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Mylenium hit the nail squarely on the head. There are no inches in video, web page, or even a gif file. The PPI metadata does nothing to or for the image in either setting, and you can't even add PPI info to a GIF file. It does not support the metatag.
You usually set the scanning resolution in DPI or Dots Per Inch when you scan something. For every inch of the original object scanned, give me X number of pixels. When you set up an image for printing, you assign PPI or Pixels Per Inch to tell the printer how many pixels you want on every inch of paper. The resolution of an image has nothing to do with the PPI setting. That only controls the resolution of the print. Here's proof. Download both of these images:
Open them both in Photoshop or After Effects. They are the same size and the same resolution. Check the PPI. One is 1PPI, and the other is 9999PPI. Add both of these images to any app designed to print on paper like Apple Pages or MS Word. The 1 PPI image will fill the page and then some, and you will be lucky even to be able to see the 9999PPI image because it will be so small. Load them into After Effects or Premiere Pro, or as you can see, put them on a web page, and they will be the same size.
When preparing images for video production, you want to make sure that the part of the image you want to fill the screen is at least close to the same width as the composition. Scale any raster (bitmap) image or video file over about 120% you will start to see a significant loss in quality. The same thing happens with a 3D layer and a camera. If the camera is closer to the layer than the Zoom value of the lens (press 'aa' to see the zoom value of a camera layer), you are scaling up the layer. Moving the camera in is scaling the image.
When you originally scanned the image, if you wanted to zoom in to a one-inch wide section of the video using any video editing or graphics app, the scanner resolution should be the same as the width of the comp. One inch of the original image would have to be scanned at 1920 DPI for your project. When you open that image in Photoshop, the PPI would probably be set to 320 PPI unless the scanner software adds a PPI metadata tag to the image.
I hope this helps. Properly preparing images for video production is one of the least covered workflows in class and in tutorials, but it is one of the most important things you should learn how to do. There is much more to it than just properly setting up scanner resolution.
Thank you for your answer. luckily my scanner has the option to scan 1200 dpi!
These are small illustrations, and when open it in Ps the resulition shown is 1200 dpi.
My last question is: I usually assemble 5-6 of these small images as diferent layers in a Ps document. What would be there the optimal size of that blank document? Would it still be HD (1920x1080 72 ppi) or should this master document also be configured to twice the size/twice the resolution?
Many thanks
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