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Changing DPI question

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Zvi Twersky
Changing DPI question
on May 25, 2015 at 10:09:14 am

So I've read a lot about DPI and resolution so I'll dive right into the question that's bothering me. (I'm rounding numbers so it's easier to understand.)

Since DPI is just the amount of dots printed per inch on a paper, when you change the DPI of an image in Photoshop WITHOUT re-sampling, the image resolution doesn't change. This is great and understood.

I tested this by opening a new document, setting the DPI to 300, and size to 80x30 cm. Dragged into it a 9000x3500 resolution image. Saved it to JPEG. Now I changed the DPI to 10 and saved again to JPEG. Yes, in Photoshop, the image size increased dramatically but the resolution stayed the same. Now I again changed the DPI back to 300 and the image size changed back to 80x30 and resolution stayed the same.

So first, please tell me this: by doing the above even 100 times back and forth, is it correct to say that I'm not changing any pixels or resolution in the image and that at the end, it would be as if I have an intact image of 80x30 cm, 9000x3500, 300 DPI with no effect from the tests?

Ok, hopefully you will say that the above is correct. Now for the question that I'm not understanding...
If DPI doesn't effect resolution, read this scenario:

I opened a new document with 80x30 cm but with only 200 DPI. Then I dragged in the same 9000x3500 image... went to image size and saw that the resolution was only 6300x2300 and of course, changing the DPI to 300 won't change this when NOT re-sampling.

So why if I open a document with 300 DPI will the resolution of the image stay 9000x3500 even if I later lower the DPI to 10 but if I open a document with 200 DPI, the dragged image resolution will immediately drop to 6300?



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Steve Crook Jr
Re: Changing DPI question
on May 26, 2015 at 1:43:50 pm

This is an informed guess based on a bit of experience. But STILL a guess:

When you drag in artwork at its native resolution (9000px x 3500px in your case) and change the DPI in the image size dialog BUT ensure the width and height are the same as the initial values, Photoshop does not resample the image. Fairly straightforward and consistent with your results.

When you drag your artwork into a document that has a lower resolution (as defined by pixels in either/both width by height, NOT DPI) Photoshop automatically downsamples the artwork to fit.

Basically, Photoshop makes it fit and tosses the extra info.

Again, this is just how it seems to work for me, and your tests seem to confirm it.

Your best bet to retain highest quality is to use File > Place Embedded -or- Linked so it is a smart object and retains all info no matter what you do to it. It resamples as needed, on the fly. (I added this last bit to maybe answer some other questions.)

Hope I helped a little bit...

Steve Crook, Jr.
http://www.stevecrookjr.me

I am a simple creative professional that can get my Adobe suite and a few other creative tools to do what I want. Barely. :)


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Bill Stephan
Re: Changing DPI question
on May 27, 2015 at 12:44:33 am

Rule of thumb: When dropping an image into another image with different specs, higher dpi = smaller image size, lower dpi = larger image size. Resolution is always converted to that of the document you paste into. Then use transform to adjust the size of the pasted image.

If you change the specs yourself in Image Size, then PS will recalculate anything you want.

Bill Stephan
Senior Editor/DVD Author
USA Studios
New York City


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Darby Edelen
Re: Changing DPI question
on Jun 6, 2015 at 4:12:27 am

I'm not quite sure I understand. If you make an 80x30cm document at 200dpi then of course the resolution is different than an 80x30cm 300dpi document.

Just because we're talking about dpi doesn't mean it's entirely unrelated to image resolution:

Inches x Dots per Inch = Pixels

When you specify both the print size and the dpi there is only one possible pixel resolution for the document.

In your first examples when you change from 300 to 10 dpi you are keeping the pixel resolution constant but changing both the print size and dpi. Here's a simplified example:

3000 pixels = 300dpi x 10in

3000 pixels = 10dpi x 300in

When you change the dpi without resampling the pixel resolution stays the same so the print resolution must change to keep the formula accurate.

However, when you're creating a new document you are defining the image size for the first time, if you define it using print size and dpi then Photoshop will simply calculate the appropriate pixel resolution:

10in x 300dpi = 3000 pixels

10in x 200dpi = 2000 pixels

All of these values are linked to each other.

Darby Edelen


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