Should I upload 1 file or 3?

Question

I just discovered digital prints on etsy about a month ago and I've been scrambling for the last 3 weeks to learn everything i can about PPI/DPI, Resizing images, GIMP, pixel wxh, file formats, megapixels, colorspaces etc. so I can get my stuff up there.

About a week ago after being totally lost I'm finally putting it all together. The one thing I'm hung up on is this. I resize my image on a google play app called Photo Editor, I size it to A2 Ya I even learned about International paper sizes and ANSI lol. I size it to A2- at 300 PPI. I have nice big A2 size 7-15MB files that MyOW's says are all around 30 mega-pixels.

So then I open that image in GIMP just to make sure its dimensions and resolution are correct (phone apps are buggy so) Here's something I noticed and I think it's important but I dont understand it.

I decided I would take the A2 about 16x22 inches 300 DPI file and save that as my medium sized image file. saved it.

Then i turned the Original file into an 8x10 inch at 600DPI.

And finally a Large size A0 about 33x46 inches at 150DPI.

Every time I converted the PPI/DPI and Dimensions it seemed like it wasnt taking long at all for GIMP to change the image like it would when i was experimenting with it trying to understand the process. It was almost instantaneous, it made me wonder,since i sized the original image at 300 dpi at A2 size- do I even need to make the small and large files? Like if I just uploaded the A2,file and they wanted a small size and they took it to their print shop/printed themselves an 8x10 inch would it appear to be/or even really be printed at 600dpi since its smaller?

And if they took it somewhere to print the A2 file at A0 size it would appear to be/or even really be printed at 150DPI?

Like is there some mathematical formula going on here where doing the other 2 files is pointless? Or should I go ahead and upload all 3 files- 1 for every size option for every image so I know the customer is getting the size they want at the quality I want them to be? Thank you so much mates, wish i had found this site 3 weeks ago. GL- W1TCH

Answer 1

When you change the DPI of an image in Image>Print size you only change "metadata": this PPI setting is merely an indication for whatever does the print (and can be ignored....). Gimp won't change the real size of the image that way.

In Image>Scale image you can set both a physical size (top) and a resolution (bottom) and Gimp will scale the image to the required size in pixels (always shown in small print under the top selector) knowing that the three values are inseparably bound by the equations (that are all different forms of the same one):

PhysicalSize = SizeInPixels / PPI
SizeInPixels = PhysicalSize * PPI
PPI = SizeInPixels / PhysicalSize

Answer 2

@xenoid has given the correct answer here, so I won't repeat it.

However there's something else I noticed about your question and your workflow.

TL:DR - Don't get so hung up on DPI for large format prints.

300DPI is a bit excessive for an A2 poster. You could easily get away with less. The larger the print, the less DPI you need because the natural viewing distance between viewer and image will be greater. Viewers tend to stand further back from a large print to see it all. To print something A2 all you need is something around 12 Megapixels for a good quality print. In fact a 12 Megapixel image is good enough to print at any size, even a billboard which is typically viewed from across a street.

Resampling an image that is already good enough to print, to an image size of 30 Megapixels, is not a good idea. Resampling degrades image quality. You might think you are increasing the quality, but you're not - all your doing is adding more pixels, not more image data.

I often print A1 posters with 12 Megapixel photographs at around 120 dpi, on a large format inkjet printer, and they look perfect. A2 prints still look good at around 170 dpi. I never resample the images.

300DPI is the print resolution typically used in quality magazines/publications and small prints which will be viewed up close at normal reading distances.

There's an excellent if somewhat old website with information that will help you understand DPI and resampling better as it relates to digital raster images. http://www.rideau-info.com/photos/mythdpi.html - Also another link here which shows the relationship between DPI and viewing distance: http://resources.printhandbook.com/pages/viewing-distance-dpi.php