If I double the size of the image and at the same time halve the DPI setting, does the quality stay the same?
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?
It is much easier if you change you thinking pattern as follows:
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.
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.