If I double the size of the image and at the same time halve the DPI setting, does the quality stay the same?


Question

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

2014/12/31
1
0
12/31/2014 5:32:00 PM

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.

2014/12/31
2
12/31/2014 1:30:00 PM