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My file has resolution of 150 ppi, i duplicate that file and made another one of resolution of 300 ppi, with resample checkbox turned off. As a consequence of that, pixels from both images haven't changed and are the same, while only print size changed.

When i exported both files, one with 150ppi and one with 300 ppi, they were the same size 1.0 mb, even the information on jpeg is measuring the same pixels dimensions.

But when i place both of my images in illustrator, the one with 300ppi has smaller size, also when i placed it in indesign the one with 300 ppi is smaller, and the one of 150 ppi is larger.

How so, when they have the same pixel dimensions?

Can someone explain me logic behind this.

Thanks.

  • DPI/PPI is not a measure of quality in digital images. Read this: The Myth of DPI. It explains everything including why software like Illustrator/Indesign change the size. – Billy Kerr Oct 21 at 10:34
  • PPI is a rate, like miles per hour. And just like travelling a certain distance takes half the time at 100 mph than at 50, an image will be half the size at 300 ppi than 150. HTH. – MG_ Oct 21 at 11:07
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Yes.

Both files have the same number of pixels, so the same file size and image quality.

But when you import your images to InDesign, which takes into account image resolution to print them (since DPI is dots per inch, meaning it's useful to measure how large an image will print in a physical medium) the image with 300dpi will look smaller, because it has more dots per inch. Each pixel will be printed smaller on paper so it can fit more dots per inch.

An image 1000px x 1000px at 1000DPI will print at 1" x 1". The same exact image at 500DPI will occupy 2" x 2" when printed on paper. Simple math, you're just printing the pixels larger. Got it?

Photoshop doesn't care about the resolution, you're editing the image pixels there. 1000px are still 1000px no matter what resolution. It might make a difference when adding text, since the font sizes are usually pt and Photoshop will scale it differently based on the resolution, but if you set a font size to 20px it will also look the same regardless of the resolution. The resolution setting is there because you'll need it when printing.

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Just to make sure you know: ppi means pixels per inch.

Let's pretend we have an image which is 300×300 px. Its resolution is set to 150 ppi.

Then we make a duplicate of that image and set the resolution to 300 ppi (without resizing it).

These two images are exactly the same! They contain the same amount of pixels, the same information. Only difference is the resolution.

The resolution set in Photoshop is only a setting to tell other applications what the intended resolution is. It tells nothing about the image. It could be set to anything you like.

When you place an image in InDesign by clicking once (not dragging) the image will be placed at the resolution saved in the image.

So a 300×300 px image with the resolution set to 150 ppi will get the width:

300 px / 150 ppi = 2 inches

150 pixels are printed for each inch and since the image is 300 pixels wide the physical width of the image becomes 2 inches.

If we place the duplicated image (still 300×300 px but with the resolution set to 300 ppi) the width will be:

300 px / 300 ppi = 1 inch

It's the same image, but here 300 pixels are printed for each inch, so the image has higher resolution, but therefore also smaller physical size.

In InDesign you can select an image and in the Info Panel or the Links Panel you can see the Actual ppi and Effective ppi of that image (you might need to change the settings of each panel to get that info).

The Actual ppi is just the resolution saved in the image. Remember this is only a setting and has no real meaning.

The Effective ppi is the interesting one. This tells you which resolution the image has at the physical size you have chosen for it in your document. Always use this value to check if your images has the resolution you want for this particular print.

  • Wow, thanks you for putting an effort to explain this topic. I am so grateful! Thanks! Things are now more clear. – bladerunner12 Oct 21 at 17:05
  • You're welcome. I've explained this a thousand times for clients. I see this as practice. :-) – Wolff Oct 21 at 17:11
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Both illustrator and indesign emulate physical media (paper). Since you told that the other image has a denser pixel pitch (PPI) on output it imports as smaller (untill you decide otherwise). It simply has same amount of pixels in a smaller area.

It is still the same image, so size of disk is exactly the same.

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