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I know this is very likely been asked before, but I don't know if it's the words or phrases I'm using or what but I can't find it. If it was resolved before, please just point me in the right direction.

I have to print a 20cm x 20cm image at 150dpi but the one I took with my camera is 10x10 at 300 dpi. (I'm using rounded numbers to make the example simpler)

Can I increase the size while sacrificing dpi and retain the same amount the quality? Is there a formular to calculate how much I can increase size per N dpi sacrificed? Can I do this in Photoshop?

Thanks

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It is much easier if you change you thinking pattern as follows:

  1. Instead of thinking in terms of physical units and DPI. Think about total pixels. After all for the digital images that all that counts. All DPI stuff just gets people perennially confused. It does not matter until you plan to print, a digital image is dimensionless.

  2. When you start to prepare for printing, you check that you have enough pixels. If you dont, then dont sweat it. There's little you can do about it anyway your camera makes the amount of pixels it does that's it. So what you do is you divide your piel dimensions with the PPI you want to target. Most likely 150-300 but choosing one is different subject. This gives you the biggest size that you can comfortably get for your image range. If you need bigger make it bigger and hope it looks ok:ish.

The only time you think size in terms of PPI/DPI/LPI is if, and only if, you can dominate the image capture and you know ahead where its going to be used, even then it's just best to stop thinking DPI but calculate how many pixels you want.

Once you think in pixels you realize that 150 DPI image 10 inches wide has as many pixels as a 300 DPI image 5 inches wide. They in fact have the same pixels, and are in fact the same image if they have a common source with just a different scaling factor. So its more natural to think this way.

Quality if the image is simply, in a way, available pixels. Visual quality on the other hand depends on who viewing it where how and so on. And print quality depends on many factors. 150 dpi on sublimation device is very good quality even up close, whereas not so much if its offset print.

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DPI, or more accurately PPI (pixels per inch) is just information how to scale pixels to a physical length measure, ie. PPI = number of horizontal pixels / physical width of the end result (or vertical pixels per height of the image, respectively). Hence, if you increase the size of your image and reduce the amount of PPI with the same ratio at the same time, your image data will not lose any quality. As long as you don't resample your image, the quality will always stay the same, because no image data will be touched, just the metadata.

Of course, the perceived quality of the image from the same distance drops, if you print the same image with half the PPI (and therefore double the width and height). But larger prints are usually made to be seen from farther away, so when a 300 PPI image is perfecly good down to a distance of approximately 10 cm (about the minimum human eye can focus to), 100 PPI image is still perfectly good from the distance of 30 cm.

Photoshop
Resize the image in Photoshop with resampling disabled (ie. tick off the Resample in the Image Size window). When you change the size, the PPI value will be adjusted automatically. Or vice versa, set the PPI to a desired minimum value and see how large the image would be.

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  • do you know de ratio of milimiters or inches per dpi that can be balanced? – Juan Dec 31 '14 at 2:56
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    If you want to print a photo, you should use a dpi value of 150 or higher. Otherwise you photo will have a pixelated look. Check you printer or the print service, if you use one, for recommended dpi values. Technically, every value is possible. – Mario Dec 31 '14 at 7:05
  • Of course, "quality will stay the same" means the quality of the data stays the same. The quality of the end result is always worse, if the same data is printed to a larger area. – Elmo Allén Dec 31 '14 at 12:59
  • Yes, that's what I meant. Thank you for mentioning it. But it's not that simple, because in the end the key quality factor is the viewing distance. – Mario Dec 31 '14 at 13:05
  • Yes, I mentioned it because the original question was a bit ambiguous about that. And yes, when halving the DPI and doubling the view distance, the perceived quality stays the same. I'll make an edit to the answer, if you will. – Elmo Allén Dec 31 '14 at 13:12
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I have to print a 20cm x 20cm image at 150dpi but the one I took with my camera is 10x10 at 300 dpi

It's math. The above two images are exactly the same in terms of image data:

20 x 150 = 10 X 300

The only time they'd differ is when printing from software that looks at the DPI meta data. If the software obeys the DPI setting, it will then do some math for you and make sure it prints it at the physical size the DPI calls for.

On the other hand, if you are importing these two images into a program such as InDesign and physically size them the same in InDesign, then when you print, they will be exactly the same.

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