable with the same meaning, pixels per inch. There is more that can be said about that, but it is largely not important today, because printer drivers have mostly given up the notion of asking about print quality in "ink drops per inch", which is something entirely different. To bypass that, print quality today is the Good, Better, Best categories.

Using a photo editor, you can simply change the dpi number to be 300 dpi. This is called "scaling the printed size" in inches, which is what dpi does. That will then specify some specific printed size in inches, which may or may not be acceptable, depending on your image size in pixels, and on your purpose for using the image.

Yes, if you change dpi to be 300 dpi and then save the file, yes, it will then say 300 dpi in the file. Some editors today may call it 300 ppi, same thing.

This scaling to 300 dpi does NOT affect the number of bytes, UNLESS you also change either the resampled size in pixels, or change the JPG Quality factor.

It does change the scaled print size inches, to be the number of inches that 300 pixels per inch will print from the pixels that you have.

Answer 2

For JPEG images the size of the image in pixels plus the complexity of the images (and some other factors) influences the size of the image file in kilobytes, but there isn't necessarily a 1:1 link. So don't worry about that too much.

Also, the PPI is metadata (information about the image, instead of information in the image), so Photoshop doesn't always change the size of the image file when the PPI is changed. And yes, the metadata as set by Photoshop should be readable by most modern software and seen as DPI by applications that express PPI that way.

All that said, here is a little more of a dive into image resolution and images size, focusing on Photoshop but most image editing applications have similar tools:

When looking at the image size window (Image -> Image Size...), there will be four things that are important:

1) Width: the width of the image in pixels, which can be expressed as inches, which Photoshop converts to using the Resolution value (Inches here means, "if printed 100%" at the specified PPI density, this would be the size of the image).

2) Height: the height of the image in pixels (the same comments about width apply here as well).

3) Resolution: which is our PPI (if you have inches selected as the unit).

4) A checkbox that says "Resample". If Resample is not checked, that tells Photoshop to only change the metadata PPI settings - not the actual image pixels. If Resample is selected, then Photoshop will increase or decrease the number of pixels in the image to match your change in resolution - but will keep the printed Width and Height constant.

For example, let's say I've got a 1600x1000 pixel image at 144 ppi, which Photoshop says is 11.111 in x 6.944 in.

If I uncheck Resample and change the Resolution to 300 ppi Photoshop tells me that the image is now 5.33 in x 3.33 in. What this is telling us is that if we printed the image at a print density of 300 ppi the picture would only be 5.33 in x 3.33 in in size. The actual pixel dimensions of the image won't be changed.

If I check Resample and then change the Resolution to 300 ppi, Photoshop adds pixels to the file - we are now at 3333 x 2083 px, but the Height and Width stay the same (11.111 in x 6.944 in). Here the pixel dimensions will be changed (and the file size will probably be a lot larger as well. Telling Photoshop to "blow up" the image like this can make the image blurry -- how much depends on how many pixels are added and the complexity of the image.

So, what does this mean to you? Really, your instructions are incomplete - you would need to know 300 ppi at what minimum pixel size or print size, as 300 ppi by itself isn't very meaningful (Do they want an 8x10? A 2x3? PPI/DPI is normally only of concern to people printing images).

If it were me, I would uncheck the Resample checkbox, change the PPI (knowing that really no pixels are being changed) and save a copy out to send. If that gives you images that are too small, then you would need to turn Resample on and possibly lose some image quality.

Answer 3

dpi/ppi is a print scale, kbytes is a file size. They have as much to do with each other as the print quality of a book and its weight. Namely there is arguably some feeble relation depending on a whole lot of other variables but you would not think of those two as being related.

In fact, the dpi setting can be manipulated with Exif (metadata) editing programs that don't actually touch a single pixel of the image data. Loading and saving in typical image editing programs will cause recompression of the JPEG and consequently a loss of quality. However, photo managing programs (rather than image editing programs) may offer the ability to manipulate just the Exif data, for example for changing dates or adding GPS locations or adding a title or copyright notice, stuff that does not actually change the image data but gives additional information.

Answer 4

The true measure is pixels per inch, but people use DPI and PPI interchangeably (until they figure out that their 2400DPI printer uses several dots for a single pixel...).

There is a relationship between size in pixels, print size in inches, and definition in PPI[*]:

print size = size in pixels / definition 
definition = size in pixels / print size
size in pixels = print size * definition

(these three formulas are equivalent).

In other words, these three numbers are related, and you cannot change one without changing another.

When you are requested to provide 300PPI images (which is standard quality for laser printing), it means that the images should have enough pixels to be printed at 300DPI at their intended print size. For instance if the image is printed 5" wide it should be 5" * 300PPI = 1500 pixels wide.

Common pitfalls are:

• Getting the image size wrong: your image editor is showing the image "dot-for-dot", one pixel of the image being one pixel of the screen. This in effect shows you the image with your display screen definition (100-200DPI) so your image is bigger on the screen than in print. In other words, the physical size on your screen is not the size on the paper (unless you tell your editor to show you the image at true size, but this is rarely a default).
• Thinking that you can just change the PPI in the image file. As told above, this will make it print at a smaller size.
• Scaling up a small image to get the adequate number of pixels. This will make the image blurry, the final result won't be better than the initial image printed at a lower definition.

[*]The file size in KBytes/MBytes is unrelated. It has some relationship with the size in pixels (images with more pixels are usually encoded with more bytes) but it also depends on the image file format, the compression level, and the compressibilily of the image data (minute detail like grass or sand is harder to compress than relatively uniform areas like car bodies).

Answer 5

I am rephrasing somethings that other users already have said. But I will focus on the usage of Photoscape.

1. Regarding the "definitions".

Weight and PPI almost nothing to do with each other. Is like comparing the taste of the cake looking the size of it.

Photoscape does not convert anything into nothing. When you load an image you simply are reading data into a viewable image. Nothing has happened.

It shows you some data, but showing you this data does not mean the other data is converted.

PPI is just an instruction, it is a side note, you put in an image.

It is like adding, to a photo, a Post-it that says "Print this big" or "print this photo of this other size." It does nothing to the photo itself. You can even throw away the post-it and the photo will be fine. Only without your info about the size, it is supposed to be printed. So the other person (or program) just define it as they seem fit.

2. Regarding how Photoscape transform this PPI info.

I will "bite my tongue", but on this program, yes. PPI is the same as DPI. This usage of letters actually means different things, but I will not go into that.

I am no expert on Photoscape. (I just download it to test this) but it seems you need to actually change the real dimensions of the photo, for this PPI info to actually be changed, and after that re-saving (and recompressing) the image for this data to be saved. I do not recommend at all these two steps.

3. If you are using windows

I recommend you to use Irfanview. It is an extremely small program but with superpowers and with a lot of nice stuff.

On this case I recommend you to use the JPG lossless operations. This means the original data that actually forms your image will be kept, but only the small Post-it (Metadata) will be added.

As you familiarize on the usage, I recommend working on a copy of your files.

A. Install it and install the plugins.

B.Run the program, open the image and press Shift+J to open the JPG lossless operations.

Keep this marked options to change the PPI settings.

And Ouch... It also uses DPI and PPI interchangeably... :o(